AEROSOLS: A New Climate Change Challenge

[From the March/April edition of Foreign Affairs (page 135). I am surprised by the breadth and complexity of the aerosol issues, which add a major dimension to my understanding of the climate change dilemma.]

The Next Front on Climate Change

How to Avoid a Dimmer, Drier World

Veerabhadran Ramanathan, Jessica Seddony, and David G. Victor

After dithering for decades, governments finally seem to be pay ing serious attention to the problem of global climate change.

Late last year, at the Paris climate conference, they adopted a major new agreement to limit global warming, beginning a process to strengthen commitments to reduce greenhouse gas emissions over time.

For many observers, the promises of the Paris conference offer too little, too late, because emissions are high and still rising and because there will be major disruptions to the climate even if countries meet their emissions reduction pledges. Nevertheless, it had been 18 years since the world’s governments left a major climate summit with an agreement in hand, so just getting to yes in Paris has offered climate diplomacy fresh credibility.

Until now, governments have focused on limiting the greenhouse gases that cause global warming and its attendant hazards, such as rising sea levels and stronger storms. But there is more to climate change than higher temperatures. Many of the activities that cause greenhouse gas emissions—burning coal for power, diesel for transport, and wood for cooking, for example—also yield ultra-small particles known as aerosols, which blanket vast areas in a haze that blocks and scatters sunlight. By reducing the solar energy that reaches the earth’s surface, aerosols reduce evaporation and slow the water cycle that governs where, when, and how much rain falls.

For years, climate scientists have believed that a warmer world would be wetter, because higher temperatures hasten evaporation and increase rainfall. But even when these higher temperatures are accounted for, a world dimmed by aerosols will in fact be drier in many places—including some areas, such as the Sahel and other regions in sub-Saharan Africa, that have long suffered from drought because they rely on rainfall to sustain subsistence agriculture. According to many of the most reliable models, such as those produced by the National Center for Atmospheric Research and Princeton University’s Geophysical Fluid Dynamics Laboratory, China, North America, and South Asia are also in danger of more frequent and severe droughts owing to aerosols. Indeed, for much of the world, aerosol-induced dimming and drying are among the most immediate dangers posed by pollution.

The good news is that swift action on aerosols is possible, with huge potential benefits. Many of the tools needed to make rapid cuts to aerosol emissions are already available, and policymakers around the world—notably in Europe and the United States, and also in East Asia—have shown how to use them. Since aerosols have a short atmospheric life span, the climatic benefits of emissions cuts would appear quickly, within only a couple of decades. What is more, speedy action on aerosols would bring huge global health benefits: roughly seven million people die each year from causes related to particulate pollution, and cutting down on aerosols would dramatically reduce the death toll. In light of these potential benefits, governments around the world should ensure that aerosols play a central role in their environmental policies by encouraging the development and deployment of cleaner technologies for power generation, transportation, and household cooking, heating, and lighting.

Measures to limit aerosol pollution tend to receive less public attention than the broader campaign against greenhouse gases, but they, too, should be an essential component of global action against climate change.


Climate scientists have known about the dimming effect of aerosols since at least the 1970s, but most research has focused on their effects on temperature. Darker aerosols, such as diesel soot and other kinds of black carbon, absorb sunlight and accelerate warming. But lighter aerosols, such as the sulfates and nitrates formed from coal, gasoline, and other fuel emissions, cool the planet by reflecting sunlight back into space. That explains, in part, why the world hasn’t seen more of a temperature increase from the greenhouse gases already present in the atmosphere. (This masking effect is powerful enough that some advocates of geoengineering have proposed injecting more reflective aerosol particles into the atmosphere in order to cool the earth.)

Focusing on how aerosols affect temperature, however, has distracted policymakers from the important and distinct effects that aerosols have on the water cycle. These effects are most pronounced in the Northern Hemisphere, which is the source of most of the world’s aerosols and thus suffers the most dimming from these pollutants. But because air currents tend to carry pollution, water droplets, and water vapor far from their origins, aerosols produced in one region can also affect rainfall far afield.

Since the 1880s, when reliable record keeping began, global temperatures have increased by about 0.9 degrees Celsius. And as the planet has warmed, rainfall at latitudes above 45 degrees has generally increased. But twice since the mid-twentieth century, surges in aerosol emissions have significantly disrupted this pattern, reducing rainfall in a number of regions.

The first disruption was the result of the sulfur dioxide emissions produced by the massive combustion of coal and other fuels across Europe and North America in the mid-twentieth century, driven by rapid industrial growth after World War II. From the 1950s to the late 1980s, global emissions of sulfur dioxide (which in the atmosphere becomes sulfate, a reflective aerosol) nearly doubled, reducing the amount of sunlight reaching the earth’s surface by about two percent on average. As a direct result of this dimming, average rainfall in the Northern Hemisphere declined by between three and four percent over the same period. Indeed, there is strong evidence that sulfur dioxide emissions in the United States and western Europe contributed to the Sahelian megadroughts that began in the 1960s and continued through the 1990s, a period during which precipitation in the Sahel and some other parts of sub-Saharan Africa fell by between 25 and 50 percent relative to twentieth-century averages.

Thanks to stringent air pollution laws introduced in the 1970s and strengthened steadily in the following years, the blanket of aerosols over Europe and North America has thinned since the 1980s. From 1980 to 2000, the average amount of sunlight that reached the earth’s surface in these regions increased by about four percent-enough to lift average annual precipitation on land areas in the Northern Hemisphere by a similar magnitude.

[In 2010, China and India received between 10 and 15 percent less sunlight than they did in 1970.]

A second surge in aerosols is now playing out in East Asia and South Asia. These regions, which have rapidly industrialized over the past four decades, have seen a two- to fourfold increase in sulfur dioxide and black carbon emissions since the 1970s. As a result, in 2010, China and India received somewhere between ten and 15 percent less sunlight than they did in 1970. As the wind has carried sulfates and black carbon over thousands of miles, the dimming effect has extended to the atmosphere over the Indian Ocean, reducing the evaporation of seawater and thus weakening the monsoons that bring much-needed water to East Asia and South Asia every year. From 1950 to 2002, the most recent period for which estimates are available, there was a seven percent decrease in average annual rainfall over the Indo-Gangetic Plain, the fertile belt of land crossing eastern Pakistan, northern India, and Bangladesh that is home to more than one billion people, many of them dependent on rain-fed agriculture. Over the same period, summer monsoon rainfall in parts of northern China decreased by more than ten percent.

The desiccation of China’s north and the region’s recent drought, in 2010 and 2011, have affected not only agriculture but also other water dependent activities, such as hydroelectric power generation. The consequences have worried Chinese authorities to such a degree that they are building canals and pipelines that will eventually divert some 1.6 trillion cubic feet of water to the region each year. Some of China’s repressive policies toward water-rich Tibet are motivated by the Chinese government’s desire to maintain control over the nation’s fragile water supplies and their hydropower potential.

China has the capacity and the financial means to protect itself from erratic precipitation by investing in water infrastructure. So do other relatively wealthy countries, which can also respond to droughts by importing more water-intensive products and refocusing domestic economic activity on crops and industries that are less dependent on precipitation. Strategies such as these, along with aggressive measures to improve water-use efficiency, have allowed California, for example, to grow its economy even as it suffers its worst drought in modern history.

But things are different in much of the developing world, where water infrastructure and state capacity are more limited and a higher proportion of the population depends on locally sourced food produced on rain-fed land. In South Asia, for example, 60 percent of the agricultural land is rain-fed. That proportion reaches 90 percent in Latin America and 95 percent in sub-Saharan Africa. Many countries in these regions can’t easily turn to infrastructure and trade to solve their food production problems because of limited budgets and because they lack the capacity to rapidly shift production to new crops and industries. And many of these countries are particularly dependent on agriculture: nearly half of all employment in India is in farming, and even in richer Brazil, agricultural laborers account for 15 percent of the work force. All told, more than 400 million farmers, along with their dependents, count on rain-fed agriculture for their livelihoods. Countries such as Brazil, Colombia, Sudan, and Zimbabwe, which rely on hydropower for between 60 and 80 percent of their electricity generation, face additional risks from the dimming.

Forty percent of the world’s population is already expected to live J under severe water stress by 2050. That proportion will likely increase as aerosol-induced dimming further disrupts the water cycle. And as governments around the world are beginning to realize, water scarcity is not only an economic and humanitarian challenge but also a geopolitical one: as supplies of fresh water dwindle, states will begin to jockey for access to them, as they already have, for example, in north eastern Africa, where Egypt has squabbled with Ethiopia over its construction of a massive hydroelectric dam on the Blue Nile.


Although the costs of aerosol-induced dimming are high, the policies needed to reduce the pollution that causes it are relatively clear. Cutting aerosols will require action in three main sectors: electric power generation, transportation, and household energy services for the poor.

With regard to electric power generation, most of the concern about aerosols centers on burning coal, which is responsible for more than 70 percent of the world’s sulfur dioxide emissions. Given its environmental and health impacts, conventional coal power is increasingly hard to justify. So if coal is to remain part of the global energy mix in the coming decades, coal-fired power plants will need to become more efficient and include equipment to remove sulfur dioxide and other pollutants from their emissions. As the technology to do so improves, new coal plants will also need to capture and store carbon dioxide emissions—an expensive prospect. At the same time, governments and firms will have to invest more in other energy sources. Natural gas, which emits much lower levels of most pollutants (including aerosols) than coal does, is one option, and in North America, the shale boom has dramatically cut the cost of supplying it. Making gas friendlier for the climate and the water cycle will require more work to plug leaks in the natural gas supply and transmission system (since those leaks release methane, a potent green house gas), and it will require greater frugality in the use of water to drill and frack shale gas wells. Of course, there are also many options beyond natural gas, such as nuclear, solar, and wind power.

Regulators in California and the European Union, meanwhile, have already pioneered policies that cut aerosol emissions from transportation. They have mandated cleaner fuels and combustion technologies, such as low-sulfur diesel and exhaust systems equipped with efficient particulate filters and catalytic converters. Officials elsewhere should follow their lead, and they should pair these regulations with rigorous compliance regimes, which are currently lacking in many countries.


Eliminating combustion altogether, perhaps through electric vehicles, could be a next step. In the meantime, subsidy reforms can help limit the use of some of the dirtiest fuels. Changes to India’s fuel-pricing regime, for example, have encouraged car buyers there to shift from diesel to gasoline engines, which emit far fewer aerosols. Transitioning large commercial and public-transportation vehicles to natural gas could also help.

Cutting aerosol emissions produced by burning dirty fuels in the world’s poorest households is another way to reduce global dimming. Just over one billion people, most of them in the developing world, rely on kerosene to light their homes, and three billion use solid fuels, such as crop residue and dung, for cooking and heating. Burning these fuels with traditional technologies generates aerosols that damage lungs along with the climate: the particulates emitted by biomass based cooking and heating are responsible for about a third of the dimming in South Asia. Cleaner technologies for cooking, heating, and lighting, such as energy-efficient cookstoves and solar lanterns, are readily available, and making them universally accessible would offer huge health and environmental benefits to the world’s poor. Ensuring such access by 2030 would cost up to $50 billion per year—a high price, but one that should be manageable if it is shared among a number of states, including rich countries, which would themselves benefit from lower aerosol emissions in the developing world.

Since aerosols have a short atmospheric life span, pursuing policies such as these could significantly reduce global dimming within ten or 20 years. That would dramatically limit the risk of droughts and irregular monsoons. It would also heat up the planet by reducing the atmosphere’s reflective aerosol “mask,” however, so any effort to reduce global dimming must be accompanied by significant cuts to carbon dioxide and other greenhouse gas emissions.


As governments build on what they achieved at the Paris climate conference, they must set politically feasible targets for future action. Focusing on aerosols could help. Whereas greenhouse gas emissions will bring about relatively distant and diffuse dangers, aerosols cause immediate and localized harm. That should raise the incentives for governments to  act against them, and it should raise the willingness of their constituencies to accept such action. Indeed, in the case of aerosol reductions, the parochial interests that have so often stymied broader climate diplomacy need not hinder progress. That is why some countries that have long been reluctant to do much about global pollution—from China and India to Brazil and the United States—have pursued bolder policies when it comes to pollutants that have localized effects, such as aerosols.

As states sharpen their pledges to reduce greenhouse gas emissions in the coming years, they should also make distinct pledges to cut aerosols. (So far, few states have done so: of the 186 emissions-reduction pledges submitted before the Paris climate conference, only a handful, including Chile’s and Mexico’s, mentioned aerosols.) And they should broadcast the promise of these reductions to build public support for the policies needed to achieve them. As such policies take hold, they will generate rapid, tangible benefits, encouraging even more progress on the changing climate’s other challenges.

Unfortunately, even the most effective climate diplomacy will leave the world’s poorest states exposed to the higher temperatures, rising sea levels, and disruptions in rainfall caused by industrial pollution. As a result, governments will have to work to adapt. Today, the countries with the highest emissions—among them, China, Japan, the United States, and the members of the European Union—are on track to raise around $100 billion per year by 2020, much of which will be used to help vulnerable states adjust to the dangers of a changing climate.

As for how to spend these funds, a variety of efforts will be needed, and states should be willing to experiment to determine which programs work best, sharing the know-how they gain with one another. As they do so, they should invest in infrastructure and technologies that address the effects of both warming and dimming, such as irrigation methods that can better protect farmers from erratic rainfall and new kinds of drought-resistant crops. Indeed, innovation in water conservation technologies remains massively underfunded, despite their huge promise. Finally, governments should remove protectionist policies in their countries’ agricultural sectors, which limit the ability of consumers to access foreign sources of food when erratic rainfall and higher temperatures harm local production.

The dimming caused by aerosols has already made the world’s water supplies less secure. It is both economically and technologically feasible to reverse this process. Doing so will require a concerted global effort, but failing to do so will compound the risks of drought and poverty already in store as a result of the world’s changing climate.


VEERABHADRAN RAMANATHAN is Distinguished Professor of Atmospheric and Climate Sciences at the Scripps Institution of Oceanography at the University of California, San Diego.
JESSICA SEDDON is Founder and Managing Director of Okapi Research and Advisory and a Senior Fellow at the Centre for Technology and Policy at the Indian Institute of Technology Madras.
DAVID G. VICTOR is a Professor at the School of Global Policy and Strategy at the University of California, San Diego, and the author of Global Warming Gridlock: Creating More Effective Strategies for Protecting the Planet.
© Foreign Affairs

About Bruce

Work for sustainable development of small islands and the Chesapeake Bay; ex-Peace Corps (Volunteer and staff) in LA & Caribbean; cruised Caribbean on S/Y Meander for three years; like small tropical islands, French canals, Umbria, Tasmania, and NZ. Married 52 years to the late Kincey Burdett Potter (see President of the now-sunsetting Island Resources Foundation.
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