problems of megacities essay

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Urbanization and the Megacity

Across the globe and in a short amount of time, we’ve given up the tractor for the city bus, the open landscape for one of brick and mortar. We are now an urban planet. In fact, by 2008 over 50 percent of the global population was living in urban areas. It was 3 percent in 1800. Throughout history, cities have attracted people as centers of culture, religion, learning, and economics. Looking back, the first wave of urban migration took place in what are today’s more developed countries, especially in Europe and North America. But looking ahead, 90 percent of the future urban increase is expected to take place in Asia and Africa, and it is projected that more than two-thirds of all people will be calling cities home by 2050.

Urbanization is often linked with economics – increased job opportunities, a centralized market, better pay and higher individual wealth have all drawn people into cities. And for a long time, these pull factors are what caused cities to grow. The Industrial Revolution caused a shift from agriculturally based societies to industrial, and thus geographically centered, societies. But that dynamic is changing. Today, most urban growth is natural increase – due to more births than deaths among those already dwelling in cities. Additionally, formerly small settlements are being reclassified as urban areas as the populace living there grows from within.

Implications of Growing “Too Fast”

Depending on cities’ ages and locations, there is much variation in wealth and infrastructure. Many of the newer urban areas, located in Latin America, Asia and Africa, have an entirely different look, feel, and outlook than their older European or North American counterparts. How fast an area grew, or is growing, is a key component.

When a city grows at a manageable rate, which is often considered roughly 1 percent annually, its infrastructure can keep pace with an increasing population and its demands. Necessities such as roads and public transportation, appropriate sewers and water treatment facilities, clinics, schools and housing have time to be planned and built alongside the increase in human numbers. The risk of fast urban growth, especially in an economically strained country, is that the necessary infrastructure often cannot expand fast enough to keep up with residents’ needs. Without infrastructures in place to provide basic needs, residents can be forced to create their own provisions with whatever is available.

The Rise of Slums

In less developed countries, densely populated slums form both on the edges and within the largest cities. Due to a poor economy and weak infrastructure, cities such as Mumbai, India do not have the means to support the overwhelming urban population. According to the 2018 UN World Urbanization Prospects Report, Mumbai ranked as the seventh largest city in the world, with 20 million people in the entire metropolitan area. Even more striking, over half of Mumbai’s metro residents live in slums surrounding the city, causing huge public health, environmental, and land use problems.

Slum dwellers survive with practically no sanitation, water, urban amenities, employment, or security, and roughly one-seventh of the world’s population lives under these conditions. The lack of running water and sanitation, plus malnutrition and inadequate housing, leads to deadly conditions in the slums and shantytowns that surround many cities in Africa, Asia, and Latin America. The spread of HIV/AIDS and other infectious diseases in areas where so many people live in such close physical proximity is a critical public health issue for urban areas throughout the developing world. When combined with high unemployment rates and inadequate schools, these public health issues create a poor quality of life for many of the city’s residents.

The Emergence of Megacities

The urban shift over time has led to the emergence of the megacity – a city with a population of 10 million or more. New York City and Tokyo were the first known megacities, both reaching an urban conglomeration of over 10 million by the 1950s. But today they are far from alone in their size. In 2018 there were 33 megacities across the planet – from Sao Paulo, Brazil to Lagos, Nigeria and London, England to Shanghai, China – and all major global regions except Oceania are marked with megacities.

Most of the cities that have reached the 10 million marker in recent years are located in Asia and Africa. In fact, it’s where six of the eight newest megacities can be found and where nine of the 10 projected 2030 megacities will be located. These regions are also home to the fastest growing megacities. The population of Kinshasa, capital of the Democratic Republic of the Congo, has doubled roughly every 5 years since 1950. From 2010 to 2015, Kinshasa’s population grew by over 23 percent, and today over half of the more than 13 million residents are under 22 years old. A combination of factors has led to this growth including migration from rural areas, high fertility rates, and widening of the city’s boundaries. The population is outpacing almost all support structures in the city where the threat of food shortages, traffic congestion, and insufficient education facilities have become a stark reality.

Environmental Pros and Cons

A large urban population may seem environmentally troublesome with cities viewed as a disruption to the natural world. But environmentalism and urbanization are not incompatible. Dense urban areas have a much smaller ecological footprint – many people live in apartments or smaller connected houses rather than ranch-style homes in sprawling neighborhoods. Multifamily dwellings have the added benefit of being more energy efficient and they require less resources per person. Cities are also walkable and have public transportation options that can make cars less of a necessity. And above all, densely populated areas make it possible to protect other open spaces to serve as wildlife habitat, farmland, conservation areas, or oxygen-producing forests.

But of course, there are ecological downsides to cities as well. Concentrations of people mean concentrations of pollutants and trash. Cities produce up to 70 percent of global CO2 emissions and smog is becoming a common feature in many urban landscapes. Large swaths of continuous pavement prevent water drainage and boost temperatures. Without proper infrastructure, cities also risk having waste – both trash and human waste – clogging waterways and causing damage. And with cities across the globe producing more than 2 billion tons of waste annually, that’s a lot for one area to handle.

Planning an Urban Future

It is predicted that most future urban growth will happen in settlements currently home to between 100,000 and 250,00 people, and if this is to be done sustainably, planning is a must. Future high-growth areas require strategic urban planning individually tailored to a city’s history, culture, value system, and other specificities; a single cookie-cutter approach won’t work, nor will the plans of the 20th century. But by keeping an eye towards social justice concerns, natural resource use, environmental hazards, and other issues of modern cities, urban plans can help ensure the health and well-being of tomorrow’s city dwellers.

distribution of world urban Population by area

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Megacities and the Developing World

Author: George Bugliarello

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The problem with megacities.

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The triumphalism surrounding the slums and megacities frankly disturbs me. It is, of course, right to celebrate the amazing resilience of residents living in these cities' massive slums. But many of the megacity boosters miss a more important point: that the proliferation of these sorts of communities may not be desirable or even necessary.

Cities may be getting larger, particularly in the developing world, but that does not make them better. Megacities such Kolkata (in India), Mumbai, Manila, Sao Paolo, Lagos and Mexico City -- all among the top 10 most populous cities in the world -- present a great opportunity for large corporate development firms who pledge to fix their problems with ultra-expensive hardware. They also provide thrilling features for journalists and a rich trove for academic researchers.

But essentially megacities in developing countries should be seen for what they are: a tragic replaying of the worst aspects of the mass urbanization that occurred previously in the West. They play to the nostalgic tendency among urbanists to look back with fondness on the crowded cities of early 20th Century North America and Europe. Urban boosters like the Philadelphia Inquirer 's John Timpane speak fondly about going back to the "the way we were" -- when our parents or grandparents lived stacked in small apartments, rode the subway to work and maintained a relatively small carbon footprint.

Unfortunately such places were often not so nice for the people who actually lived in them. After all, they have been moving from higher to lower density locations for over fifty years, a trend still noticeable in the new Census. As my mother, who grew up a slum-dweller, says of her old Brooklyn neighborhood: "Brownsville was a crappy neighborhood then, and it's a crappy neighborhood now."

My mother considers herself a tried and true New Yorker, but she and my late father chose to raise their kids on Long Island. She now lives in an apartment in Rockville Centre, somewhat farther out on the Island. One could imagine many slum-dwellers in developing countries would also choose a less crowded environment for themselves and their children, if that option existed.

Most slum-dwellers, at least from what I have seen in India, move to the megacity not for the bright lights, but to escape hopeless poverty in their village. Some argue that these migrants are better off than previous slum-dwellers since they ride motorcycles and have cell phones.

But access to the wonders of transportation and "information technology" is unlikely to compensate for physical conditions that are demonstrably worse than those my mother endured.  At least Depression-era poor New Yorkers could drink water out of a tap and expect consistent electricity, something not taken for granted by their modern day counterparts in Mexico City, Manila or Mumbai.

More serious still, the slum-dwellers face a host of health challenges that recall the degradations of Dickensian London. Residents of mega-cities face enormous risks from such socially caused maladies as AIDS and other sexually transmitted diseases, urban violence, unsafely built environments, and what has been described as  "the neglected epidemic" of road-related injuries. According to researchers Tim and Alana Campbell , developing countries now account for 85% of the world's traffic fatalities.

One telling indication of the difficulties the newcomers face is the relatively low level of life expectancy in the city -- roughly 57 years -- which is nearly seven years below the national average.

Even with solid economic growth, these megacities are not necessarily becoming better places to live. In 1971, slum dwellers accounted for one in six Mumbai-kers; now they constitute an absolute majority. Inflated real estate prices drive even fairly decently employed people into slums. A modest one-bedroom apartment in the Mumbai suburbs, notes R. N.  Sharma of the Mumbai-based Tata Institute of Social Sciences, averages around 10,000 rupees a month, double the average worker's monthly income.

Traffic congestion is also worsening. Nearly half of Mumbai commuters spend at least one or two hours to get to work, far more than workers in smaller rivals such as Chennai, or Hyderabad. Fifty percent of formal sector workers expressed the desire to move elsewhere, in part to escape brutal train or car commutes; only a third of workers in other cities expressed this sentiment.

What does this say about the future for megacities?  When conditions become oppressive enough, people generally respond by finding a better place to live. Poor village dwellers in Bihar may not all stay in the countryside, but they -- and many better-skilled immigrants -- may find other, less intense urban options.

Recent research suggests that these immigrants will increasingly move to the urban fringe or to smaller cities. A massive research effort published earlier this year for the Lincoln Institute of Land Policy found that since 1990 "built-up area densities" have been dropping by roughly 2% a year, including in the developing world.

An impressive new study by the McKinsey Global Institute, called " Mapping the Economic Power of Cities ," has found that "contrary to common perception, megacities have not been driving global growth for the past 15 years." Many, the report concludes, have not grown faster than their host economies.

McKinsey predicts these cities will underperform economically and demographically as growth shifts to   577 "fast growing middleweights," many of them in China and India.  We can see this already in the shift of industrial growth to smaller cities in India. There may be an additional 25 million jobs added to the Indian auto industry by 2016, according to recent estimates, it appears most will go to other states, such as Gujarat, West Bengal and Tamil Nadu, enriching cities such as Chennai and Ahmedabad, nut not Mumbai.

These realities lead some advocates in developing countries to question the logic of promoting megacities. Tata's Sharma notes that as manufacturing and other industries move to smaller, more efficient cities, they remove many middle-income opportunities. Instead, the gap between the megacity's rich and poor expands more rapidly.   "The boom that is happening is giving more to the wealthy.  This is the 'shining India' people talk about," Sharma says. "But the other part of it is very shocking, all the families where there is not even food security. We must ask: The ‘Shining India' is for whom? "

Ashok R. Datar, chairman of the Mumbai Environmental Social Network and a long-time advisor to the Ambani corporate group, suggests that Asian megacities should stop emulating the early 20th Century Western model of rapid, dense urbanization. "We are copying the Western experience in our own stupid and silly way," Datar says. "The poor gain on the rich. For every tech geek, we have two to three servants.

Datar suggest that developing countries need to better promote the growth of more manageable smaller cities and try bringing more economic opportunity to the villages.  One does not have to be a Ghandian idealist to suggest that Ebenezer Howard's "garden city" concept -- conceived as a response to miserable conditions in early 20th Century urban Britain -- may be better guide to future urban growth.

Rejecting gigantism for its own sake, "the garden city" promotes, where possible, suburban growth, particularly in land-rich countries. It also can provide a guide to more human-scale approach to  dense urban development. The "garden city" is already a major focus in Singapore, where I serve as a guest lecturer at the Civil Service College. Singaporean planners are embracing bold ideas for decentralizing work, reducing commutes and restoring nearby natural areas.

These ideas may be most relevant to cities on the cusp of rapid growth, such as Hanoi. As we walk through the high-density slums on the other side of the dike that protects Hanoi from the Red River, Giang Dang, founder of the nonprofit Action for the City, tells me that rapid growth is already degrading the quality of Hanoi's urban life, affecting everything from the food safety to water to traffic congestion. Houses that accommodated one family, she notes, now often have two of three.

Expanding Hanoi's current 6 million people -- already at least twice its population in the 1980s -- to megacity size -- say between 10 million and 15 million -- may thrill urban land speculators but may not prove  so good for city residents.  Like Datar, Dang favors expanding conditions both smaller cities, and the Vietnamese countryside.

"The city is already becoming unlivable," she  insists. "More people, more high-rises will not make it better. Maybe it's time to give up the stupid dream of the megacity."

Such voices are rarely heard in the conversation about urban problems.  But the urban future requires radical  new thinking.  Rather than foster an urban form that demands heroic survival, perhaps we should focus on ways to create cities that offer a more a healthful and even pleasant life for their citizens.

Joel Kotkin is a distinguished presidential fellow in urban futures at Chapman University. He is also an adjunct fellow at the Legatum Institute in London and serves as executive editor of newgeography.com . He writes the weekly New Geographer column for Forbes . His latest book, The Next Hundred Million: America in 2050 , was published February 2010 by Penguin Press.

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Introductory lecture: air quality in megacities

First published on 30th October 2020

Urbanization is an ongoing global phenomenon as more and more people are moving from rural to urban areas for better employment opportunities and a higher standard of living, leading to the growth of megacities, broadly defined as urban agglomeration with more than 10 million inhabitants. Intense activities in megacities induce high levels of air pollutants in the atmosphere that harm human health, cause regional haze and acid deposition, damage crops, influence air quality in regions far from the megacity sources, and contribute to climate change. Since the Great London Smog and the first recognized episode of Los Angeles photochemical smog seventy years ago, substantial progress has been made in improving the scientific understanding of air pollution and in developing emissions reduction technologies. However, much remains to be understood about the complex processes of atmospheric oxidation mechanisms; the formation and evolution of secondary particles, especially those containing organic species; and the influence of emerging emissions sources and changing climate on air quality and health. While air quality has substantially improved in megacities in developed regions and some in the developing regions, many still suffer from severe air pollution. Strong regional and international collaboration in data collection and assessment will be beneficial in strengthening the capacity. This article provides an overview of the sources of emissions in megacities, atmospheric physicochemical processes, air quality trends and management in a few megacities, and the impacts on health and climate. The challenges and opportunities facing megacities due to lockdown during the COVID-19 pandemic is also discussed.

Introduction

In 1970, only 37% of the world’s population lived in urban areas; this increased to 55% by 2018 and is projected to increase to 68% by 2030, with almost 90% of the growth happening in Asia and Africa, 3,4 as shown in Fig. 1 . In 2018, about 23% of the world’s population lived in 550 cities with at least 1 million inhabitants, of which 50 cities had populations between 5 million and 10 million, and 33 cities had more than 10 million inhabitants (loosely defined as megacities). The world is projected to have 43 megacities (representing 8.8% of the global population of 8.5 billion) by 2030, with most of them located in developing countries, 3 many are facing the challenge of growing their economies and managing the environment to provide better quality of life and cleaner and breathable air for the population.

Activities from megacities are responsible for the emissions of primary pollutants, including gaseous species such as volatile organic compounds (VOCs), nitrogen oxides (NO x ), carbon monoxide (CO), sulfur dioxide (SO 2 ), ammonia (NH 3 ) and air toxins ( e.g. , benzene, 1,3-butadiene), some of which contribute to the formation of secondary pollutants such as ozone (O 3 ) and secondary aerosols; semi-volatile species ( e.g. , polycyclic aromatics, dioxins, furans); particulate matter ( e.g. , combustion soot, dust); and metals ( e.g. , lead, mercury). Despite the large amount of research that has been conducted, including air quality monitoring, field measurements and modeling studies, the sources and processes of emissions generated in megacities that lead to high concentrations of major pollutants such as ozone and secondary particulate matter, are still not well understood, thus limiting our ability to mitigate air pollution and make air quality forecasts to alert the residents of potentially unhealthy air pollution episodes.

This article will provide an overview on the sources of emissions in megacities, atmospheric physicochemical processes, air quality trends and management programs in a few megacities, and the impacts on health and climate change. The final section describes the challenges and opportunities facing the megacities due to lockdown during the COVID-19 pandemic. There is a very large volume of literature articles and topics related to megacities, this article will not be able to cover all of them, but will present the main ideas and include references where more information is available.

Sources of emissions in megacities

The following sections provide an overview of some of the major sources and control strategies.

Transportation

Several megacities have implemented measures to reduce emissions from on-road vehicles, including improvement of vehicle technology and fuel quality, implementation of strict international vehicle emission standards, replacement of diesel with natural gas, and introduction of hybrid and electric vehicles. Some megacities have introduced more efficient mobility through a number of strategies, such as improving the efficiency and security of the public transportation network to encourage use of public transit, expanding infrastructure for non-motorized transportation (walking and cycling); reducing traffic congestion by limiting the circulation of vehicles ( e.g. , “No drive day” in Mexico and many megacities) (SEDEMA, http://www.aire.cdmx.gob.mx) and road pricing ( e.g. , London) (https://tfl.gov.uk/modes/driving/congestion-charge).

In the last decades, technological advances have been responsible for significant changes observed in the energy performance, fuel efficiency, and emissions reductions of vehicle fleets (https://ww2.arb.ca.gov/). Few other polluting sources have been so dramatically affected by improvements in emission control technology as motor vehicles. As a result of the technological and regulatory measures implemented, many megacities with high vehicle turnover or retrofit rates have experienced overall vehicle emission reductions despite large increases in fleet size, particularly for gasoline-powered vehicles. However, reductions of emissions from some megacities in developing countries have been more difficult to achieve, partly due to limited access to economic instruments that promote the acquisition of emission control technologies and fleet turnover programs. Reducing transport-related emissions will continue to present a challenge with growing urban population and transportation demand for both people and freight. It is important to integrate land-use planning, infrastructure development and transportation management in the design of policy options.

In addition to on-road vehicles, non-road vehicles such as agricultural and construction equipment can substantially contribute to emissions of particulate matter, CO 2 , CO, NO x and VOCs. In contrast to on-road vehicles, there is no regulation on the emissions levels for in-use non-road vehicles and they are often kept in service for several decades. Their relative emissions contributions increase over time as emissions from on-road vehicles continue to be reduced by advanced technologies. 6–8

Area sources

Municipal solid waste (MSW) is the third largest source of global anthropogenic methane emissions, generating about 800 million tons of CO 2eq annually; 10 it is also a significant source of black carbon and CO 2 . Many cities have recycling and waste separation programmes – some are converting organic waste into compost. Landfill is the most widespread method for final waste disposal, followed by capturing of the landfill gas for power generation as part of the integrated waste management program. Many megacities, with large urban populations, are host to some of the world’s largest landfills. Along with urban population growth, the amount of solid waste generated in megacities is expected to continue to increase, which will pose a major challenge for MSW management.

The use of solid fuels for cooking and heating continues to be a major source of emissions with adverse health effects. Some cities are promoting the use of cleaner fuel, such as liquefied petroleum gas (LPG). However, emissions from LPG leakage in homes, businesses and during distribution can contribute to substantial emissions of VOCs (mainly propane and butane), as shown in Mexico City. 11,12 Many cities are promoting energy efficiency programs for public and private buildings, including incentives for using renewable-energy technologies, such as solar heating systems and solar water heaters.

As the emissions of urban VOCs from transport-related sources have decreased due to technological advances and regulatory measures, volatile chemical products (VCPs) from sources such as consumer products (personal care and household products), aerosol coating, painting, solvent use and pesticides have gained in importance. A study by McDonald et al. 13 found that VCPs have emerged as the largest photochemical source of urban organic emissions, highlighting the need for regulatory actions to control the sources.

In some megacities, especially in developing regions, the agricultural sector is a large source of emissions, generated from raising domestic animals, operation of heavy-duty farming machinery, application of nitrogen based fertilizers and chemical pesticides, and burning of crop residues. Agriculture has evolved as a major source of global ammonia emissions. Enteric fermentation from ruminant livestock is one of the largest sources of methane; numerous studies are underway to mitigate the enteric methane production as well as livestock manure management. 10

Biomass burning

There are many sources and fire types related to biomass burning emissions; some are natural sources such as forest fires, while others, such as emissions from burning of crop residue, municipal solid waste, residential wood burning for cooking and heating, and biofuel for brick production, are the result of human activities. Different approaches have been used to estimate emission factors for biomass burning, including direct measurements over fires in field experiments, 18 aircraft measurements, 19–22 and laboratory measurements. 23 Andreae and Merlet 24 compiled emission factors for about 100 trace-gases and aerosols emitted from burning of savannas and grasslands, tropical forest, extratropical forest, domestic biofuel, and agricultural waste burning. The compilation was updated by Andreae 14 to include the number of species and the burning types. Akagi et al. 25 have also compiled emission factors for open and domestic biomass burning for use in atmospheric models.

In spite of the significant progress in emission factor measurements, detection and quantification of fires, there is still a need to improve the accuracy in the activity estimates, both for open burning and biofuel use. Once released, the gas and particle emissions undergo substantial chemical processing in the atmosphere. In some cases, this processing may lead to compounds that are more detrimental to human health. In the case of wildfires, some of the large number of compounds from fire smoke are not found in a typical urban atmosphere; more research is needed to better understand the chemical processes forming secondary pollutants, 26 especially as smoke plumes are transported into urban population centers. 27

Agricultural residue burning has been a common practice in many regions around the world to control pests and weeds and to prepare the land for the next crop, which releases a large amount of aerosols and trace gases to the atmosphere. 14,24,25 In some countries in South America, e.g. , Argentina and Brazil, the burning of stubble has decreased substantially due to investment in direct drilling, known as no-till, which seeds into untilled soil without removing stubble; restrictions on burning; and the use of machinery for harvesting. 10 Prescribed burning is an important forest management tool to reduce fuel loading and improve ecosystem health. Most of the burning tends to occur in the non-summer months and is a major source of combustion products to the atmosphere. Wildland fires are a natural occurrence (but often caused by arson and deliberate clearing of rain forest); they are a critical part of ecosystems. However, the area burned in recent years has markedly increased, as demonstrated by the fires in the Amazon, Indonesia, and the western USA. Several studies have documented the importance of climate change on the increasing frequency and size of fires in the western USA, especially in California. A warmer and drier climate is expected to lead to more frequent and more intense fires near or within populated areas. 27–29

Many megacities in developing countries still use biomass and fossil fuels (wood, agricultural wastes, charcoal, coal and dung) for residential and industrial cooking and heating; these are a major source of several gases and fine particles in developing countries, and in the wintertime in developed regions. 18,25,30 Another important source is open garbage burning, which occurs not only in rural, but also in urban areas, especially in cities that do not have adequate solid waste disposal facilities such as landfills. 10 In some cities, small scale brick production is an important source of urban pollution because of the burning of high polluting fuels such as wood, coal and dung, emitting significant levels of black carbon, organic carbon and other pollutants. 31–33

The chemical composition of biomass burning particles shows a wide variety of organic compounds, fragments, and functional groups, 26,34,35 in addition to the classic tracer levoglucosan. 36 Recent work shows the important contribution of secondary organic aerosol to biomass emissions. 37 Health effects of biomass burning, similar to hydrocarbon burning, has been shown to include carcinogenic compounds in varying amounts depending on fuel, burning conditions, and secondary contributions.

Atmospheric physicochemical processes in megacities

In the early 1950s, Arie Haagen-Smit and his coworkers discovered the nature and causes of Los Angeles smog: principally that a major component of smog is O 3 formed by NO x (produced by combustion sources, cars, heaters, etc. ) and VOCs (from evaporation of gasoline and solvents, etc. ) in a complex series of chemical reactions that also produce other oxidants and secondary aerosols. 43 They also reported that synthetic polluted air exposed to sunlight could cause plant damage observed by Middleton et al. 44 Since then, high O 3 levels have been observed in many urban areas throughout the world, and photochemical smog – induced primarily from transport and industrial activities – is now recognized as a major persistent environmental problem and a priority research area for atmospheric scientists.

Fig. 2 shows an overview of our current understanding of the complex physicochemical processes taking place in the atmosphere.

Gas phase chemistry

In urban environments, there are many VOC sources of anthropogenic and biogenic origin. VOC oxidation initiated by OH during the daytime produces a number of organic products of oxygenated functional groups, such as aldehyde, ketone, alcohol, carboxylic acid, hydroperoxide, percarboxylic acid, and peroxyacyl nitrate groups. 51,52 The relative abundance of these products depends on the VOC structure, the NO x level, temperature, relative humidity (RH), and the solar intensity. Some compounds formed in the first oxidation step undergo additional oxidation reactions to yield multi-functional groups, and the resulting multi-generations of products are of lower volatility and higher solubility in comparison with their parent compounds.

NO x plays an important role in determining the fate of peroxy radical intermediates (HO 2 and RO 2 ). Under relatively clean (low-NO) conditions, peroxy radicals will react with other peroxy radicals or (in the case of RO 2 ) will isomerize; under polluted urban conditions, peroxy radicals will react with NO, forming NO 2 , which rapidly photolyzes in the daytime, producing O 3 . 53 However, the ambient levels of OH, HO 2 , and RO 2 radicals not only depend on NO x but also simultaneously on the VOC abundance and VOC reactivity. This dual NO x and VOC–reactivity dependency ultimately controls the chemical regimes of O 3 production. Therefore, successful emission control policies strongly depend on determining the chemical regimes of O 3 production.

The sensitivity of O 3 production to changes in concentrations of precursor VOCs and NO x is complex and nonlinear. 53–55 Under high VOC concentrations and low NO x concentrations, O 3 production rates increase with increasing NO concentrations (NO x limited) due to increases of NO 2 through reactions of NO and peroxy radicals. But at higher NO x , O 3 production rates decrease with increasing NO x (NO x saturated) due to OH and NO x termination reactions that form HNO 3 and alkyl nitrates. The additional NO x also serves as a sink for OH radicals, slowing down the oxidation of VOCs and suppressing O 3 production. NO x can also sequester O 3 in temporary reservoirs such as NO 2 and N 2 O 5 ; in these conditions, lower NO x emissions can lead to higher O 3 concentrations. This result could suggest that O 3 production in polluted urban areas, as is the case in many megacities, may be in the NO x saturated regime. 56–59 However, recent studies indicate that O 3 production can have marked spatially different chemical regimes within megacities due to the heterogeneous distribution of VOC and NO x sources, VOC reactivity, and meteorological conditions. 60,61

During the night-time, the oxidation of NO via O 3 and organic radicals has two main effects: it depletes night-time O 3 levels and accumulates NO 2 and NO 3 radicals that subsequently form N 2 O 5 and HNO 3 through heterogeneous reactions. 62 This condition increases the early morning NO 2 /NO ratios and affects O 3 production during the next day by increasing the contribution of excited oxygen atoms via NO 2 photolysis. The accumulated NO 2 and nitrate can also form HONO through surface-catalyzed reactions, 63 further impacting the accumulation of free radicals.

Particulate matter

The abundance and chemical constituents of PM 2.5 vary considerably in urban cities, depending on the complex interplay between meteorology, emissions, and chemical processes. 67,68 Fig. 3 shows the average measured chemical composition of submicron PM (PM 1 ), which typically comprises most of the PM 2.5 for various megacities, urban areas, and outflow regions around the world. 69 A substantial fraction of urban PM 1 is organic aerosol (OA), which is composed of primary OA (POA, organic compounds emitted directly in the particle phase) and secondary OA (SOA, formed from chemical reactions of precursor organic gases). SOA is typically a factor of 2 to 3 higher than POA for these locations.

More recently, ultrafine particles (UFP, particles with diameter 0.1 μm or less) have become increasingly important in urban air because they are produced predominantly from local combustion processes with major contributions from vehicular exhaust and new particle formation (NPF) in cities. 70–73 Particles that are smaller than 1 μm have both longer lifetimes and higher probability of penetration into alveolar sacs in the lungs, and even smaller “nanoparticles” (<100 nm in diameter) have been shown to have some of the most toxic exposures. Recent evidence suggests that nanoparticles and transition metals, which are also associated with fossil fuel combustion, may play an important role. 74–79

Currently, particle mass concentration has been used for regulatory air quality standards. However, this metric accounts mainly for larger particles with larger mass, while particle number concentration (PNC) has been used as a metric for UFP, which are smaller with little mass. de Jesus et al. 80 evaluated the hourly average PNC and PM 2.5 from 10 cities over a 12 month period and observed a relatively weak relationship between the two metrics, suggesting that control measures aiming to reduce PM 2.5 do not necessarily reduce PNC. It is important to monitor both PM 2.5 and UFP for health impact assessment.

For developing effective pollution control strategies and exposure risk assessment, it is necessary to know the contribution of the various sources of pollutants. Several techniques have been used in source apportionment studies of PM, including chemical mass balance (CMB) and positive matrix factorization (PMF) analysis on filter-based chemical speciation data, carbon mass balance modeling of filter-based radiocarbon ( 14 C) data, aerosol mass spectrometry or aerosol chemical speciation monitoring coupled with PMF. While significant progress has been made in evaluating the sources of pollutants, some sources remain poorly characterized, such as food cooking and open trash burning (see e.g. , Molina et al. 81 for Mexico City).

Pandis et al. 82 investigated the PM pollution in five cities (Athens, Paris, Pittsburg, Los Angeles and Mexico City) and found that reductions of emissions from industrial and transportation related sources have led to significant improvements in air quality in all five cities; however, other sources such as cooking, residential and agricultural biomass burning contribute an increasing share of the PM concentrations. These changes highlight the importance of secondary PM and the role of atmospheric chemical processes, which complicate the source apportionment analysis. Xu et al. (DOI: 10.1039/D0FD00095G ) evaluated the fine OC and PM 2.5 in Beijing using different methods (CMB, PMF and AMS/ACSM-PMF) and found that the fine particles were mainly secondary inorganic aerosols, primary coal combustion and biomass burning emissions. Although there are some consistencies, modeled contributions for several sources differed significantly between the different methods, particularly for cooking aerosols.

New particle formation

Extensive efforts have been made to elucidate the fundamental mechanism relevant to atmospheric NPF from field measurements, laboratory experiments, and theoretical calculations. Previous field studies include measurements of ultrafine particles down to approximately 1 nm in size, gaseous concentrations of nucleating precursors (such as H 2 SO 4 , NH 3 , and amines), and pre-nucleation clusters. 95,96 Numerous laboratory experiments have been conducted to understand aerosol nucleation. 92,97–99 In addition, theoretical investigations of aerosol nucleation have been carried out to determine the stability and dynamics of pre-nucleation clusters using thermodynamic data from quantum chemical calculations. 100–102

NPF events occur with a frequency of 50, 20, 35, and 45% in spring, summer, fall, and winter, respectively, in Beijing. 102–104 NPF events have been occasionally measured in Houston during several campaigns. 105,106 In addition to the correlation with elevated SO 2 , 107 the contribution of secondary condensable organics to NPF is implicated in Houston. 108 On the other hand, NPF events are rarely measured in Los Angeles. 109 One plausible explanation is that the heavy accumulation of pre-existing particles and low levels of SO 2 lead to unfavorable conditions for aerosol nucleation in the Los Angeles basin. 110 NPF events are frequently observed during field campaigns in Mexico City 81,111,112 and are usually accompanied with a high level of SO 2 , 113 indicating that the oxidation of SO 2 contributes to the formation and growth of freshly nucleated particles. The polluted layer substantially ventilated from the Mexico City basin represents another potential factor in driving NPF in the afternoon, which is characterized by a decrease in pre-existing particle concentrations preceding the NPF events. 114

A recent study shows the striking formation of NPF in urban air by combining ambient and chamber measurements. 91 By replicating the ambient conditions ( i.e. , temperature, relative humidity, sunlight, and the types and abundance of chemical species), the existing particles, photochemistry, and synergy of multi-pollutants play a key role in NPF. In particular, NPF is dependent on preexisting particles and photochemistry, both of which impact the formation and growth rates of freshly nucleated nanoparticles. Synergetic photooxidation of vehicular exhaust provides abundant precursors, and organics, rather than sulfuric acid or base species, dominating NPF in the urban environment.

Another laboratory chamber study by Wang et al. 115 reported that airborne particles can grow rapidly through the condensation of ammonium nitrate (NH 4 NO 3 ) under conditions typical of many urban environments in wintertime, such as Beijing and Delhi. NH 4 NO 3 exists in a temperature-dependent equilibrium with gaseous NH 3 and HNO 3 , but NH 4 NO 3 can quickly condense onto newly formed clusters at temperatures below 5 °C, allowing the clusters to reach stable particle sizes before they are scavenged by other existing particles in the atmosphere. Moreover, at temperatures below −15 °C, NH 3 and HNO 3 can nucleate directly to form NH 4 NO 3 particles. The formation of new particles through NH 4 NO 3 condensation could become increasingly important as the SO 2 emissions continue to reduce due to pollution controls implemented in many cities. This may in turn imply the importance of controlling NO x and NH 3 emissions.

Based on the observations in Beijing over a period 14 months, Kulmala et al. (DOI: 10.1039/D0FD00078G ) found that almost all present-day haze episodes in Beijing originate from NPF, suggesting that air quality can be improved by reducing the gas phase precursors for NPF, such as dimethyl amine, NH 3 and further reductions of SO 2 emissions, as well as anthropogenic organic and inorganic gas-phase precursor emissions.

Secondary organic aerosol

Atmospheric models typically underestimate the SOA mass measured in field studies if only traditional SOA precursors are considered. 122,129–131 Inclusion of non-traditional SOA precursors, such as organic gases from POA evaporation and di-carbonyls, has helped to bring better closure between models and observations. 132–136 However, there are still inconsistencies between modeled and measured SOA yields, which can be explained by several factors, including incorrect emission inventories, missing precursors, and unaccounted processes of gas-to-particle conversion. 13,68

Nault et al. 69 investigated the production of anthropogenic SOA (ASOA) in urban areas across three continents (see Fig. 3 ) and observed that it is strongly correlated with the reactivity of specific VOCs; the differences in the emissions of aromatics and intermediate- and semi-volatile organic compounds (IVOC and SVOC) influence the ASOA production across different cities. Emissions from fossil fuel sources ( e.g. , gasoline, diesel, kerosene, etc. ) and volatile chemical products (VCPs, such as personal care products, cleaning agents, coatings, etc. ) contribute nearly similar amounts to estimated ASOA, further supporting the important role of VCPs in urban air quality. 13

Sulfate formation

The aqueous-phase conversion of dissolved SO 2 to sulfate driven by O 3 and hydrogen peroxide (H 2 O 2 ) is an important chemical formation pathway in cloud/fog water, but the two SO 2 oxidation pathways still cannot close the gap between field observations and modeling studies. 139 Aqueous SO 2 oxidation by O 2 catalyzed by transition metal ions (TMI) in models has improved sulfate simulations, 140 and recent studies have further revealed the enhanced effect of TMI during in-cloud oxidation of SO 2 ( ref. 141 ). However, during wintertime haze days free of cloud or fog in North China, rapid sulfate production has been observed 142,143 showing that the sulfate formation mechanism is still not well understood.

A laboratory/field study of wintertime haze events in Beijing and Xi’an has indicated that the aqueous oxidation of SO 2 by NO 2 is key to efficient sulfate formation under the conditions of high RH and NH 3 neutralization, 143,144 Li et al. 145 proposed a SO 2 heterogeneous formation pathway, in which the SO 2 oxidation in aerosol water by O 2 catalyzed by Fe 3+ , is limited by mass resistance in the gas-phase and gas–particle interface, and closes the gap between model and observation. A recent experimental study has highlighted that the oxidation of SO 2 by H 2 O 2 in hygroscopic, pH-buffered aerosol particles occurs more efficiently than under cloud water conditions, because of high solute strength. 146 Furthermore, another recent study 147 has unraveled a novel sulfate formation mechanism, showing that SO 2 oxidation is efficiently catalyzed by black carbon (BC) in the presence of NO 2 and NH 3 , even at low SO 2 levels (down to a few ppb) and an intermediate RH range (30–70%). The sulfate formation mechanism during wintertime haze days in China is still controversial considering the uncertainties of the aerosol pH value, rather low oxidants level, and possible loss of active sites in BC.

Nitrate formation

Aerosol radiative effect.

Aerosol radiation interaction has also significantly contributed to the PM pollution during haze days. 68 It is well established that ARI cools the surface but heats the air aloft, increases the atmospheric stability, enhances accumulation and formation of PM 2.5 in the planetary boundary layer (PBL), and eventually deteriorates the air quality during haze days. 155–157 Wu et al. 157 have revealed that the ARI contribution to near-surface PM 2.5 concentrations increased from 12% to 20% when PM 2.5 concentrations increased from 250 to 500 μg m −3 during a persistent and severe PM pollution episode in the North China Plain. However, modification of photolysis caused by aerosol absorbing and/or scattering solar radiation (referred to as aerosol–photolysis interaction or API) changes the atmospheric oxidizing capability and influences secondary aerosol formation. Coatings also affect the ratio of absorption to scattering, and these control changes in radiative forcing. 158 Simulations have revealed that API hinders secondary aerosol formation and substantially mitigates the PM pollution caused by ARI. 159

It is worth noting that ARI or API is highly sensitive to the single scattering albedo (SSA) that is dependent on aerosol composition, particularly regarding absorbing aerosols including BC and brown carbon (BrC). 159 Primary BC and BrC aerosols undergo chemical transformation in the atmosphere, by coating with organic and inorganic constituents, commonly referred to as the aging process. 158 Aging of primary aerosols not only changes the particle mixing state ( i.e. , from externally to internally), but also alters the particle properties, including the morphology, hygroscopicity, and optical properties, further enhancing aerosol absorption capability. 117,160–162

Air quality management in megacities

Since 1987, the WHO has produced air quality guidelines designed to inform policy makers and to provide appropriate targets in reducing the impacts of air pollution on public health. Currently, many countries have established ambient air quality standards to protect the public from exposure to harmful level of air pollutants and are an important component of national risk management and environmental policies. National standards vary according to the approach adopted for balancing health risks, technological feasibility, economic, political and social considerations, as well as national capability in air quality management. Some countries set additional standards for lead and CO. Table 1 presents the current air quality standards for O 3 , PM 10 , PM 2.5 , SO 2 , CO, Pb and NO 2 for China, India, Mexico and the USA, together with the WHO guidelines. 163

The WHO estimated that about 90% of the world’s population breathe polluted air, many of the world’s megacities exceed WHO’s guideline levels for air quality by more than 5 times. 164 Fig. 4 shows the annual average of PM 2.5 for a three-year period (2017, 2018, and 2019) for the megacities where data is available; this includes London, Seoul and Chengdu.

The data were compiled by IQAir 165 from real-time, hourly data from government monitoring stations, validated PM 2.5 monitors operated by private individuals and organizations. Ideally, the monitoring data used to calculate the average annual PM concentrations should be collected throughout the year, for several years, to reduce bias owing to seasonal fluctuations or to a non-representative year. However, data for most cities are not available for trend analysis. Some of the megacities were not included in the report; the data were taken from WHO 166 for a single year reported in 2015 or 2016.

The PM 2.5 levels for all the megacities shown in Fig. 4 , with the exception of New York, are above the WHO guideline value of 10 μg m −3 . The megacities with the highest PM 2.5 concentrations are located in South Asia; however, comparison of the three-year data show reduction in the PM 2.5 levels in the cities from 2018 to 2019. Much of this can be attributed to increased monitoring data, economic slowdown, favorable meteorological conditions and government actions. For example, 2019 marked the launch of India’s first National Clean Air Program, which set PM 2.5 and PM 10 targets and outlined new strategies for tackling air pollution. However, India still has a relatively limited air quality monitoring network, with many communities lacking access to real-time information. 167

The data shown in Fig. 4 are the annual average; however, there is a large seasonal variation for some cities (Delhi, Lahore, Dhaka, Kolkata), as shown in Fig. 5 , due to geographical location and prevailing meteorology. The PM 2.5 concentrations are the highest in November to January, and the lowest from July to September, as monsoon rains wash out airborne particulates, leading to cleaner air. During the winter, emissions from residential heating, burning of crop residues, and intensive brick production lead to higher PM 2.5 concentrations in these cities. The landlocked geography of Delhi and the coastal location of Mumbai influence the distribution of air pollutants in the two cities. 168

Lahore ranks as one of the megacities with the highest annual PM 2.5 concentrations, weighted by city population. Until recently, there was no government monitoring in Pakistan. The data provided in the IQAir 165 report (2019) comes from low-cost sensors operated by individuals and non-governmental organizations. Recently the Pakistan government cited air pollution as a key priority and has reinstated the monitoring infrastructure in Lahore. Current anti-smog measures include stricter emission standards on factories and penalties for high-polluting vehicles and farmers burning crop stubble.

Although more countries are taking action and more cities are now included in the air quality database, there are still many cities that do not have ambient monitoring and their residents do not have access to air quality information where pollution levels may be high. For example, South America is the most urbanized region of the world; five of the megacities are located in this continent: Bogotá (Colombia), Buenos Aires (Argentina), Rio de Janeiro (Brazil), São Paulo (Brazil) and the metropolitan area of Lima-Callao (Peru). Recently, Gómez Peláez et al. 169 reviewed the air quality trends of the criteria pollutants collected by the automatic monitoring networks of 11 metropolitan areas in South America, including four megacities (Rio de Janeiro, São Paulo, Buenos Aires, and Lima). Despite concerted efforts to monitor air quality, the data provided by environmental authorities in some cities are of poor quality, making it difficult to assess the air quality trends and take action for critical air pollution episodes. Integration of the emission from the whole continent and their application in an air quality model are essential to investigate the effect of long-range transport and to construct air quality and emission control strategies for the entire region. Integrated coordination due to transboundary pollution transport, mainly from the biomass burning in the Amazon basin, is essential, especially considering the record-breaking number of Amazon fires in 2019 and again in 2020. Analysis of an aerosol particles’ chemical composition and optical properties during the biomass burning season in 2014 showed that, depending on the wind direction, smoke plumes from central Brazil and southern regions of the Amazon basin can be transported over São Paulo. 170

In February 2020, the United Nations Environment Programme (UNEP), together with the UN-Habitat and IQAir, launched the world’s largest air quality platform, bringing together real-time air pollution data from over 4000 contributors, including governments, citizens, communities, and private sectors. 171 This partnership covers more than 7000 cities worldwide and aims to empower governments to take action to improve air quality, allowing citizens to make informed health choices, and businesses to make investment decisions promoting a cleaner and greener environment.

The following describe the air quality trends and air quality management programs for Los Angeles, the Mexico City metropolitan area and four Chinese megacities. While the differences in the governance, economics, and culture of the megacities greatly influence the decision-making process, all have overcome severe air pollution and have made significant progress in reducing concentrations of harmful pollutants by implementing comprehensive integrated air quality management programs. The experience can be valuable for other megacities.

Air quality in the Los Angeles basin

Following the recognition of Los Angeles photochemical smog as a severe environmental problem in the 1940s, comprehensive emissions control efforts have been implemented by the air quality management authorities, the California Air Resources Board (CARB) and the South Coast Air Quality Management District (SCAQMD) principally, particularly in the transportation sector, which plays a major role in the air pollution problem. The urban center is decentralized; major commercial, financial and cultural institutions are geographically dispersed, relying on a vast network of interconnected freeways. The emissions control measures included the introduction of unleaded gasoline and an eventual complete ban of lead in gasoline, three-way catalytic converters, stringent NO x control for ozone and PM 2.5 , low-sulfur fuels, and diesel particle filters. Other regulations such as controls on power plants and boilers have reduced smog-forming oxides of nitrogen emissions, rules on consumer products such as paints and solvents have limited volatile organic compounds, and other controls on gasoline components, chrome platers, dry cleaners, and other sources have reduced levels of airborne toxics. 6,172

Other emission sources include goods movement sources, such as railroads, ocean-going vessels, commercial harbor craft, cargo handling equipment, drayage trucks, and transport refrigeration units. California adopted the first-in-the-nation regulation requiring ocean-going vessels to use cleaner fuel when near the California coast in 2008, which has been effective in reducing SO 2 emission from ships. 173 Emissions from ports have also been reduced by making shore power available to docked ships that previously idled their engines, while the more polluting drayage trucks are either removed from service or retrofitted. 6 The Advanced Clean Car Regulation (https://ww2.arb.ca.gov/our-work/programs/advanced-clean-cars-program) is the latest of a series of technology-forcing standards aimed at limiting passenger vehicle emissions and reducing smog as well as mitigating climate change. 174 As a result of the stringent emissions reduction measures, peak ozone levels and PM 2.5 concentrations in Los Angeles today are about one third of their level in 1970. Nevertheless, the ozone concentration is frequently still above the current USA ambient 8 h ozone standard of 70 ppb (see Fig. 6 ).

One of the main challenges is that a substantial fraction of the ozone in Southern California is transported into the region from outside its border, which is not subject to local control. This includes the baseline ozone concentrations, which are not affected by continental influences, such as ozone transported from the Pacific 175 and the background ozone (the ozone concentration that would be present if anthropogenic precursor emissions were reduced to zero), which are affected by continental influences such as deposition to continental surfaces, vegetation, production from natural ozone precursors ( e.g. from trees, soils and lightning). 176 This could be as high as 89% of the USA NAAQS (62.0 ± 1.9 ppb) and that about 35 years of additional emission control efforts will be needed to meet the NAAQS.

Altuwayjiri et al. (DOI: 10.1039/D0FD00074D ) investigated the long-term variations in the contribution of emission sources to ambient PM 2.5 organic carbon (OC) in the Los Angeles basin and the effect of the regulations targeted tailpipe emissions during 2005–2015. They found a significant reduction in the absolute and relative contribution of tailpipe emissions to the ambient OC level, while the relative contribution of non-tailpipe emissions (road dust resuspension, tire dust, and brake wear particles) increases over the same period, suggesting that the regulations were effective but also underscore the importance of regulating non-tailpipe emissions.

Recent wildfires in California have markedly increased, worsening air quality in much of the region. A warmer and drier climate is expected to lead to more frequent and more intense fires near or within the populated areas, threatening to undo the significant improvement in air quality after decades of implementing the Clean Air Act. 27–29 Long-term monitoring and reevaluation of forest management strategies will be needed to address the wildfire problem as climate change continues to bring hotter and drier conditions conducive to wildfire activity. 177

Air quality in the Mexico City metropolitan area

Field measurements studies conducted in the MCMA during MCMA-2003 ( ref. 112 ) and MILAGRO-2006 ( ref. 81 ) showed that ozone formation was generally VOC-limited within the urban core, while mostly NO x -limited in the surrounding area depending on the prevailing meteorology, 56–58 and that O 3 production might continue in the outflow for several days due to the formation of peroxyacetyl nitrate (PAN), which could regenerate NO x and contribute to regional O 3 formation. 181 A recent study by Zavala et al. 60 shows that there is an overall reduction in the VOC–OH reactivity during the morning hours in the urban area with large spatial variability, implying a large spatial variability in O 3 production, which in turns suggests spatially different O 3 sensitivity regimes to precursor gases. While alkanes (from leakage and unburned LPG used for cooking and water heating) are still key contributors to VOC–OH reactivity, the contribution from aromatic and alkene species has decreased, consistent with reduction of VOCs from mobile sources. Changes in ozone production suggest that increases in the relative contributions from highly oxygenated volatile chemical products, such as consumer and personal care and solvent use, are responsible for sustained high O 3 levels in recent years. The study also found a significant increase in NO 2 /NO ratios, suggesting changes in the night-time accumulation of radicals that could impact the morning photochemistry. The results suggest a need for a new field study of radical budgets in the MCMA, expanding measurements of VOCs to include oxygenated species, and using the data to support modeling studies in the design of new air quality improvement programs.

The MCMA faces additional challenges from regional contributions. Urban expansion of the MCMA has produced the megalopolis, consisting of Mexico City and contiguous municipalities of six surrounding states. The Megalopolis Environmental Commission (CAMe, https://www.gob.mx/comisionambiental) was created in 2013 to coordinate the regional policies and programs; however, the different administrative and legislative jurisdictions and the available resources have created an ongoing challenge. With the exception of the MCMA, there is limited air quality monitoring and air pollution studies in the other states, making it difficult to evaluate the regional air quality and the impacts of pollutants in the region. 9

Burning of regional biomass is a major contributor of fine particles to the MCMA during air pollution episodes (http://www.aire.cdmx.gob.mx). During the dry season, agricultural and forest fires in the surrounding areas are frequent, the wind transports air masses enriched with organic aerosols, VOCs, as well as other reactive gases to the MCMA, severely impacting the air quality. 20 Lei et al. 182 evaluated the impact of biomass burning in the MCMA and found that biomass burning contributed significantly to primary organic aerosol (POA), secondary organic aerosol (SOA), and elemental carbon (EC) locally and regionally but has relatively little effect on O 3 . Despite the important contribution of biomass burning to local and regional air quality, the authorities did not include fire mitigation in the air quality management strategies. In May 2019, following a severe air pollution episode caused by regional wildfires, the authorities announced new action contingencies, adding PM 2.5 threshold level, in addition to O 3 and PM 10 , to the contingency plan.

Air quality in Chinese megacities

As a global hub for manufacturing, the heavy industries, including production of iron and steel, other nonmetal materials and chemical products, play an important role in the Chinese economy, resulting in huge consumption of energy and large emissions of air pollutants. In addition, more and more people are moving from rural to urban areas, leading to a fast expansion of cities and an increasing demand for vehicles, contributing to severe air pollution.

Faced with increasing pressure on the environment in urban development, the Chinese government launched the Action Plan for Air Pollution Prevention and Control (Action Plan) in September 2013 (http://www.gov.cn/zwgk/2013-09/12/content_2486773.htm), which stated the development targets and roadmap for 2013–2017. The Action Plan provides the framework for air pollution control measures in cities, covering capacity building, emission reduction measures and supporting measures. The implementation of a series of control measures, including coal combustion pollution control, vehicle emission control and VOCs control, have resulted in the reduction of most pollutants and a large decrease in PM 2.5 concentrations. 167 However, according to a government report, 183 74.3% of 74 key cities exceeded the NAAQS of annual mean PM 2.5 concentrations (35 μg m −3 ) in 2017.

As shown in Fig. 7 , while the concentration of most pollutants have decreased for each city, O 3 was not effectively controlled (red line). Also, the annual mean PM 2.5 concentrations still exceeded the NAAQS of China, except Shenzhen, the first city that met the PM 2.5 standard. The exceeding of the PM daily average concentration often occurred during the cold winter from November to February, while the maximum daily 8 h average concentration of O 3 is more likely to exceed in the summer during the afternoon, according to the local monitoring data (China National Environmental Monitoring Center, http://www.cnemc.cn/en/).

Although the major sources of emissions differ among the four cities, in general, vehicle emissions remain the primary source of air pollution and contribute significantly to VOC, NO x , CO, and PM 2.5 (including BC). The primary source of SO 2 is fossil fuel combustion from industry and mobile vehicles; while road and construction dust is the main source of PM 10 , and agriculture is the primary source of NH 3 .

Vehicle emission control has been a priority of air quality management, and the cities have continuously tightened emission standards for new gasoline and diesel vehicles; promoting the use of electric vehicles through subsidies. 167 Shenzhen is the first megacity in China and in the world to adopt electric vehicles for the entire public transportation system. However, as a coastal city, diesel trucks carrying large amounts of cargo is the primary source of local emissions and ocean-going vessels, contributing a large portion of SO 2 due to the use of low-quality heavy fuel oil. Emission control policies for the port area and the ocean-going vessels are areas also being implemented. In addition to local sources, pollutants transported from the outskirts have contributed to the pollution levels of the cities. 167

Although the PM levels have decreased significantly due to the stringent measures implemented by the government authorities, 184–186 summertime O 3 concentrations have increased in the megacity clusters in China. 187,188 Several studies have investigated the anthropogenic and meteorological factors that are responsible for the O 3 pollution in China. 189–194 Flat VOCs emissions and reduced NO x emissions have slightly increased the O 3 concentration in most urban areas of eastern China. A significant anthropogenic driver for the O 3 enhancement is the over 40% reduction of PM 2.5 in the North China Plain (NCP), which slows down the aerosol sink of hydroperoxyl radicals, thus stimulates the O 3 production. 61,193 However, meteorological influences have been thought to be comparable to or even more important than the impact of changes in anthropogenic emissions. Increased solar radiation reaching the surface level due to the decrease of cloud cover, cloud optical thickness as well as the aerosol optical depth has promoted photochemical reactions and resulted in O 3 enhancement. Higher temperature, as a result of enhanced solar radiation, has been recognized as an important factor corresponding to the increasingly serious O 3 pollution for enhancing biogenic emissions and decreasing O 3 dry deposition. 195

The dominant cause of increasing O 3 due to changes in anthropogenic emissions was found to vary geographically. In Beijing, NO x and PM emission reductions were the two main causes of O 3 increase; in Shanghai, NO x reduction and VOC increase were the major causes; in Guangzhou, NO x reduction was the primary cause; and in Chengdu, the PM and SO 2 emission reductions contributed most to the O 3 increase. 195,196 While NO x reduction in recent years has helped to contain the total O 3 production in China, VOC emission controls should be added to the current NO x –SO 2 –PM policy in order to reduce O 3 levels in major urban and industrial areas.

In addition to O 3 , there have been several extreme haze events in China during wintertime in recent years, as a consequence of diverse, high emissions of primary pollutants ( e.g. , from residential heating) and efficient production of secondary pollutants. 68,197–201 In particular, the North China Plain (Beijing–Tianjin–Hebei) and Chengdu–Chongqing region have suffered from severe haze pollution. 68,197,198 Unfavorable meteorological conditions enhancing the air static stability and shallow planetary boundary layer due to aerosol–radiation and aerosol–cloud interactions, could also aggravate severe haze formation. 68,157,159

Atmospheric NH 3 plays an important role in fine particle pollution, acid rain, and nitrogen deposition. In contrast to those in developed countries, agricultural NH 3 emissions largely overlap with the industrial emissions of SO 2 and NO 2 in northern China. A model study showed that the average contribution of the agricultural NH 3 emissions in the NCP was ∼30% of the PM 2.5 mass during a severe haze event in December 2015. 68 Control of NH 3 would mitigate PM pollution and nitrogen deposition. However, another study 202 found that NH 3 control would significantly enhance acid rain pollution and offset the benefit from reducing PM pollution and nitrogen deposition.

The examples of the four Chinese and two North American megacities illustrate the complexity of managing urban pollution. In spite of significant progress in cleaning the air, there are still remaining challenges. While each city has its own unique circumstances – geographical location, meteorology, emission sources, financial and human resources, the need for an integrated, multidisciplinary assessment of the complex urban air pollution problem is the same. In light of the multicomponent nature of air pollution, application of integrated control strategies that address multiple pollutants, supported by ambient monitoring, emissions characterization, air quality modeling, and comprehensive rather than separate strategies for each single pollutant, would be more cost-effective. 203

Impacts of degraded air quality in megacities

Health effects.

Over the past few decades, data on air quality has become increasingly available and the science underlying the related health impacts is also evolving rapidly. Effects of air pollution on human health have been investigated with epidemiology, animal studies, and human exposure studies. Populations at greater risk include children and the elderly and those that have pre-existing conditions such as diabetes, or cardiovascular and respiratory diseases. While many countries have established air quality standards for criteria pollutants, or follow WHO guidelines ( Table 1 ), there is an ongoing debate as to the maximum permissible limit of a particular pollutant concentration. As more information becomes available, the standards have been strengthened to protect public health.

It is evidenced from epidemiological and clinical studies that exposure to particulate matter, especially PM 2.5 , is linked to cardiorespiratory disease and adverse birth outcomes. 208–211 Although there is a large volume of research on the adverse effects of PM exposure, composition of the particles and the mechanisms causing such association are still not well understood. The physicochemical characteristics of PM vary according to emission sources, secondary atmospheric chemical reactions and meteorological conditions. Other factors can also affect the toxicity of PM, such as the metal content of the particles and their reactivity. For example, some physiological studies of health effects have shown that the causes of cell degradation from exposure to fine particles are most likely from specific toxic compounds, such as polycyclic aromatic hydrocarbons (PAHs) and black carbon.

More recently, ultrafine particles were found to possibly exert higher toxicity than larger particles due to their small size; they generally enter the body through the lungs but are translocated to essentially all organs. 212,213 Nanoparticles and transition metals, which are also associated with fossil fuel combustion, may also play an important role. 74–79 Although exposure to UFP is commonly attributed to vehicular exhaust, monitoring in Ghana showed higher exposure from trash burning and domestic cooking. 213

Several approaches have been used to elucidate the mechanism of toxicity, one is the use of in vivo experimental models to evaluate the effects of PM on the respiratory, cardiovascular and nervous system, another one is in vitro models, which has proven useful for investigating mechanistic responses, such as inflammatory/immune alterations and genotoxicity. 214 Besides the well-documented impacts on respiratory and cardiovascular health, the evidence is accumulating around exposures during pregnancy and adverse birth outcomes, cancer, brain alterations and interactions between infectious agents and air pollution. 215,216 H. Bové et al. 217 reported the presence of black carbon particles as part of combustion-derived PM in human placenta, suggesting that ambient particulates could be transported towards the fetus, representing a potential mechanism for the adverse health effects of pollution from early life onwards.

Many studies have investigated the association between oxidative potential of air pollutants with adverse health outcomes; however, there are some contradictory results. For example, Quintana et al. 218 reported the oxidative potential correlated with PM 10 Cu/Zn content but not with the in vitro biological effects from samples collected in Mexico City during the MILAGRO field campaign. Weichenthal et al. 219 examined the relationship between PM 2.5 oxidation burden and cause-specific mortality; the results suggest that glutathione-related oxidative burden may be more strongly associated with lung cancer than the mass concentrations. Strak et al. 220 examined the role of particle size, composition and oxidative potential; the results suggest that changes in particle number concentrations (PNC), NO 2 and NO x were associated with acute airway inflammation and impaired lung function, while PM mass concentration and PM 10 oxidative potential were not predictive. A study conducted in central London also indicated the association of PNC with cardiovascular effects, while non-primary PM components (nitrate, sulfate, chloride and organic carbon) were associated with adverse respiratory outcomes. 221

Results from the various studies and epidemiological evidence suggest that each megacity will have contributing factors that create different air pollution impacts on health, 222 among those could be specific chemical mixtures in the atmosphere, meteorology, socioeconomic conditions/disparities. An important research topic is the health impact related to exposure from smoke from biomass burning (wildfires, burning of agricultural residues and trash, etc. ). PM 2.5 , O 3 , and other compounds in smoke have clearly demonstrated human health impacts; however, the episodic nature of smoke exposure and the large and variable mix of compounds make health studies even more challenging than traditional air pollution episodes. It is important to better understand the long-term consequences, such as birth outcomes, neurological and cognitive effects, and progression and incidence of chronic disease related to smoke exposure and to establish exposure guidelines. 27

To estimate the health impact to be expected from measures affecting air quality, it is important to conduct health risk assessment. An important step is the exposure–response function, such as the exposure–mortality model (EMM) which is based on total concentration of PM 2.5 and does not consider the unequal toxicity of different components of PM 2.5 . Xue et al. (DOI: 10.1039/D0FD00093K ) developed a component-specific EMM (CS-EMM) using the census data, the concentration of ambient PM 2.5 and satellite-based concentrations simulated by a chemical transport model. The CS-EMM was found to perform better than the EMM. Among the components, although BC contributed only 6.4% of PM 2.5 , it corresponded to a 46.7% increase in PM 2.5 -associated deaths. This new approach will allow policy makers to target the toxic source of air pollution and design cost-effective control strategies.

Recently Apte et al. 66 estimated the population-weighted median decrement in life expectancy from PM 2.5 (ΔLE). If PM 2.5 concentrations worldwide were limited to the WHO air quality guideline concentration of 10 μg m −3 , global life expectancy would be on average 0.59 years longer. This benefit would be especially large in countries with the highest current levels of pollution, with approximately 0.8–1.4 years of additional survival in countries such as Egypt, India, Pakistan, Bangladesh, China, and Nigeria. In contrast, many high income countries already nearly meet the WHO guideline and would have much smaller LE benefits; for example, the ΔLE of 0.38 years for the USA is about 3 times lower than that of countries with higher PM 2.5 concentrations. The result of this study illustrates that reducing air pollution at all levels of economic development could lead to substantial gains in life expectancy, a benefit similar in magnitude to that of eradicating lung and breast cancer.

Regional and global climate impacts

The contributions of megacities to global anthropogenic emissions have been estimated to be 12%, 7% and 4% for CO 2 , CH 4 and N 2 O respectively for the base year (2005), and are projected to increase significantly in 2050, 224 while the contribution to BC, OC, CO, NO x and SO 2 are relatively small (3 to 5%). With the exception of CO 2 , all the estimated emissions are disproportionately smaller compared to the population. This could be due in part to some of the energy production taking place outside the cities. However, there is a large uncertainty in estimating the emissions and their geographic distributions; further research is needed to better understand the role of megacities in the Earth’s environment.

Recently, short-lived climate forcers (SLCFs), also known as short-lived climate pollutants (SLCPs), have received increasing attention due to their relatively short residence time in the atmosphere and the multiple benefits of reducing them using existing available technologies. 10,225–227 The major SLCFs with lifetimes under a few decades are BC (∼days to weeks), CH 4 (∼a decade), tropospheric O 3 (weeks to months) and some hydrofluorocarbons (HFCs, average 15 years). Due to their nature, these substances can be rapidly controlled, providing near-time climate benefits and air quality improvement. It is important to emphasize that despite these near-term benefits, reducing warming in the longer term will also require action now to reduce current and future CO 2 emissions.

Anthropogenic CH 4 is emitted into the atmosphere from ruminant livestock, rice cultivation, microbial waste processing (landfills, manure, and waste water), coal mining, and oil and natural gas systems. Methane has about 34 times the Global Warming Potential (GWP) of CO 2 (100 year horizon). Due to its shorter lifetime, it is even more effective over a 20 year time horizon. Methane is included as one of the six greenhouse gases (CO 2 , CH 4 , N 2 O, HFCs, perfluorocarbons (PFCs), and sulfur hexafluoride (SF 6 )) controlled under the Kyoto Protocol. Black carbon is emitted directly into the atmosphere in the form of PM 2.5 ; from diesel engines, industrial sources, residential coal and solid biofuels for cooking and heating, and agricultural and forest fires and open burning of solid waste. Black carbon could be the second largest contributor to global warming after CO 2 . 228–229 Although there are large uncertainties about the magnitude of BC climate impacts, it is very likely that mitigating sources with a high proportion of BC, such as diesel engines, will have positive climate benefits, in addition to significant improvement in public health. Many countries have included or are in the process of including BC reduction in their national determined contributions (NDC) to the United Nations Framework Convention for Climate Change (UNFCCC, https://unfccc.int/); Mexico was the first country to commit to reducing black carbon. HFCs are synthetic chemicals produced for use as substitutes for ozone-depleting substances in refrigeration, air-conditioning, insulating foams, aerosols, solvents, and fire protection. However, most HFCs currently in use have high GWP. In 2016, the Parties to the Montreal Protocol agreed to the Kigali Amendment to phase down the production and consumption of HFCs (Ozone Secretariat, https://ozone.unep.org).

Many megacities are located along coastal areas, on floodplain and in dry areas, and are increasingly experiencing the effects of extreme weather and climate-related events, such as heat waves, hurricanes, heavy downpours, flooding, droughts, and more frequent and intense wildfires. Despite these risks, many cities have not yet incorporated climate action plans to existing urban planning due to a lack of resources to prepare for the extreme events as well as public awareness on climate change and its impacts. The continued urban expansion and infrastructure development provide an opportunity for cities to manage risks and develop strategies for climate mitigation and adaptation at the local level while at the same time improving the air quality. Organizations such as C40 Cities, a network of the world’s megacities, is supporting cities to collaborate and share knowledge to drive measurable and sustainable action on climate change (http://c40.org).

Megacities not only influence the environment as large sources of pollutants, but also change the urban landscape and meteorological conditions by replacing vegetation and green areas with asphalt and concrete for roads, buildings and other structures to accommodate the growing population, creating the urban heat island effects. 230,231 The temperature difference between the urban area and the rural surroundings is usually larger at night than during the day and most apparent when winds are weak. The increased demand for air conditioning to cool buildings and homes relies on power plants to meet the demand, leading to an increase in air pollution and greenhouse gas emissions. Higher air pollution, reduced night-time cooling and increased daytime temperature can adversely affect human health and comfort. Some megacities, such as Mexico City, are using green roofs and vertical gardens to help reduce urban heat island effects by shading building surfaces.

Impact of COVID-19 on megacities’ air quality

Transmission of SARS-CoV-2 is considered to be predominantly by respiratory droplets produced when an infected person coughs, sneezes, or talks. Most public health guidelines have focused on social distancing measures, regular hand-washing, and other precautions to avoid large respiratory droplets. 238 Several studies have implicated airborne transmission of SARS-CoV-2 via respiratory microdroplets as a probable route for the spreading of the disease. 239–244 Following an open letter from more than 200 scientists appealing to international and national bodies to consider the risk of airborne transmission, 245 the WHO revised its guidelines and also recognized the threat of airborne transmission, particularly in inadequately ventilated indoor spaces. 246

While national responses to the unprecedented COVID-19 pandemic have been varied, most countries have enacted strict measures to contain the spread of the disease to protect lives and preserve health systems, including lockdowns, quarantines, and travel restrictions, bringing global economic activity, particularly that of developing economies, to a major pause. Thus, in addition to the enormous human toll, the pandemic has led to a deep global recession. 247,248 The stress from the pandemic and the resulting economic recession have negatively affected the mental health and well-being of people all over the world. 249

The response of the scientific community to COVID-19 has resulted in the publication of a large volume of articles at extraordinary speed; with many studies made available in parallel to peer review. In light of the global health emergency of the pandemic, rapid publication ensures that new evidence is shared in a timely manner. However, this also poses a challenge, especially the publicly released preprints that have not been fully evaluated for scientific quality. As noted by Palayew et al. , 250 it is important for the scientific community to take measures to safeguard the integrity of scientific evidence and avoid the risk of misinterpretation and misleading application in public policy.

The drastic measures implemented around the world to contain the spread of COVID-19 have led to significant reductions in the emissions of air pollutants, notably NO x and CO 2 emissions from fossil fuel combustion. Many cities have seen dramatic improvement in air quality. Delhi, one of the most polluted megacities, experienced the clearest skies in years as pollution dropped to its lowest level in three decades (https://earthobservatory.nasa.gov/images/146596/airborne-particle-levels-plummet-in-northern-india).

The dramatic reduction in air pollution associated with COVID-19 lockdowns and other restrictions imposed by governments in cities around the world has provided an opportunity for atmospheric scientists to conduct a unique natural experiment to gain a better understanding of the complex interactions between emissions, meteorology, and atmospheric processes, as well as the efficiency of control measure surrogates ( e.g. , reduced gasoline- and diesel-fueled vehicle traffic as a stand-in for large-scale zero emission vehicle deployment, reduced fossil fuel-derived electricity demand for renewable energy) that could lead to long-term emission reductions. Most of the studies have been conducted in China since the first stringent lockdown was enacted by the Chinese authorities in response to the initial outbreak of SARS-CoV-2 in Wuhan. 251–259 A similar reduction in air pollution levels has been reported in other megacities, for example, Sao Paulo, 260,261 Barcelona, 262 Rio de Janeiro, 263,264 and Delhi. 265,266 Sharma et al. 267 analyzed the air quality of 22 cities in India, including Delhi, Kolkata, Mumbai and Chennai, and found that the concentrations of PM 2.5 decreased while O 3 increased in most regions during the lockdown period. Jain and Sharma 266 assessed the impact of nationwide lockdowns on the air quality in five megacities of India: Delhi, Mumbai, Chennai, Kolkata, and Bangalore. The study evaluated the criteria pollutants PM 2.5 , PM 10 , NO 2 , CO and O 3 before and during the lockdown period (March–April 2020) and compared them with air quality in the same period of the previous year. The results showed a statistically significant decline in all the pollutant concentrations except for O 3 . The increase in O 3 levels during lockdown may be attributed to more favorable conditions for photochemical reactions due to increased solar insolation (due to the reduced primary pollutant levels) and a decrease in NO 2 , which is consistent with the VOC-limited regime of India for O 3 production. 268

While the COVID-19 lockdown improved air quality in many regions across the world, 269 secondary air pollutant levels in some megacities has not improved due to the complex interplay among emissions, meteorology, and atmospheric chemistry, as illustrated in the following example. 45,253 Le et al. 253 examined the changes in emissions during the COVID-19 lockdown in four megacities in China: Wuhan, Shanghai, Guangzhou, and Beijing. Satellite and ground-based observations revealed up to 90% reductions of NO 2 and SO 2 concentrations. PM 2.5 concentrations were also reduced in Wuhan, Shanghai and Guangzhou. In contrast, PM 2.5 concentrations in Beijing–Tianjin–Hebei (BTH) increased substantially during lockdown; the region experienced several severe haze episodes. Ozone followed similar trends to that of PM 2.5 . Synergistic observation analyses and model simulations show that anomalously high humidity during this period promoted aerosol heterogeneous chemistry, along with stagnant airflow and uninterrupted emissions from power plants and petrochemical facilities, contributing to severe haze formation. Due to nonlinear O 3 production chemistry, reduced NO x resulted in O 3 enhancement in urban areas, increasing the atmospheric oxidizing capacity and facilitating secondary aerosol formation. The results of this study suggest that it is not sufficient to control emissions from vehicular traffic and manufacturing activities, a comprehensive regulation of precursor gases from all possible emission sources, such as power plants and heavy industries, must be considered for long-term improvement of air quality. The study also highlights the importance of meteorological factors when planning short-term stringent emission controls. Sun et al. 257 analyzed the responses of primary and secondary aerosols to the changes in emissions during the outbreak in Beijing, along with the effects of emissions reductions during the Chinese New Year holiday of the previous years. The results showed substantial reductions in primary aerosols associated with traffic, cooking, and coal combustion emissions but much smaller decreases in secondary aerosols, suggesting the need for better understanding of the mechanism driving the chemical responses of secondary aerosols to emissions changes under complex meteorological conditions. Zhu et al. (DOI: 10.1039/D0FD00091D ) conducted hourly measurements of PM 2.5 and chemical speciation at an urban site in Shanghai before and during the restriction. They observed an overall reduction in PM 2.5 , with a similar amount from OC, while nitrate accounts for most of the decrease. The reduction was due mostly from the decrease in vehicle traffic volume and fuel consumption; however, this was partially offset by an increase in secondary sources during lockdown, indicating the challenge of predicting PM 2.5 improvement based on emissions reduction from primary sources.

As noted by the WHO, 204 while more countries are taking action to improve the air quality, air pollution levels still remain dangerously high in many regions of the world. Several studies have found that air pollution substantially increases the risk of infection and the severity of COVID-19 symptoms. 270 Furthermore, people with pre-existing conditions from past air pollution exposure are more vulnerable to COVID-19. A study in the USA reported an increase in COVID-19 death rates in areas with higher long-term average PM 2.5 pollution levels, emphasizing the importance of enforcing existing air pollution regulations during and after the COVID-19 crisis. 271 High levels of pollution have also been found to be a co-factor in the high lethality risk of COVID-19 disease in Northern Italy. 272

An important consequence of the improvement in air quality during the COVID-19 pandemic is the health benefits in non-COVID-19 illnesses. Chen et al. 273 reviewed the daily concentrations of NO 2 and PM 2.5 in 367 Chinese cities, and estimated that the improved air quality led to substantial cases of avoided death from cardiovascular diseases.

While the unintended consequences of the COVID-19 crisis have brought some temporary non-COVID-19 related health benefits, the drastic measures of shutting down the global economy to clean the air are not sustainable. In fact, as some of the restrictions were lifted and the recovery began, satellite images from NASA show that much of the air pollution has returned (https://earthobservatory.nasa.gov/images/146741/nitrogen-dioxide-levels-rebound-in-china). Nevertheless, the unprecedented global pandemic demonstrates that it is possible to achieve better air quality by implementing emission reduction strategies that have been proven to be effective; furthermore, it raises public awareness about the benefits of cleaner air and calls for governments to take actions for the longer term.

Conclusions

Pollutant emissions from vehicles and industrial activities have reduced in many megacities by applying technology-forcing policies. However, establishing stringent regulations and their enforcement is more difficult in megacities with limited economic and human resources. International collaboration and cooperation are strongly encouraged, including strengthening local capacity in air quality monitoring and emissions inventory development, so that megacities confronting severe air pollution challenges will have the opportunity to learn from the experience of those cities that have successfully addressed them.

Conflicts of interest

Acknowledgements.

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Water Challenges of Megacities

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In late July I attended the 9th International Association of Hydrological Sciences (IAHS) Groundwater Quality Conference (GQ16), which this year was held in Shenzhen, China, a city that has grown from less than 30,000 residents to over 20 million in the last 40 years. The location and the conference theme of “Safeguarding Groundwater Quality in a Changing World” provided ample opportunity to ponder the looming water quantity and quality challenges that megacities such as Shenzhen, which have populations of 10 million or more, will face in the coming decades. Shenzhen has the advantages of relatively new infrastructure and a city government that prides itself on promoting an environmentally friendly lifestyle. An entire floor of the Shenzhen Industrial Museum is devoted to green city planning; interactive exhibits illustrate strategies to protect water quality and sustain the coastal marine environment. However, during my wanderings through the city, I saw numerous streambeds that were either almost dry or choked with sediment- and algae-laden water.

Several of the conference presenters made remarks that recognized these persistent problems, which are not unique to Shenzhen. Shiyi Chen, President of the South University of Science and Technology (SUSTech), used his introductory remarks to describe a major water quality improvement campaign that is underway in the city. Liu Zhiquan, Deputy Director General of the Chinese Ministry of Environmental Protection, noted Chinese cities’ heavy reliance on groundwater as well as the significant investments currently being made in water quality research and monitoring at the national level. Chunmiao Zheng, Dean of SUSTech’s School of Environmental Science and Engineering and Chair of the GQ16 conference, noted that despite the current priority being placed on water resources, nitrate contamination of groundwater persists throughout China. Citing a recent article in The New York Times [Buckley and Piao, 2016] that concluded that groundwater contamination, not smog, may be China’s most serious environmental problem, Zheng called for increased recognition of water quality challenges coupled with an emphasis on pollution prevention and on systems approaches that consider soil, surface water, and groundwater holistically. Looking ahead to looming water quantity shortfalls, Yan Zheng of SUSTech and Lamont-Doherty Earth Observatory argued that using reclaimed water for managed aquifer recharge needs to play a larger role in China’s water management strategies.

China is not the only country grappling with these issues, as other conference participants emphasized. Frank Schwartz of Ohio State used part of a keynote address to highlight the “ticking time bomb” of water-related health problems in megacities. Other presenters at the meeting described the development of national and regional monitoring networks and groundwater resource evaluation efforts in India, Korea, and the European Community. Presenters discussed emerging contaminants such as pharmaceuticals, nanomaterials, and caffeine both in terms of their potential impacts on health and as valuable tracers of the urban footprint on groundwater resources. The role groundwater exploitation plays in exacerbating urban populations’ exposure to high concentrations of naturally-occurring substances such as arsenic and iodine was another conference theme.

Beyond GQ16

Many in the Water Resources Research (WRR) audience are already familiar with the water challenges associated with megacities, but the hydrologic community is still far from identifying viable, long-term solutions. In a pair of papers published in 2010, Srinivasan, Gorelick, and Goulder used simulation-optimization models to explore the evolution of a drought-driven water crisis in the Indian megacity of Chennai and to evaluate a variety of policies that could contribute to increased resiliency of the urban water supply [ Srinivasan et al . 2010a , 2010b ]. As part of a set of debate articles on the future of the hydrologic sciences, Lall [2014] argued that we must move beyond our current understanding of local-scale hydrologic systems to develop a “planetary focus” to our science, in part because megacities create “hot spots” in the hydrologic flow field: “[The] surface and groundwater hydrology modifications by such cities require their own paradigm for understanding at a level that transcends a particular city.” A number of the articles published last year as part of a special issue celebrating the 50th anniversary of WRR recognized the need for new approaches to water management and protection in megacities. For example, Kumar [2015] proposed a new framework, “hydrocomplexity,” that would take an integrated approach to water security threats posed by stressors such as the rapid expansion of urban and peri-urban areas. In the same issue, Cosgrove and Loucks [2015] wrote a commentary about current challenges and research directions in water management, in which they highlighted problems of urban waste disposal and the cross-scale interdependencies of freshwater, wastewater, flood control, and stormwater in urban areas. They concluded that “there is a need to identify, and then implement, ways to rehabilitate urban ecosystems. This will require innovative institutional mechanisms, and a balance between autonomy and cooperation.”

While my visit to Shenzhen heightened my awareness of the water challenges posed by megacities, I am hopeful that collaborative efforts involving the hydrologic community along with other physical, biological, and social scientists and practitioners will yield the innovative strategies that are needed to sustain both water quantity and water quality in an increasingly urban world. I look forward to seeing those strategies explored further by my fellow hydrologists and the broader geoscientific community.

—Jean M. Bahr, Editor, Water Resources Research; email: [email protected]

Bahr, J. M. (2016), Water challenges of megacities, Eos, 97 , https://doi.org/10.1029/2018EO061279 . Published on 26 October 2016.

Text © 2016. The authors. CC BY-NC-ND 3.0 Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.

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Key things to know about U.S. election polling in 2024

Confidence in U.S. public opinion polling was shaken by errors in 2016 and 2020. In both years’ general elections, many polls underestimated the strength of Republican candidates, including Donald Trump. These errors laid bare some real limitations of polling.

In the midterms that followed those elections, polling performed better . But many Americans remain skeptical that it can paint an accurate portrait of the public’s political preferences.

Restoring people’s confidence in polling is an important goal, because robust and independent public polling has a critical role to play in a democratic society. It gathers and publishes information about the well-being of the public and about citizens’ views on major issues. And it provides an important counterweight to people in power, or those seeking power, when they make claims about “what the people want.”

The challenges facing polling are undeniable. In addition to the longstanding issues of rising nonresponse and cost, summer 2024 brought extraordinary events that transformed the presidential race . The good news is that people with deep knowledge of polling are working hard to fix the problems exposed in 2016 and 2020, experimenting with more data sources and interview approaches than ever before. Still, polls are more useful to the public if people have realistic expectations about what surveys can do well – and what they cannot.

With that in mind, here are some key points to know about polling heading into this year’s presidential election.

Probability sampling (or “random sampling”). This refers to a polling method in which survey participants are recruited using random sampling from a database or list that includes nearly everyone in the population. The pollster selects the sample. The survey is not open for anyone who wants to sign up.

Online opt-in polling (or “nonprobability sampling”). These polls are recruited using a variety of methods that are sometimes referred to as “convenience sampling.” Respondents come from a variety of online sources such as ads on social media or search engines, websites offering rewards in exchange for survey participation, or self-enrollment. Unlike surveys with probability samples, people can volunteer to participate in opt-in surveys.

Nonresponse and nonresponse bias. Nonresponse is when someone sampled for a survey does not participate. Nonresponse bias occurs when the pattern of nonresponse leads to error in a poll estimate. For example, college graduates are more likely than those without a degree to participate in surveys, leading to the potential that the share of college graduates in the resulting sample will be too high.

Mode of interview. This refers to the format in which respondents are presented with and respond to survey questions. The most common modes are online, live telephone, text message and paper. Some polls use more than one mode.

Weighting. This is a statistical procedure pollsters perform to make their survey align with the broader population on key characteristics like age, race, etc. For example, if a survey has too many college graduates compared with their share in the population, people without a college degree are “weighted up” to match the proper share.

How are election polls being conducted?

Pollsters are making changes in response to the problems in previous elections. As a result, polling is different today than in 2016. Most U.S. polling organizations that conducted and publicly released national surveys in both 2016 and 2022 (61%) used methods in 2022 that differed from what they used in 2016 . And change has continued since 2022.

One change is that the number of active polling organizations has grown significantly, indicating that there are fewer barriers to entry into the polling field. The number of organizations that conduct national election polls more than doubled between 2000 and 2022.

This growth has been driven largely by pollsters using inexpensive opt-in sampling methods. But previous Pew Research Center analyses have demonstrated how surveys that use nonprobability sampling may have errors twice as large , on average, as those that use probability sampling.

The second change is that many of the more prominent polling organizations that use probability sampling – including Pew Research Center – have shifted from conducting polls primarily by telephone to using online methods, or some combination of online, mail and telephone. The result is that polling methodologies are far more diverse now than in the past.

(For more about how public opinion polling works, including a chapter on election polls, read our short online course on public opinion polling basics .)

All good polling relies on statistical adjustment called “weighting,” which makes sure that the survey sample aligns with the broader population on key characteristics. Historically, public opinion researchers have adjusted their data using a core set of demographic variables to correct imbalances between the survey sample and the population.

But there is a growing realization among survey researchers that weighting a poll on just a few variables like age, race and gender is insufficient for getting accurate results. Some groups of people – such as older adults and college graduates – are more likely to take surveys, which can lead to errors that are too sizable for a simple three- or four-variable adjustment to work well. Adjusting on more variables produces more accurate results, according to Center studies in 2016 and 2018 .

A number of pollsters have taken this lesson to heart. For example, recent high-quality polls by Gallup and The New York Times/Siena College adjusted on eight and 12 variables, respectively. Our own polls typically adjust on 12 variables . In a perfect world, it wouldn’t be necessary to have that much intervention by the pollster. But the real world of survey research is not perfect.

Predicting who will vote is critical – and difficult. Preelection polls face one crucial challenge that routine opinion polls do not: determining who of the people surveyed will actually cast a ballot.

Roughly a third of eligible Americans do not vote in presidential elections , despite the enormous attention paid to these contests. Determining who will abstain is difficult because people can’t perfectly predict their future behavior – and because many people feel social pressure to say they’ll vote even if it’s unlikely.

No one knows the profile of voters ahead of Election Day. We can’t know for sure whether young people will turn out in greater numbers than usual, or whether key racial or ethnic groups will do so. This means pollsters are left to make educated guesses about turnout, often using a mix of historical data and current measures of voting enthusiasm. This is very different from routine opinion polls, which mostly do not ask about people’s future intentions.

When major news breaks, a poll’s timing can matter. Public opinion on most issues is remarkably stable, so you don’t necessarily need a recent poll about an issue to get a sense of what people think about it. But dramatic events can and do change public opinion , especially when people are first learning about a new topic. For example, polls this summer saw notable changes in voter attitudes following Joe Biden’s withdrawal from the presidential race. Polls taken immediately after a major event may pick up a shift in public opinion, but those shifts are sometimes short-lived. Polls fielded weeks or months later are what allow us to see whether an event has had a long-term impact on the public’s psyche.

How accurate are polls?

The answer to this question depends on what you want polls to do. Polls are used for all kinds of purposes in addition to showing who’s ahead and who’s behind in a campaign. Fair or not, however, the accuracy of election polling is usually judged by how closely the polls matched the outcome of the election.

By this standard, polling in 2016 and 2020 performed poorly. In both years, state polling was characterized by serious errors. National polling did reasonably well in 2016 but faltered in 2020.

In 2020, a post-election review of polling by the American Association for Public Opinion Research (AAPOR) found that “the 2020 polls featured polling error of an unusual magnitude: It was the highest in 40 years for the national popular vote and the highest in at least 20 years for state-level estimates of the vote in presidential, senatorial, and gubernatorial contests.”

How big were the errors? Polls conducted in the last two weeks before the election suggested that Biden’s margin over Trump was nearly twice as large as it ended up being in the final national vote tally.

Errors of this size make it difficult to be confident about who is leading if the election is closely contested, as many U.S. elections are .

Pollsters are rightly working to improve the accuracy of their polls. But even an error of 4 or 5 percentage points isn’t too concerning if the purpose of the poll is to describe whether the public has favorable or unfavorable opinions about candidates , or to show which issues matter to which voters. And on questions that gauge where people stand on issues, we usually want to know broadly where the public stands. We don’t necessarily need to know the precise share of Americans who say, for example, that climate change is mostly caused by human activity. Even judged by its performance in recent elections, polling can still provide a faithful picture of public sentiment on the important issues of the day.

The 2022 midterms saw generally accurate polling, despite a wave of partisan polls predicting a broad Republican victory. In fact, FiveThirtyEight found that “polls were more accurate in 2022 than in any cycle since at least 1998, with almost no bias toward either party.” Moreover, a handful of contrarian polls that predicted a 2022 “red wave” largely washed out when the votes were tallied. In sum, if we focus on polling in the most recent national election, there’s plenty of reason to be encouraged.

Compared with other elections in the past 20 years, polls have been less accurate when Donald Trump is on the ballot. Preelection surveys suffered from large errors – especially at the state level – in 2016 and 2020, when Trump was standing for election. But they performed reasonably well in the 2018 and 2022 midterms, when he was not.

During the 2016 campaign, observers speculated about the possibility that Trump supporters might be less willing to express their support to a pollster – a phenomenon sometimes described as the “shy Trump effect.” But a committee of polling experts evaluated five different tests of the “shy Trump” theory and turned up little to no evidence for each one . Later, Pew Research Center and, in a separate test, a researcher from Yale also found little to no evidence in support of the claim.

Instead, two other explanations are more likely. One is about the difficulty of estimating who will turn out to vote. Research has found that Trump is popular among people who tend to sit out midterms but turn out for him in presidential election years. Since pollsters often use past turnout to predict who will vote, it can be difficult to anticipate when irregular voters will actually show up.

The other explanation is that Republicans in the Trump era have become a little less likely than Democrats to participate in polls . Pollsters call this “partisan nonresponse bias.” Surprisingly, polls historically have not shown any particular pattern of favoring one side or the other. The errors that favored Democratic candidates in the past eight years may be a result of the growth of political polarization, along with declining trust among conservatives in news organizations and other institutions that conduct polls.

Whatever the cause, the fact that Trump is again the nominee of the Republican Party means that pollsters must be especially careful to make sure all segments of the population are properly represented in surveys.

The real margin of error is often about double the one reported. A typical election poll sample of about 1,000 people has a margin of sampling error that’s about plus or minus 3 percentage points. That number expresses the uncertainty that results from taking a sample of the population rather than interviewing everyone . Random samples are likely to differ a little from the population just by chance, in the same way that the quality of your hand in a card game varies from one deal to the next.

The problem is that sampling error is not the only kind of error that affects a poll. Those other kinds of error, in fact, can be as large or larger than sampling error. Consequently, the reported margin of error can lead people to think that polls are more accurate than they really are.

There are three other, equally important sources of error in polling: noncoverage error , where not all the target population has a chance of being sampled; nonresponse error, where certain groups of people may be less likely to participate; and measurement error, where people may not properly understand the questions or misreport their opinions. Not only does the margin of error fail to account for those other sources of potential error, putting a number only on sampling error implies to the public that other kinds of error do not exist.

Several recent studies show that the average total error in a poll estimate may be closer to twice as large as that implied by a typical margin of sampling error. This hidden error underscores the fact that polls may not be precise enough to call the winner in a close election.

Other important things to remember

Transparency in how a poll was conducted is associated with better accuracy . The polling industry has several platforms and initiatives aimed at promoting transparency in survey methodology. These include AAPOR’s transparency initiative and the Roper Center archive . Polling organizations that participate in these organizations have less error, on average, than those that don’t participate, an analysis by FiveThirtyEight found .

Participation in these transparency efforts does not guarantee that a poll is rigorous, but it is undoubtedly a positive signal. Transparency in polling means disclosing essential information, including the poll’s sponsor, the data collection firm, where and how participants were selected, modes of interview, field dates, sample size, question wording, and weighting procedures.

There is evidence that when the public is told that a candidate is extremely likely to win, some people may be less likely to vote . Following the 2016 election, many people wondered whether the pervasive forecasts that seemed to all but guarantee a Hillary Clinton victory – two modelers put her chances at 99% – led some would-be voters to conclude that the race was effectively over and that their vote would not make a difference. There is scientific research to back up that claim: A team of researchers found experimental evidence that when people have high confidence that one candidate will win, they are less likely to vote. This helps explain why some polling analysts say elections should be covered using traditional polling estimates and margins of error rather than speculative win probabilities (also known as “probabilistic forecasts”).

National polls tell us what the entire public thinks about the presidential candidates, but the outcome of the election is determined state by state in the Electoral College . The 2000 and 2016 presidential elections demonstrated a difficult truth: The candidate with the largest share of support among all voters in the United States sometimes loses the election. In those two elections, the national popular vote winners (Al Gore and Hillary Clinton) lost the election in the Electoral College (to George W. Bush and Donald Trump). In recent years, analysts have shown that Republican candidates do somewhat better in the Electoral College than in the popular vote because every state gets three electoral votes regardless of population – and many less-populated states are rural and more Republican.

For some, this raises the question: What is the use of national polls if they don’t tell us who is likely to win the presidency? In fact, national polls try to gauge the opinions of all Americans, regardless of whether they live in a battleground state like Pennsylvania, a reliably red state like Idaho or a reliably blue state like Rhode Island. In short, national polls tell us what the entire citizenry is thinking. Polls that focus only on the competitive states run the risk of giving too little attention to the needs and views of the vast majority of Americans who live in uncompetitive states – about 80%.

Fortunately, this is not how most pollsters view the world . As the noted political scientist Sidney Verba explained, “Surveys produce just what democracy is supposed to produce – equal representation of all citizens.”

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Scott Keeter is a senior survey advisor at Pew Research Center .

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Megacities and Large Urban Agglomerations in the Coastal Zone: Interactions Between Atmosphere, Land, and Marine Ecosystems

• Published: 18 October 2012
• Volume 42 , pages 13–28, ( 2013 )

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• Roland von Glasow 1 ,
• Tim D. Jickells 1 ,
• Alexander Baklanov 2 ,
• Gregory R. Carmichael 3 ,
• Tom M. Church 4 ,
• Laura Gallardo 5 ,
• Claire Hughes 6 ,
• Maria Kanakidou 7 ,
• Peter S. Liss 1 ,
• Laurence Mee 8 ,
• Robin Raine 9 ,
• Purvaja Ramachandran 10 ,
• R. Ramesh 10 ,
• Kyrre Sundseth 11 ,
• Urumu Tsunogai 12 ,
• Mitsuo Uematsu 13 &
• Tong Zhu 14  

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Megacities are not only important drivers for socio-economic development but also sources of environmental challenges. Many megacities and large urban agglomerations are located in the coastal zone where land, atmosphere, and ocean meet, posing multiple environmental challenges which we consider here. The atmospheric flow around megacities is complicated by urban heat island effects and topographic flows and sea breezes and influences air pollution and human health. The outflow of polluted air over the ocean perturbs biogeochemical processes. Contaminant inputs can damage downstream coastal zone ecosystem function and resources including fisheries, induce harmful algal blooms and feedback to the atmosphere via marine emissions. The scale of influence of megacities in the coastal zone is hundreds to thousands of kilometers in the atmosphere and tens to hundreds of kilometers in the ocean. We list research needs to further our understanding of coastal megacities with the ultimate aim to improve their environmental management.

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Multiple climate change-driven tipping points for coastal systems

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Acknowledgments

We thank the International Geosphere–Biosphere Programme (IGBP), the Scientific Committee on Oceanic Research (SCOR), The Surface Ocean and Lower Atmosphere Studies programme (SOLAS), and the International Global Atmospheric Chemistry proect (IGAC) for funding an expert workshop.

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School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK

Roland von Glasow, Tim D. Jickells & Peter S. Liss

Danish Meteorological Institute, Copenhagen, Denmark

Alexander Baklanov

Department of Chemical & Biochemical Engineering, The University of Iowa, Iowa City, IA, 52242, USA

Gregory R. Carmichael

School of Marine Science and Policy, University of Delaware, Newark, DE, 19716-3501, USA

Tom M. Church

Departamento de Geofísica & Centro de Modelamiento Matemático, Universidad de Chile, Blanco Encalada 2002, Piso 4, Santiago, Chile

Laura Gallardo

Environment Department, University of York, Heslington, York, YO10 5DD, UK

Claire Hughes

Environmental Chemical Processes Laboratory, Department of Chemistry, University of Crete, P.O. Box 2208, 71003, Heraklion, Greece

Maria Kanakidou

Scottish Marine Institute, Scottish Association for Marine Science (SAMS), Oban, Argyll, PA37 1QA, UK

Laurence Mee

The Ryan Institute for Environmental, Marine and Energy Research, National University of Ireland, Galway, Ireland

Robin Raine

Institute for Ocean Management, Anna University, Chennai, 600 025, India

Purvaja Ramachandran & R. Ramesh

Center for Ecology and Economics (CEE), NILU-Norwegian Institute for Air Research, Instituttveien 18, P.O. Box 100, 2007, Kjeller, Norway

Kyrre Sundseth

Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan

Urumu Tsunogai

Center for International Collaboration, Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8564, Japan

Mitsuo Uematsu

State Key Laboratory for Environmental Simulation and Pollution Control, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China

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von Glasow, R., Jickells, T.D., Baklanov, A. et al. Megacities and Large Urban Agglomerations in the Coastal Zone: Interactions Between Atmosphere, Land, and Marine Ecosystems. AMBIO 42 , 13–28 (2013). https://doi.org/10.1007/s13280-012-0343-9

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Meeting the Challenges of Megacities in the Developing World: A Collection of Working Papers (1996)

Chapter: options for improving labor markets for megacities in the developing world, introduction.

Development experts are concerned with fostering economic growth, alleviating poverty, protecting the environment, and improving the general standard of living of people in developing countries. Much of this work is directed toward either creating employment or raising labor earnings because at the bottom of the earnings scale, labor earnings comprise the most significant portion of total income, and hardship and poverty are often the direct results of insufficient access to adequate employment opportunities.

Megacities are frequently the centers of production for a country’s goods and services and the location of the vast majority of a country’s paid-employment opportunities. Every year, megacities attract considerable numbers of rural migrants, who, together with the rapidly growing native population, quickly expand the labor force and generate an enormous policy challenge: to create sufficient employment opportunities to absorb the large number of new workers. With public-sector wage bills being forced to contract in many countries, much of the needed increase in employment is likely to occur within the small-scale “informal sector” or within self-employment. 1 Traditionally, nonagricultural self-employment has declined with modernization, and the urban labor force has become increasingly occupied in regular protected wage employment. However, the recent change in economic conditions, coupled with rapid population growth, may be inducing profound changes in the structure and nature of employment.

This paper addresses the problem of the need to expand productive capacity in developing-country megacities in order to create a billion new jobs over the next 35 years. The remainder of the paper is organized as follows. The next section addresses the question of whether labor markets in megacities have special characteristics. The next two sections describe the labor force and the labor market, respectively, in developing-country megacities. 2 This is followed by a review of problems in developing-country megacity labor markets that may be amenable to policy intervention. Next is an examination of the role of policy in addressing these problems. A discussion of the potential role of science and technology is then presented. The paper ends with a summary and conclusions.

WHAT IS SPECIAL ABOUT LABOR MARKETS IN MEGACITIES?

Just as megacities have characteristics that distinguish them from other urban areas, there are several distinguishing features of megacity labor markets. First, the urban scale involved means that the tradeoff between scale economies and congestion costs is different than for smaller urban areas. Agglomeration economies create opportunities for improved productivity and for less costly information dissemination per capita, but congestion costs are also likely to be greater. The latter may weaken, especially in higher-income megacities, as access to the World Wide Web and other computer sources improves, although the extent to which telecommunications can substitute for transportation is controversial (see the discussion later in this paper).

Second, the composition of economic activity in megacities is distinctive in a number of ways, permitting easier comparisons across megacities than among different locations (e.g., urban and rural areas) within countries. The most distinctive characteristic of economic activity in most megacities is a large quaternary (i.e., high-order financial and service) sector, often global in scope. In addition, the informal sector represents a large share of employment, though typically not as large a share as in smaller cities; the manufacturing sector may be more or less important than in other cities, usually depending on the type of development strategy (e.g., import substitution or export orientation) and when it was adopted; and the government sector is substantial in the majority of cases where the megacity is the national capital.

Third, megacities are more open to global influences than are many other urban areas (although some natural resource locations, such as mining towns, are even more influenced by global considerations). The increasing globalization of recent years has made the world’s megacities much more interdependent than before, and international capital mobility and the ease of information connectivity have resulted in a set of megacities competing for internationally footloose economic activities. There will be winners and losers in this competition, and labor market efficiency can help determine the outcome.

Fourth, megacities have a mix of advantages and disadvantages that affect their competitiveness and the demand for labor:

A better-educated, better-trained, and more diverse labor pool, but more expensive labor costs

More access to sources of capital, with favorable influences on the labor market, given the complementarity of factors of production

A concentration of educational and training facilities, increasing the potential for human resource investments

Much higher urban absorption costs per capita (i.e., the capital costs of accommodating people in the megacity, including housing, infrastructure, and job creation) than in other urban areas and up to six times higher than in rural

areas (Richardson, 1987), implying that megacity concentration aggravates the investment constraints that cripple many developing countries

Severe traffic congestion, with its adverse implications for job accessibility and labor market performance

A complicated nexus (though not necessarily fully supported by evidence) from megacity living to higher incomes to lower fertility to higher female labor force participation

Higher land costs that may favor locational choices in other areas (e.g., distant sites within the core region or other regions)

Information barriers to both migration and labor market entry that may be more difficult as a result of the scale effect

Megacities contain a large share of both a country’s industrial sector and the national population, but economic activities are much more heavily concentrated than population. In Seoul, for example, there is a heavy concentration of the nation’s modern and internationally oriented economic activities: 61 percent of managerial personnel, 96 percent of the top 50 corporate headquarters, 64 percent of research scientists, all stock brokerage offices, and 66 foreign bank offices (Yeung, 1995). Istanbul accounts for only one-tenth of Turkey’s population and is not the national capital, yet it accounts for 45 percent of manufacturing, 40 percent of commercial activity, 25 percent of all vehicles, 44 percent of the country’s hotel rooms, and 50 percent of all university students (El-Shakhs and Shoshkes, 1995). Before the revolution in Iran, Tehran accounted for 83 percent of registered companies, 60 percent of wholesale employment, 55 percent of telephones, 53 percent of city bank branches, 47 percent of construction investment, 41 percent of insurance companies, and 40 percent of retail employment.

In the new global economy, function becomes more important than population size. For example, in Asia, Bangkok and Singapore are smaller but more interconnected than Beijing or Shanghai (Yeung, 1995). In these new world cities, the sectoral composition of the labor force may be quite different than in the traditional megacities (see the discussion in the next section).

On the other hand, developing-country megacities rank higher in terms of population (and employment) than production. Of the world’s top 20 cities in gross urban product, only 2 are in developing countries: Seoul (a marginal candidate for developing-country city status) ranks twelfth with $93 billion, and São Paulo ranks twentieth with $70 billion, as compared with $854 billion for Tokyo, $448 billion for New York, and $326 billion for Los Angeles. Among other developing-country megacities, Singapore accounts for 65 billion; Hong Kong for $60 billion; Bangkok and Rio each for about $40 billion; and Manila, Cairo, Jakarta, Shanghai, and Calcutta each for $10-15 billion (Prud’homme, 1995).

THE MEGACITY LABOR FORCE

As suggested above, megacities have different economic structures from other urban areas because of their higher degree of connectedness to the international economy, the existence of high-order (i.e., quaternary) services, and the presence of a cosmopolitan elite. This differentiation has implications for the types and composition of the labor force found in megacities.

Types of Labor

While it is common to discuss the urban labor force in terms of a “formal” and an “informal” sector, recent analysis has suggested that this split is too simplistic and fails to capture the diversity in capital-widening activities, earnings, labor quality, and entry barriers (Kannappan, 1985; Cohen and House, 1996). Some analysts draw instead a distinction between “vulnerable” (Harriss, 1989) and “protected” sectors (Mazumdar, 1983). An alternative but related classification might distinguish five categories: (1) protected labor (contracts and restrictions on entry); (2) competitive but regular labor; (3) unprotected labor (e.g., casual labor, domestic service, wage labor in petty trades); (4) self-employment and family labor; and (5) marginal activities (e.g., hawking, semilegal, or illegal activities). Only numbers (3), (4), and (5) would be included in most definitions of the informal sector.

Thus while we refer to the informal sector in the discussion that follows, we are mindful of this underlying complexity, as well as of the need to recognize the informal sector itself as a heterogeneous entity. Indeed, much of the debate surrounding the functioning of urban labor markets is concerned with how to conceptualize the diversity of income opportunities and heterogeneity of the informal sector. Some experts have emphasized the diversity and dynamism of the informal sector (e.g., House, 1987) or the problems these characteristics imply for devising and implementing effective policy prescriptions (Tokman, 1989). On the other hand, despite the heterogeneity of the sector, many studies have found similar identifying characteristics, suggesting a sector of family-based enterprises ranging in size from one-person operations to medium-sized firms. These firms usually operate in relatively competitive markets and use labor-intensive technologies. Workers in these enterprises are often young and poorly educated. Many rural-urban migrants view the informal sector as a point of entry into the urban economy and as a means of lowering their costs of searching for a formal sector job. It is in this broad, inclusive sense that we use the term here.

Sectors of Employment

The informal sector.

Because of its size in low-income megacities, the informal sector plays a critical role in their economies. It is sometimes described as a residual sector--implying not a small segment of the labor force or a subsistence component, but rather a large, unprotected sector

with considerable economic potential and dynamism, sometimes even contributing to export performance.

The informal sector employs more than 50 percent of the labor force in most developing-country megacities. 3 In Indian cities, unregistered (i.e., informal sector) manufacturing firms employ more than do those officially registered by the government. The informal sector is generally larger in sub-Saharan Africa and Latin American than in Asia (World Bank, 1995a), influenced by two main policy considerations: pro-urban strategies that encourage faster rural-urban migration and labor market policies that dampen formal sector job growth by weakening the role of wage adjustments as market signals.

The informal sector is also very important for non-household-head workers (see Browder et al., 1995, on Bangkok, Jakarta, and Santiago). Its growth during national recessions is an important labor market adaptation strategy. It acts as a buffer for avoiding more negative effects elsewhere in the labor market, such as downward adjustments in wages and increased open unemployment. On the other hand, it may not be an avenue to formal sector jobs. In a labor survey in Bombay, 70 percent of “casual” workers had not changed jobs, whereas 57 percent of factory workers had (only 13.5 percent had started as casual workers). There is little evidence for the “graduation hypothesis” from the informal sector. Contacts (family, caste, neighborhood) are very important. “Regular work looks like an enclosure, to which a limited number hold the keys” (Holmstrom, 1984:203). On the other hand, especially in Asian countries, many capable informal sector workers “graduate” to more remunerative family enterprises (Blau, 1986; Foster and Rosenzweig, 1994; Soon, 1987).

One view about government interventions in the informal sector is that they should be minimal (“benign neglect”). An alternative view is that if governments are to develop informal sector strategies, these should be related to long-term development objectives. In Cairo, for example, “reconstructing, improvement and upgrading low-income, deteriorated urban areas have been temporary remedial procedures to face violence, terrorism, and inflamed social unrest. . . . it is important that the government recognizes and deals with the informal sector. The informal sector should not be seen as either an economic or physical liability (as it is indeed still perceived as such by the government), but rather as an asset that could be directed, upgraded, and recruited to be an efficient, impelling force in systemic development” (Yousry, 1995:25).

Manufacturing

In many developing-country megacities, the role of the modern and/or large-scale manufacturing sector is relatively small, and the informal sector employs more people. For example, manufacturing accounts for only 15 percent of employment in Delhi and for less than 15 percent in Jakarta, while in the latter case one-fifth of the employed population is own-account workers, and about one-half of the labor force is in the informal sector. In

Manila, the manufacturing sector accounts for 20 percent of employment and the personal services sector for about 35 percent. 4 Even in São Paulo, with its reputation as a megacity industrial dynamo, fewer than one-third of all jobs are in the manufacturing sector (more or less equally divided among the 500-plus firms, the 100-499 firms, and the less-than-100 firms).

Government Employment

There is a large segment of urban employment in the government sector in many developing countries (e.g., Egypt, Argentina). In 1988, 23 percent of urban employment (much higher in Cairo) was in the public sector, although the share may be falling slightly as privatization strategies are implemented. There is evidence of overmanning, and sinecures are common (PADCO, 1982). Efficiency has been impeded by low wages (especially for critical skills) and the problem of the principal agent (i.e., monitoring of work performance).

The growth of corruption is a by-product of low and declining public-sector salaries (see Kpundeh, 1994). For example, real public-sector wages fell by 36 percent in Egypt during 1981-86 and by 80 percent in Ghana during 1977-85. In a sample of African countries, the ratio of private- to public-sector salaries for engineers ranged from 1.6 to 8.6 and averaged 3.7 (World Bank, 1995c).

Urban Services

The housing, transportation, and water and sanitation sectors are also important for job creation. Examples are informal sector construction workers, transportation operatives (120,000 bicycle rickshaw drivers in Dhaka and “phut-phut wallas”--drivers of three-wheel scooters--in Delhi), home-based auto repair shops, and community participation in public works projects. Thus efficiency in these three sectors can have a major impact in terms of higher labor productivity, as well as improved quality of life.

The High-Order Financial and Service or “Quaternary” Sector

As noted above, most megacities have a high-order service sector specializing in banking, finance, information processing, and professional services. This sector may be small in size, but it is important as a source of employment for highly trained professionals and as a direct link with the global economic system. As international markets are liberalized, this sector can play a major role in the efficient functioning of international trade and finance.

Women and Children

Women and children are employed in many sectors, but their potential vulnerability in the work force and in society justifies specific attention.

Female Labor

As might be expected, women are paid less than men in developing-country megacities, although in many cases female employment has grown more rapidly, especially in the informal sector. In a survey of urban West Bengal, India (mainly Calcutta), one-fifth of male workers but two-fifths of women were in the two (out of six) lowest-income (expenditure) classes. Male workers earned about double what female workers earned. About one-fifth of adult females were in the labor force (Bardhan, 1989). In addition, the female labor force participation rate was lower in locations where the manufacturing sector was more important, where male job opportunities were better, and where skilled jobs were more plentiful (women, mostly unskilled, were more likely to work in the informal sector). The taboo against women working outside the home was weaker for nonmanual jobs requiring education and for lower castes. The female labor force participation rate was lower for large households (where women were looking after children and old women), better-off households (income effect), households with more adult male workers, and wage-employed households (women having more job opportunities in self-employed households). In Bombay, the adult female labor force participation was 35 percent among casual-labor households, but less than 10 percent among factory-worker households. Of course, women (and children) were extensively employed in family-based economic activities, such as tailoring and laundering.

The gender gap in share of informal sector employment in favor of women varies between 5 percent (Korea, Thailand) and 28 percent (Bolivia, Egypt) (World Bank, 1995a). In Jakarta and Santiago, household heads earn two to three times the combined income of other household members (Browder et al., 1995). In São Paulo, the income disparity between male and female workers is also 2:1, although female employment has grown spectacularly, probably because of the need to augment household income in an era of crisis (Arriagada, 1990). In Mexico City between 1960 and 1986, the female labor force participation rate increased from 28 to 35 percent, while the male labor force participation rate declined from 81 to 70 percent. A major factor was the relative growth of the tertiary sector. In Latin America, the economic recessions of the 1980s had a positive impact on women’s participation in the labor force. In the longer run, rapid economic development was frequently associated with the growth of female employment, more education for girls, and lower fertility.

Studies of cities below the megacity size class confirm the above gender wage gaps. Telles’ (1993) study of Brazil’s nine metropolitan areas found a formal-sector female/male wage ratio of 0.73, which widened to 0.47-0.54 in the informal sector; Cohen and House

(1993) found a wage gap of 0.77 in formal-sector work in Khartoum, while Ashraf and Ashraf (1993) estimated a gap of 0.60 in Rawalpindi. There is also debate about explanatory factors behind the wage gap, emphasizing either education and skill differentials (Cohen and House) or discrimination (Ashraf and Ashraf). Reduction of the gender wage gap, implying rising female wages, is a key determinant of increasing female labor force participation because it widens the options available to women and increases the opportunity cost of staying at home. Other facilitating conditions include the development of community child care centers.

Child Labor

Children work to provide insurance against family poverty and fluctuating incomes. UNICEF has calculated that in 1991, 80 million children aged 10-14 were working, although many of them worked on farms (World Bank, 1995a). In Brazil, for example, about 18 percent of children aged 10-14 work, despite legal restrictions. On the other hand, in megacities child labor may be less of a social problem than street urchins and abandoned children.

THE MEGACITY LABOR MARKET

Labor supply.

It has been estimated that the economically active population in developing countries (largely in urban areas) will increase by about 1.2 billion between the mid-1980s and the year 2020 (International Labour Organization, 1986). The total developing-country urban labor force should expand from 598 million in 1990 to 1,521 million in 2020, although much of this growth will take place in smaller urban places. The distribution of the labor force varies considerably among different segments of the population: in the developing countries, 94 percent of males aged 20-59 are economically active, 53 percent of women aged 20-49, and at least 15 percent of children aged 10-14.

Growth in population and employment in the majority of megacities is slowing down (Beijing and Shanghai are striking exceptions; see United Nations, 1995), and this trend will continue. Moreover, in some cases the slowdown is dramatic. For example, the Rio de Janeiro Metropolitan Area grew in the 1980s at one-third the rate of the 1970s, while the share of the State (and Rio accounts for more than four-fifths of the State’s economic activity) in Brazil’s gross domestic product (GDP) fell from 13 percent in 1980 to 9 percent in 1994 (Tolosa, 1995). However, a few large cities (Dhaka and Lagos, for instance) will continue to grow rapidly (in the 4 percent per annum range), even after decelerating (United Nations, 1995). Undoubtedly, these changes will have important implications for the age structure of the labor market in developing-country megacities.

The slowdown in megacity growth will relieve labor supply pressure, but only to a modest degree. However, Williamson (1995:101-102) argues that labor supply is not a problem; on the contrary, he suggests that there has not been enough immigration into developing-country cities. Everybody (new migrants, skilled workers, capitalists) gains from additional migration except city-born unskilled workers. The problem, in Williamson’s view, is “underinvestment in cities, not overmigration.” This is an interesting opinion, but a little extreme. A more accurate statement might treat migration as a valve that regulates labor market supply in megacities, sometimes increasing and sometimes declining in response to changes in wages and in labor demand.

The Egyptian example gives an idea of the scale of the problem. In Egypt, job creation as a whole is slow, largely reflecting the low rate of private investment (7-8 percent of GDP), given that the working-age population growth rate has been and is expected to be more or less constant at 2.5 percent from 1960 through the year 2010. The urban labor force is growing somewhat more rapidly, at 3.3 percent per annum, because of migration and the rise in the labor force participation rate (Yousry, 1995).

Labor Demand

Because the demand for labor is derived from the demand for goods produced by labor, its strength in megacities depends on their ability to compete in the production and sale of tradable goods with other urban areas in the national economy and with other export-oriented cities in the international economy. Factors that impede the competitiveness of megacities, such as core city congestion and constraints on transportation and communications, will have adverse effects on the demand for labor. However, the negative impacts on the labor market are mitigated by the existence of adjustment mechanisms, such as variations in the interurban and rural-urban migration rate and the dynamism and flexibility of the informal sector. The extent of this mitigation varies from megacity to megacity. As a generalization, these cities can be divided into economically dynamic (e.g., Bangkok, Jakarta) or stagnant (e.g., Calcutta, Rio de Janeiro), with widely divergent demand for labor conditions. The labor absorption problem can be very severe in the latter cases (even in conditions of low immigration), resulting in poverty and the risk of serious social unrest.

Labor Market Equilibrium

There is an apparent inconsistency between high unemployment in megacities and rural-urban labor market equilibrium. However, complex forces are at work, such as the role of age-selective migration in stimulating future megacity rates of natural increase and in promoting increases in the domestic savings rate, the cost of public overhead capital and inelastic land supplies in choking off real wage and welfare gains, and competition between city-building investment and industrial capital accumulation (crowding out). The appearance

of disequilibrium can be deceptive because of equalizing differences and adjustments across various sectors, as well as problems involved in estimating unemployment and the size of the informal sector.

Megacity employment growth is affected by the extent of social development in rural areas and by the level of resources for industrial capital accumulation. Other factors, such as foreign capital inflows, global terms of trade, and the supply of mineral and energy resources, have played a role (Williamson, 1995). Technological progress can be important as well: “If urban sectors tend, as they do, to have relatively high rates of total factor productivity growth and if the demand for urban output is relatively price elastic, as it is at least for tradables, then final demand shifts toward the dynamic sectors, the derived demand for urban employment is augmented, urban job creation is accelerated, migration responds, and cities expand. The higher the price elasticities of demand for urban output, the greater the migration and city growth given some rate of unbalanced technological progress. The more open the economy to foreign trade, the more likely it is that these conditions will be satisfied” (Williamson, 1995:91). This explanation applies to much of developing-country city growth since the 1950s (Kelley and Williamson, 1984).

A somewhat different view is that urban economic growth is self-defeating because “increased in-migration might well undermine any gains from policies to reduce urban poverty directly” (Rodgers, 1989:2-3). However, even if this were true, aggregate welfare would still be improved because urban wages would remain higher than rural wages, and the urban/total labor force share would have increased. The formal sector urban-rural wage ratio ranges from 1.10 (Costa Rica) to 8 + (India, Ivory Coast) and is typically in the 1.50-2.00 range (World Bank, 1995a). But such data tell us nothing about what is happening in the urban informal sector, which is often characterized by rapid growth and substantial wage levels (certainly overlapping the wage distribution in the formal sector). Because of data limitations, the assertion that growth-induced migration significantly reduces welfare gains is untested.

Another option is to argue for stronger rural development interventions to increase the supply of domestically produced food and to ease the pressure on megacities from rural inmigration. However, strategies to increase rural incomes often involve labor-saving technology, thereby increasing the rural labor surplus. This may be one factor that explains the upswing in the growth rates of Beijing, Shanghai, and other large cities in China in the last decade.

Megacity Spatial (Geographical) Structure and Labor Markets

To what extent has the decentralization of large metropolitan areas in the developed countries been replicated in developing-country megacities, and how has the spatial reorganization that has occurred affected the operation of labor markets? The severity of core-city transport congestion in many megacities (e.g., Bangkok) suggests that job

decentralization could dramatically improve labor market efficiency and productivity. In Jakarta and the Hong Kong-Zhujiang Delta region in the 1980s, population growth was much more rapid in the periphery (Tangeram and Bekasi in the first case, Shenzhen and Zhuhai in the second) than in other areas, largely because of the decentralization of manufacturing (Yeung, 1995). In São Paulo during 1977-1987, tertiary employment grew 77-90 percent in peripheral locations (e.g., Ipiranga, Penha, Ibirapuera, São Miguel, Campo Limpo) as compared with 54 percent in the municipality as a whole (United Nations, 1993). Mexico City’s manufacturing sector is declining, and both population and employment growth are more dynamic outside the Federal District in the municipalities of the State of Mexico (Rowland and Gordon, 1994). In addition, Mexico City will benefit much less than the northern border cities from the effects of the North American Free Trade Agreement (Richardson and Rowland, 1994). A related issue that merits more research is whether the spatial ambit of agglomeration economies is extending in developing-country cities as it is in developed countries (a major influence on this pattern is the telecommunications revolution, which has reduced the need for face-to-face contacts).

Research Needs and Information Constraints

There is a need for more information “on the operation of urban labour market mechanisms, the labour recruitment and job rationing process, the nature of labour market segmentation, the network of ’connections’ and migration linkages, the working of intermediaries and contract systems, the formation and structure of trade unions, access to credit and marketing systems for the self-employed, the pattern of remittances and links with relatives in villages” (Bardhan, 1989:215). The fact is that we know relatively little about how labor markets work or about employment and wage levels, especially at the megacity level. The source of information on these issues is usually survey data, but surveys are sparse, and their results are not always in the public domain. Moreover, the private-sector component of labor market operations (e.g., recruitment agents, contract systems, information networks) is rarely integrated into existing databases.

The role of social structures and ascriptive networks, including ethnic ties, in constraining access to information about labor market conditions, skills, and credit is very important (Kannappan, 1988). Kinship networks reduce the risks for potential migrants and help overcome the limitations of employment exchanges and government sources of employment. At the same time, however, they restrict access to jobs and pose a challenge to equity-oriented policy interventions. On the other hand, as economies develop, the relative importance of informal networks declines, while that of more formal market networks increases. It would be wrong to draw inferences about the potential superiority of either type of network; both may be quite efficient at different phases in economic development. However, the natural transition from informal networks to market systems may be distorted by legal prohibitions. In many African countries, for example, governments retain a monopoly over employment agencies.

LABOR MARKET PROBLEMS

Labor markets in developing country megacities are vulnerable to several problems that may be amenable to policy intervention. These include unemployment, poverty and income inequality, job mobility constraints, a lack of protection, and big firm bias.

Unemployment

Unemployment in megacities is the product of many factors, such as rapid population growth, disequilibrium in the migration process, changes in megacity competitiveness, constraints on efficient job search, and skill deficiencies. In general, open unemployment rates in developing countries are misleadingly low because such a high proportion of the labor market is unstructured. However, low-productivity employment or “underemployment” is a serious problem (World Bank, 1995a). For example, Krooth and Moallem (1995) report that one-fifth of Egypt’s urban labor force is unemployed, with another fifth working part-time. Unemployment may increase with city size. To illustrate, the labor force participation rate was much lower (53.6 vs. 63.8 percent in 1986) and unemployment higher (19.3 vs. 6.7 percent) in Metro Manila than in the Philippines as a whole (Alonzo, 1989); Mexico City offers a similar example.

Poverty and Income Inequality

It is difficult to generalize about whether poverty is more severe and the distribution of income more unequal in megacities than elsewhere. The reason for this is a dearth of information on the distributional pattern of wages and on the size and distribution of nonwage income, and the fact that wage and nonwage incomes are often pooled and difficult to separate. Alegria (1994) presents evidence on the distribution of earnings by the labor force showing that Mexico City has a pattern similar to that of Monterrey, but more unequal than that of Guadalajara and (especially) Tijuana. In Brazil, only 2.7 percent of urban families were found to be in the poorest category in São Paulo, compared with 10.4 percent in Brazil as a whole and 26.6 percent in the urban Northeast (Jatoba, 1989). São Paulo also had a much lower Gini coefficient--0.48 compared with 0.58 in the northeast. However, São Paulo may be in a special position (for example, relative to Rio de Janeiro) because real wages increased strongly, by 55 percent, over the 1984-94 decade. In Egypt, the super-rich are heavily concentrated in Cairo, with the top 5 percent receiving 50 percent of the income (Yousry, 1995). Poor public investment allocation decisions between urban and rural areas can have an important influence on spatial inequality (Becker et al., 1992).

Of course, it is difficult to expect improvements in the distribution of income in the present economic climate when real wages are stagnating. As a generalization, real wages have increased only modestly, if at all, over the past two decades in all regions with the notable exception of East Asia, where real wages (in the manufacturing sector at least) have increased more than two-and-a-half times (World Bank, 1995c).

Multiple job holding is an important strategy for alleviating poverty, but its prevalence varies from country to country. It is especially common in countries with short public sector hours (e.g., 7 a.m.-2 p.m. in Jakarta). On the other hand, in Bangkok, Jakarta, and Santiago (at least with respect to peripherally located households, the samples analyzed), multiple-earner households were found to be much more common than multiple-job-holding household heads (Browder et al., 1995). Also, household earnings often combine incomes generated in different sectors of the urban economy.

Job Mobility Constraints

Various labor policies and practices result in severe constraints on job mobility. These policies and practices include job security, severance pay requirements, minimum wage policies, and housing allowances and other fringe benefits.

The Lack of Protection

The existence of a very large unprotected sector, combined with a heavily protected (in terms of labor regulations) but small formal industrial and government sector, would appear to contradict the job mobility problem as stated above. But the basic problem is the lack of equity across the labor market. The excessively protected formal sector and its associated costs may provide a strong disincentive for employers in the unprotected sector to offer even the most minimal levels of protection to their workers.

Big Firm Bias

Industrial policy in developing countries is biased in favor of large firms (both public and private). These firms are often protected from overseas competition, and they frequently, and successfully, engage in rent-seeking activities (i.e., winning subsidies and concessions from government). Their activities and preferential treatment have damaging effects on employment growth because small firms are often more dynamic and much more labor-intensive--an important characteristic in capital-constrained developing economies.

THE ROLE OF POLICY

The social sciences offer a number of robust insights into how to improve the efficiency of labor markets. For example, mandates and labor regulations will be ineffective unless they take account of rational decision making by both individuals and households. Twenty or more years of economic research and close documentation of the recent economic history of both developing countries and the former centrally planned economies has taught

us that in general, incentive mechanisms are much more efficient as instruments to guide behavior in desired directions. This section reviews some of the key lessons learned with regard to effective (and ineffective) policy interventions in the labor market.

Goals of Policy

The key government policy goal in the labor market is to raise urban labor productivity and utilization by such means as improving the efficiency of the labor market (e.g., with respect to job mobility and the search process); promoting productivity in both the urban formal and informal sectors; building up human capital; restricting interventions in the labor market to those serving an overriding public purpose in order not to erode the competitive position of developing-country megacities; and reducing unemployment.

The Role for Government

Governments are most effective in improving urban labor markets when they assume a modest (though important) role, depending on the extent of market failure and the importance assigned to distributional issues (World Bank, 1995a). Principles for intervention include the following:

Let markets work, but intervene where appropriate in key areas (e.g., provision of infrastructure, environmental externalities 5 ).

Make interventions subject to market discipline.

Intervene openly (e.g., simple rules, not discretionary action).

Invest and encourage investment in human development (education, health care, nutrition, family planning) where markets are likely to fail (because of externalities) and/or where favorable distributional effects can be expected.

Create a favorable climate for entrepreneurs (e.g., eliminating restrictions, providing infrastructure support, creating a competitive regulatory framework, and reducing price distortions).

Facilitate integration with the global economy (lower tariffs and fewer nontariff restrictions on trade and capital flows).

Promote macroeconomic stability (reduce fiscal deficits, control inflation, reform the financial sector).

Provide incentives to encourage firms to reinvest profits.

In the labor market, increase access to information (although this may be inadequate if job access is controlled through informal networks.

Reduce the complexities of labor legislation that discourage (especially) small entrepreneurs from establishing formal labor contracts.

Revise labor policy mandates that are easily evaded, are enforceable only very selectively, and clash with household and worker incentives.

In many developing countries, the state is a major actor influencing wage determination: fixing public-sector pay, repressing or promoting worker organization, and prescribing minimum wages. Some of these interventions may be inefficient in the sense of adversely affecting productivity. In addition, there are several other (in some cases possibly convincing) rationales for action: uneven market power, discrimination against women and minority groups, obstacles to information (e.g., about health and safety hazards), and insufficient insurance against risks (e.g., unemployment, sickness, and old age). Furthermore, some actions conducive to employment growth can be implemented locally. These include measures to improve the quality of the labor force, to provide and maintain sound infrastructure (particularly transportation investments), to hold local tax rates at a moderate level, and to promote environmental quality. “Cities with efficient labor markets, and with appropriate conditions for growth and innovation, will adjust more easily than others. What is done in each urban area--by urban policies--to promote the efficiency of labor markets and the business creation potential will facilitate adjustment and flexibility of the entire economy” (Prud’homme, 1995:735-6). The argument is that labor-augmenting policies may improve macroeconomic performance just as efficient macroeconomic policies can improve labor market performance. Thus, it is insufficient to take the obvious macroeconomic steps (e.g., trade liberalization, fiscal discipline, and deregulation of financial markets). These measures will not create jobs rapidly enough and may not garner long-term political support for market-oriented strategies. Complementary strategies are needed: faster provision of infrastructure in poor settlements and in metropolitan-wide transportation and telecommunications, and social policy reforms in taxation, education, housing, and social security to reduce resource misallocation and promote income equity (World Bank, 1995c). Similarly, policies to raise labor productivity may not be labor policies per se, but more general in nature. For example, “urban policies could be defined as measures to increase positive externalities and decrease negative externalities” (Prud’homme, 1995:732).

On the other hand, Rodgers (1989) has challenged the view that the labor market can be analyzed as if it were just another commodity market. Instead, he argues in favor of emphasizing social, institutional, and legal factors associated with unequal access to the labor market. However, this argument does not undermine the need for freeing up the market, but only emphasizes that such attempts will be more difficult to achieve and more prolonged. Freeing up the urban labor market will weaken the social and cultural constraints on its efficiency.

Others (e.g., Jatoba, 1989) recommend a more sweeping menu of policies, including measures to support small-scale production; stronger enforcement of minimum wage laws; wage subsidies for particular groups; interventions to prevent worker exploitation; better organization of the labor legislation concerning the right to strike, collective bargaining, entry into protected labor markets, and aid to the unemployed; and better labor market information. Such government activism would be unwise, because (among other problems) developing-country governments are not equipped to handle such a broad mandate. The narrower and more conservative principles outlined above provide a sounder basis for

intervention. We turn now to specific policy interventions that can address the problems outlined in the previous section.

Policies to Reduce Unemployment

Macroeconomic policies.

There are striking regional differences in macroeconomic policy. In East Asia, there has been generally successful export-oriented industrialization, combined with equitable income distribution policies (e.g., land reform, agricultural development policies) and an emphasis on labor-intensive measures. In Africa and Latin America (with notable exceptions, such as Botswana and Chile), on the other hand, government efforts to promote industrialization have been based on less open (to international markets) and more interventionist strategies, including trade regulation, creation of investment incentives, and direct public investment in manufacturing. These policies favor large firms and discriminate against the smaller firms that generate most jobs.

Structural adjustment policies have a mixed impact on employment. For example, a study of Costa Rica showed that structural adjustment policies helped to stimulate export promotion and private investment, and that unemployment eventually dropped (Fields, 1988). Of course to date, the forced and delayed adjustment in Mexico has had the opposite effect, although the current situation may be no worse than what would have happened in the absence of any adjustment. Structural adjustment strategies are often associated with short-term formal-sector job losses, but the key issue is whether the adjustment costs are short-run, offset in the longer run by the creation of new jobs. The benefits of export promotion tend to be concentrated in labor-intensive industries, and more emphasis on the private sector can fuel the growth of new labor-intensive businesses. Fields (1989) also points out that in the urban modern sector, the employment of unskilled workers has been limited by labor demand constraints; this implies that promotion of economic growth, especially export growth in labor-intensive industries, is critical.

Countries faced with serious debt problems, in particular, must be very careful with respect to policy choices. Wrong-headed policies can result in severe capital flight that almost always inflicts high adjustment costs on workers. For example, in the 1980s real wages fell 15-40 percent in Latin American countries, with a few exceptions, such as Argentina.

International Trade Policies

This is a controversial issue. Krugman (1991) has argued that protectionism and autarky reinforce primacy (i.e., the dominance of the largest city in a country’s urban size distribution). Open markets create a more level playing field between core and periphery

manufacturing and between urban and rural economic activities. However, at least in the short run, a dampening of labor demand has ambiguous impacts on the labor market because of doubts about the indirect effects on labor supply (via induced rural-urban migration).

An interesting feature of global interactions since 1970 is the rise in trade and capital flows and the decline in labor flows (World Bank, 1995a). The deceleration in international migration may work to dampen wage convergence, but this effect can be offset by international trade and capital flows that substitute for international labor movements. Also, international trade benefits workers because of lower consumer prices and stimuli to higher labor productivity.

Ades and Glaeser (1994) found that the share of trade in gross national product (GNP) was negatively related to the size of the largest city; in a large cross-section, a 10 percent increase in the trade share was associated with a 6 percent decline in the size of the primate city. High tariffs (measured by the ratio of import duties to total imports) were associated with increasing primacy (a 1 percent increase in the tariff variable increased primate city size by 3 percent). In an earlier study, Krugman (1991) found that a 1 percent increase in the share of GDP spent on government transport and communications investments was associated with a 10 percent reduction in primate city size, indicating that barriers to internal trade reinforce primacy. Tests of these hypotheses on Pacific Rim countries by Richardson (1995) produced ambiguous results.

Nevertheless, outward-oriented countries do grow more rapidly than inward-oriented countries. In the Pacific Rim, manufacturing accounts for three-quarters of exports, and manufacturing is overwhelmingly urban. Measures to increase urban productivity (e.g., human resource investments, infrastructure provision, and alleviation of negative externalities such as traffic congestion and air and water pollution) have direct and favorable effects on growth and export competitiveness (Richardson, 1995).

Human Resource Policies

Although there is a serious problem of the educated unemployed in some megacities, provision of better education is generally associated with rising labor productivity and higher wages; examples are offered by many East Asian countries, such as Singapore and South Korea (see Park, 1988). However, there is no clear evidence that there are public benefits to education beyond the private benefits (Psacharopoulos, 1985). 6 On the other hand, some recent studies (e.g., Behrman, 1995; Rosenzweig, 1995) have found that more education has externalities that include facilitating the spread of knowledge about new production techniques and new markets. Also, there is evidence of positive rates of return to education even in the informal sector (Cohen and House, 1995).

Government training programs have a poor record, but on-the-job training programs in the private sector can be very important. Of course, government stimulation of the private

sector can also be important. For example, in the mid-1970s, Chile changed from a centralized government training system to one where the government subsidized decentralized private training programs and supervised both government and private employment offices through a coordinating and supervising agency called SENCE (Servicio de Capacitacion y Empleo). Tax credits have induced the provision of training for about 5.5 percent of the labor force, or about 250,000 workers. Also, educational reforms in Chile since the 1980s have resulted in a 50 percent expansion in the provision of secondary education, especially by offering additional subsidies at different rates to technical, agricultural, industrial, and commercial schools and by attracting private providers into the system.

Policies to Reduce Poverty and Income Inequality

Minimum wage policies.

It is difficult to draw a clear-cut conclusion about whether minimum wages have favorable consequences. The consequences depend on the employment impact, determined in turn by market structures, the threshold level, and the degree of enforcement. To the extent that minimum wages are effective in the formal sector, they tend to protect the labor elite employed in that sector at the cost of restricting access to formal sector jobs by others.

Minimum wages relative to average incomes tend to be higher in poor countries (World Bank, 1995a). For example, some sectoral minimum wages in Bangladesh are more than double GNP per capita. To the extent that this is true, minimum wage legislation reduces employment.

Minimum wage legislation has no impact on wage levels of the unprotected informal sector that dominates income-generating activities in the megacities of low-income countries. Conversely, the minimum wages are often irrelevant to privileged formal sector workers who earn much more.

In periods of crisis, real minimum wages fall drastically (as they lag behind inflation), e.g., by more than 40 percent in the 1980s in countries such as Mexico, Chile, and Kenya. Yet minimum wage legislation is often not enforced, with substantial proportions of the labor force being paid less than the minimum wage. In Mexico City, for instance, a 1985 study by the Mexican Labor Congress found that 56 percent of workers were earning less than the minimum wage (United Nations, 1991). However, Gregory’s (1986) data did not support this conclusion, with average wages in all service sectors found to be well above the minimum. In expanding economies, minimum wages tend to be irrelevant as productivity growth fuels real wage increases.

The impact of the contribution of minimum wages is less in very big cities. Why? People are more productive in larger cities. The “natural wage” (i.e., the wage without legislative constraints) is higher, and fewer people are paid less than the minimum wage; the

minimum wage constraint is not binding (Prud’homme, 1995). Nevertheless, especially in Latin America, minimum wages can be an instrument for keeping wages down because they are used as a reference standard and are usually set far below the market wage.

Informal Sector Promotion

Measures to facilitate the expansion of informal sector activities and small-scale family businesses (discussed under “Policies to Reduce Big Firm Bias” below) can have an important role in relieving poverty. Not only do they create income-generating activities for low-income populations, but they also open up opportunities for upward mobility, given the significant overlap in the wage distributions of formal and informal sector work.

Policies to Eliminate Job Mobility Constraints

Labor unions.

Labor unions in developing countries vary widely in bargaining power and impact. Generally, they defend the interests of a relatively privileged group of regular workers, and the interests of this group may diverge widely from those of workers on the margins of the labor market. However, union membership tends to be low in developing countries, typically less than 20 percent of the urban labor force.

Labor unions are often not independent, but under indirect government control. For example, in Egypt the labor syndicates operate under the umbrella of the General Union of Labor (GUL); tensions persist between the workers and the syndicates, and between the syndicates and the GUL. In India, too, the major labor federations are affiliated with political parties, and their collective bargaining is subject to compulsory adjudication.

The influence of labor unions is mixed. On the positive side, they minimize labor turnover; improve productivity; and reduce wage dispersion and discrimination, e.g., against women workers. On the negative side, their monopolistic behavior boosts member wages (union wage premiums being in the 10-30 percent range) at the expense of nonunion labor, consumers, and stockholders, and they oppose necessary structural adjustments. Appropriate policy would attempt to create an environment that would promote the positive impacts of unions while mitigating the negative. Such policy would involve creating competitive product markets, facilitating workers’ freedom of association, and promoting decentralized bargaining (as in Seoul since the late 1980s). In addition, in some developing countries (e.g., Bangladesh, Egypt), the repression of unions has often been coupled with actions (such as featherbedding in the public sector, job security provisions, and high minimum wages) that exacerbate labor market distortions. In general, wages and the terms of working conditions should be left to voluntary agreements between workers and employers.

Social Policies and Job Security Laws

Social policies covering housing, pricing of “essential” goods, public services, and social insurance may have an important impact on poverty and welfare. When they are job-attached, however, they can impede labor market efficiency. Similarly, job security legislation mandating severance pay and long notices (as in Mexico and Sri Lanka) can impede the growth of formal-sector employment by encouraging firms to rely on casual labor and/or subcontracting.

Transportation Strategies

Many megacities suffer from inadequate transportation that undermines labor productivity. Bangkok and Cairo are merely two of many examples. Bangkok’s problem is insufficient allocation of land to road space in the central city. Dealing with the problem by investments in intersection flyovers has had a negligible impact. Decentralization of jobs and population would appear to be a more viable strategy. Cairo has also invested heavily in intersection flyovers, in elevated freeways at the 6th of October and 15th of May satellite cities, and in the Ring Road project. By 1993, more than 96 percent of the Greater Cairo Project Implementation Agency’s investments were in transportation (Yousry, 1995). However, the experience of developed-country cities suggests that it is impossible to build enough roads to control congestion. There are lower-cost and more effective approaches to overcoming the inaccessibility obstacles to higher worker productivity, including spatial reorganization (i.e., the decentralization of both jobs and people); the promotion of small-scale, private paratransit operators in peripheral settlements; and a combination of transportation demand management and transportation supply management strategies.

Spatial (Geographical) Redistribution Policies

Explicit spatial redistribution policies have had a mixed record. Job growth has slowed down in Seoul, but there is a great deal of controversy about whether this was the result of direct policies or of many other considerations, including market forces. In many other countries, however, spatial redistribution policies have been ineffective and often costly.

Secondary city strategies have value in helping those individual cities, but they have a negligible effect in reducing labor market pressures on megacities. Rural development strategies have done little to relieve megacity labor force growth, although labor-intensive public works programs may have a modest but discernible effect. The slowdown in megacity growth, where it has occurred, has been largely a natural consequence of declining national population growth rates and has had little to do with spatial redistribution.

The most striking change in direction in spatial redistribution policies is post-Mao China’s abandonment of its long-established anti-urban policy to discourage rural migrants from entering urban areas. This shift may reflect the view that reducing rural surplus labor is key to efforts to raise rural living standards, in part because of urban-rural remittances. As a result, the growth rates of China’s two largest megacities have soared--l.8 times faster growth than in the 1970s and 1980s in Beijing and 2.62 times faster growth in Shanghai (United Nations, 1995). This rapid growth has aggravated the job creation problem in those cities, and there have been recent attempts to reimpose strict controls by sharply raising the cost of urban entry.

Policies to Protect Labor

Child labor laws.

Legislative mandates against child labor are symbolic rather than effective unless incentives can be developed to shift children out of work and into school and/or to implement income security programs (e.g., food for work). Premature enforcement of labor laws is less desirable than “a comprehensive approach that takes into consideration the interaction between labor market conditions, school availability, child labor, and poverty” (World Bank, 1995b:72).

Nevertheless, child labor laws are a test of the effectiveness of government intervention in the face of market trends. Everyone agrees on the central objective--to improve child welfare; the potential disagreement is about the measures used to achieve this goal. Banning child labor outright, other than its public relations value, is ineffective if enforcement procedures are weak and if the ban conflicts with household goals. A preferred strategy is to create an environment in which child employment declines through expansion of primary and secondary education facilities, promotion of economic growth, and other non-labor market policies.

Health and Safety Legislation

The costs of occupational injuries comprise up to 4 percent of GNP in developed countries and are probably much higher in most developing countries. In general, the rural and informal sectors have received little protection from health and safety standards. Also, there are enforcement problems in the formal sector, especially with respect to small firms. In developed countries, the unions play a significant watchdog role; this could be an important contribution in low-income countries where enforcement capacity is so weak. Furthermore, developing effective liability measures may have more concrete results than relying on unenforced safety regulations.

Trying to link labor standards to international trade sanctions is ill advised because it panders to protectionist interests, unless the focus is limited to core standards (e.g., no forced labor, freedom of association, and prevention of extreme forms of discrimination). It should also be recognized that such provisions, if effective, may worsen rather than improve the income-generating opportunities and the welfare of the alleged beneficiaries.

Privatization

For the past 15 years there has been a strong trend toward privatization, including private-sector participation in shelter, infrastructure, and services. The more typical privatization strategy, however, involves the sale of the assets of inefficient public enterprises. For example, in Egypt privatization reforms were one element in the structural adjustment program adopted in 1987. A direct result was Business Sector Law 203 of 1991, which initiated cutbacks in the bloated public sector by the sale of shares and other assets of state-owned enterprises, most of them located in Cairo (Yousry, 1995).

Privatization of public-sector activities, including production establishments, can have beneficial effects on labor productivity through the incentives offered to both employers (managers) and workers. Nevertheless, it is frequently opposed by labor unions, which fear job losses, wage cuts, and the erosion of benefits. In many countries (e.g., Thailand), the strongest labor unions are found in the government sector (Rondinelli and Kasarda, 1993).

Policies to Reduce Big Firm Bias

Small-scale sector policies.

There is a case for intervention to overcome obstacles to entry, including limited access to credit, high start-up costs, a lack of technical assistance, marketing problems, an uneven playing field in competition with large firms, inadequate training, and a lack of managerial skills.

There is a clear policy choice between “removing regulatory restraints” (de Soto, 1989) and providing a “battery of costly supports” (International Labour Organization, 1986). Bromley (1993:133) argues that the former course is insufficient, while the latter is costly and difficult to implement. “The crucial issues are how to generate a macroeconomic and macropolitical climate favorable to the growth of small enterprises; how to ensure sustained and consistent application of government promotion policies; how to encourage simple, easily replicable pilot programs; and how to encourage the diffusion of successful innovations among small enterprises.” Measures to facilitate on-the-job apprenticeships in the informal sector are cost-effective (Cohen and House, 1995). However, there remains “an inherent contradiction between the flexibility and adaptability of the IFS [informal sector] and the bureaucratic rigidities that so often accompany intervention in developing countries. The IFS

is the product of a competitive environment and, by and large, it flourishes best in an environment unhampered by government regulation. If the government regards its role as supportive and facilitating, this may be enough” (Richardson, 1984:30).

Stronger efforts should be made to convert paid workers to self-employment (e.g., bicycle rickshaw drivers in Dhaka). Family-based enterprises also offer substantial benefits (stimulating capital accumulation, a long-run planning horizon, and incentive-based labor productivity).

Industrial Incentive Policies

Relatively few governments offer incentives for firms to start businesses or expand in megacities, or if offered, the incentives are higher for establishing businesses at alternative, more peripheral locations. However, most of these programs are targeted at the larger firms that are capable of meeting subsidy application requirements.

Labor Policy and Key Megacity Issues

Another way of slicing the policy cake is to relate the policy reforms discussed above to four key megacity issues: productivity, poverty, the environment, and management.

Productivity would be improved, perhaps dramatically, by policy reforms in several areas, including macroeconomic stability, open international markets, informal sector promotion, and labor policy reforms.

The informal sector is also important from the point of view of poverty alleviation, as is eliminating barriers to female employment (e.g., offering more flexible working hours or providing community-based child care).

The relationship between improvements in the labor market and protection of the environment is an important issue that has not been addressed in detail in this paper. Policies for intrametropolitan location of industry to reduce stationary-source air pollution can contribute to the spatial dispersion of the labor market. Moreover, there is substantial scope for expanding labor-intensive informal sector work in recycling and the management of waste, especially if associated with some organizational and managerial improvements in transportation. Trade-offs between environmental protection and employment can be exaggerated: introducing cleaner technology is not necessarily associated with reducing labor inputs, and servicing pollution-abatement activities is labor-intensive.

The goal of improving megacity management has implications for the upper end of the labor market, especially from the point of view of training engineers, planners, and city managers. In addition, strategies are needed for making public sector management careers

more attractive to the high-quality educated people who currently, and overwhelmingly, look to the private sector for employment.

Finally, we note that the analysis in this paper is restricted to megacity labor markets because traditionally, the development literature has emphasized the importance of the urban economies in absorbing surplus rural labor into a modern industrial sector. Nevertheless, it is important that planners not neglect the direct needs of both rural areas and other, smaller cities. Hence all the above-mentioned policies need to be evaluated at the national, and not just the megacity, level.

THE ROLE OF SCIENCE AND TECHNOLOGY

How can megacity labor markets benefit from rapidly changing technology? In several developing-country megacities, there are pools of unused educated talent. The back-office telecommunications jobs (e.g., in credit card divisions of major banks) that have decentralized to places such as South Dakota and Ireland could equally go to developing-country megacities if it were possible to finance and undertake the massive capital investments required. The latter would involve further freeing up the privatized telephone companies and facilitating the development of joint, i.e., domestic-foreign ventures.

International transportation and communications costs (for commodities, capital, labor, and communications infrastructure) have fallen by 82-97 percent since 1920. Accordingly, national economies cannot be insulated from global pressures, except by self-defeating autarkic interventions. However, megacities are much better placed than other locations within developing countries to take advantage of these falling costs because intranational transportation and communications costs have fallen much more slowly than international costs.

The telecommunications lags in many developing-country megacities threaten to widen the disparities between these cities and their developed-country counterparts. However, privatization offers an opportunity to overcome some of these telecommunications infrastructure deficits (e.g., by expanding cellular telephone services). As an example of what is possible, a large-scale telecommunications infrastructure project (Teleport) is being implemented in the decaying downtown of Rio de Janeiro. It involves a 62,000 cubic meter building on a 250,000 cubic meter site, incorporating a satellite ground station; a fiber-optic vertical transmission system; a server for voice mail, e-mail, and fax; a telecenter for voice, data, and image terminals; a video conference facility; and hotels and restaurants (Tolosa, 1995). Similarly, Tehran recently inaugurated the largest telecommunications center in the Middle East (El-Shakhs and Shoshkes, 1995).

Telecommunications may also offer developing-country megacities an opportunity to exploit the benefits of being late starters. It is well known that many of these cities (e.g., Bangkok) have woefully inadequate levels of investment in transportation. Would it be

feasible to jump a stage of technology and, instead of building more highways and transit systems, invest heavily in telecommunications, building a quaternary sector (both national and global) dependent on decentralized, satellite office centers? (Of course, in low-income developing countries, home-based information processing would be more difficult until housing and infrastructure conditions had dramatically improved.) Some analysts (e.g., Schuler, 1992) have argued that telecommunications and transportation are more complements than substitutes, but this relationship--if true--is likely to hold only in the short run, not the long run.

The two major innovations already widely in use even in developing countries are the computer and the cellular telephone. For example, Brazil is one of the fastest-growing computer markets in the world. Although most developing countries are not yet competitive in producing state-of-the-art hardware, cities such as São Paulo and Bombay have generated significant numbers of highly skilled jobs in producing software. In the United States, computerization has had destabilizing impacts-on the labor force, creating many jobs in some sectors and destroying jobs in others (e.g., bank tellers, telephone operators, and secretaries). In developing countries, on the other hand, in part because of the availability of cheap labor, the prospects for net job creation are excellent. Moreover, significant opportunities are being created in related activities, such as the expansion of electronic repair services in the informal sector and subcontracting for CD-ROM production and desktop publishing from developed-country corporations.

The cellular telephone has penetrated into surprising markets. For example, women participants in the Grameen Bank’s microenterprise loan program in Bangladesh were provided with cellular telephones to increase intervillage communications. More generally, the cellular telephone has enabled developing-country megacity businesses to overcome the frequently severe lags in obtaining regular telephone service. Diffusion could increase rapidly as cellular transmission costs drop; the rate at which this might take place depends not only on privatization, but also on the creation of a competitive rather than a monopolistic environment.

SUMMARY AND CONCLUSIONS

Developing countries (especially when the so-called transitional economies are included) span a wide range of income and development levels, and their megacities reflect these differences. Yet most of the megacities share some key labor market characteristics: a formal manufacturing sector that is usually dwarfed (in employment terms) by the service sector; a large government sector, often riddled with inefficiencies; and an informal sector whose size depends on the level of economic development, business cycle influences, and the degree of government tolerance and support. Moreover, almost all megacities have been impacted substantially by globalization and the opening up of world markets; these trends have accelerated the need for increasingly flexible labor markets.

While acknowledging that there are always exceptions to any rule, several key conclusions can be drawn from the analysis presented here.

The promotion of economic growth and economic (and political) stability offers the best guarantee of expanded employment opportunities, higher wages, and labor productivity growth. Appropriate macroeconomic policies (e.g., fiscal discipline, the control of inflation, and financial reforms), combined with both tariff and nontariff trade liberalization, offer the most promising strategy for promoting employment growth and generating enough resources to pay for the infrastructure investments needed to achieve income equity.

For this strategy to work efficiently, the economy in general and the labor market in particular need to adjust quickly to changing market conditions. Many developing countries have labor policies in place (such as minimum wage laws, job security provisions, job-related housing provision, pension systems, and centralized--often government-controlled--labor unions) that, when enforced (often they are not), aggravate labor market distortions and impede adjustments. Deregulation of the labor market offers prospects for increasing its flexibility, especially by recognizing that policy interventions should work with rather than against market forces and should pay attention to the incentives driving the behavior of individuals and households. At the same time, improved access to credit markets for potential entrepreneurs, as well as to job information and to skills acquisition for potential employees, will help megacities’ labor markets run more efficiently.

Whereas redistribution policies tampering with spatial variations in labor supply and demand are unlikely to be effective, efficiency-oriented urban policies (e.g., low-cost improvements in transportation, broadly based educational investments, privatization of government services, access to credit for small-scale enterprises, and creation of a favorable entrepreneurial environment) can promote labor productivity growth and improve the competitive efficiency of megacities, directly impacting macroeconomic performance.

With respect to the role of science and technology, more rapid diffusion of known and already widely applied technologies (e.g., computer information systems, cellular telephones) may transform the structure and composition of employment in developing-country megacities, especially given the global competitive environment.

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Cities in developing countries are experiencing unprecedented population growth, which is exacerbating their already substantial problems in providing shelter and basic services. This volume draws on the significant advances in technologies and management strategies made in recent decades to suggest ways to improve urban life and services, especially for the poor. Four challenges to developing countries' megacities are addressed: labor markets, housing, water and sanitation, and transportation, along with a synthesis of general thinking on how to meet megacity challenges and be competitive in the twenty-first century.

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Guest Essay

D.E.I. Is Not Working on College Campuses. We Need a New Approach.

By Paul Brest and Emily J. Levine

Mr. Brest is a former dean and a professor emeritus at Stanford Law School. Dr. Levine is an associate professor of education and history at Stanford.

With colleges and universities beginning a new academic year, we can expect more contentious debate over programs that promote diversity, equity and inclusion. Progressives are doubling down on programs that teach students that they are either oppressed peoples or oppressors, while red states are closing campus D.E.I. programs altogether.

For all of the complaints, some of these programs most likely serve the important goal of ensuring that all students are valued and engaged participants in their academic communities. But we fear that many other programs are too ideological, exacerbate the very problems they intend to solve and are incompatible with higher education’s longstanding mission of cultivating critical thinking. We propose an alternative: a pluralist-based approach to D.E.I. that would provide students with the self-confidence, mind-sets and skills to engage with challenging social and political issues.

Like many other universities, our university, Stanford, experienced a rise in antisemitic incidents after the Hamas attack on Israel on Oct. 7 and Israel’s response. We were appointed to the university’s subcommittee on antisemitism and anti-Israeli bias, which was charged with assessing the nature and scope of the problem and making recommendations. The upshot of hearing from over 300 people in 50 listening sessions is that many Jews and Israelis have experienced bias and feel insecure on our campus.

A parallel committee formed to address anti-Muslim, Arab and Palestinian bias reached similar conclusions for those groups.

These findings are discouraging, given that institutions of higher learning have spent several decades and vast sums of money establishing institutional infrastructures to promote diversity, equity and inclusion. Discouraging, but not surprising — because our inquiries revealed how exclusionary and counterproductive some of these programs can be.

Our committee was pressed by many of those we interviewed to recommend adding Jews and Israelis to the identities currently recognized by Stanford’s D.E.I. programs so their harms would be treated with the same concern as those of people of color and L.G.B.T.Q.+ people, who are regarded as historically oppressed. This move would be required of many California colleges and universities under a measure moving through the California Legislature. But subsuming new groups into the traditional D.E.I. regime would only reinforce a flawed system.

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