How brown haze could turn off the monsoon have been traveling to India for twenty years now—not regularly, but often enough to notice that every time I go, the air seems to be dirtier and more choked with black smoke and fumes. In the cities, much of this comes from the exhaust pipes of the millions of ill-maintained diesel-burning buses and two-stroke rickshaws that ply the gridlocked streets. The haze also contains natural sea salt and mineral dust, a fair amount of fly ash and sulfur dioxide from India's coal-burning power stations, and huge amounts of organic material and soot from the countryside. For in India's million villages, where most of its billion-strong population still live, the air is often scarcely better than it is in the cities, with smoke billowing from a hundred million cooking stoves, all burning wood, dried cow dung, and crop residues.
This smoke is becoming a major climatic phenomenon. It is merging into one giant cloud that climate researchers call India's "brown haze." Its heart is over the northern Indian plain, one of the world's most densely populated areas, which suffers near-constant smog during the winter months. This is a giant version of the old pea-soup smog that used to hit London in the days when the city was heated by coal fires. As I complete this chapter, Delhi's air is reportedly worse than ever, with thick smog preventing flights from its airport. But the haze spreads more widely, shrouding the whole of India and beyond.
The term "brown haze" was coined by scientists during the first investigation of the phenomenon. In 1999, some two hundred scientists taking part in the Indian Ocean Experiment (INDOEX) assembled in the Maldives for a three-month blitz of measuring the air over India and the Indian Ocean from aircraft and ships. The results were a surprise, even to those who had planned the project. Every winter, from November to April, a pall of smog more than a mile thick occupied a huge area south of the Himalayas, stretching from Nepal through India and Pakistan, out over the Arabian Sea and the Bay of Bengal, and even south of the equator as far as the Seychelles and the Chagos Islands. It covered 4 million square miles, an area seven times the size of India.
"To find thick brown smog 13,000 feet up in the Himalayas, and over the coral islands of the Maldives, was a shock," says Paul Crutzen, one of the masterminds of the project. Crutzen, who won a Nobel Prize for predicting a dramatic thinning of the ozone layer fifteen years before it happened, said the haze had a similar potential to cause "unpleasant environmental surprise" in India and beyond. The haze could, he said, have "very major consequences" for the atmosphere.
The INDOEX findings proved controversial in India, which felt singled out for criticism. Why pick on us? locals asked. Indian government scientists issued a detailed and largely spurious "rebuttal." The INDOEX scientists quickly switched to discussing the "Asian brown haze"—and quite rightly, for the haze is an Asia-wide phenomenon. But when I used that term at a meeting in India in mid-2005, I was quietly hissed. Even mentioning an Asian haze is considered politically incorrect today. Why single out Asia? people ask. In fact, antagonism has become so great that many Indian scientists now refuse to discuss the subject with foreigners like me, for fear of getting into political hot water.
India has been the focus of attention because its aerosol pollution is of genuinely global importance. Dorothy Koch, of Columbia University, estimates that a third of the soot that reaches the Arctic, sending pollution meters soaring from Mount Zeppelin, in Svalbard, to northern Canada, comes from South Asia. The soot is falling onto the snow and ice, making the white surface darker and so triggering melting. When her findings were published, in April 2005, one headline read: "Home fires in India help to melt the Arctic icecap half a world away." No wonder the Indians are twitchy. Suddenly a country with one of the lowest per capita emissions of greenhouse gases in the world was being fingered as a prime cause of climate change.
But, wary though they may be in public, India's scientists have been at work finding out where all the pollution comes from. At first, they assumed that most must be the product of India's fast-growing and undoubtedly polluting industries. But at the Indian Institute of Technology, in Mumbai, they mocked up rural kitchens to check emissions from cooking stoves of the kind found across the Indian countryside. They fueled the fires with wood, crop waste, and dried cattle manure; on the stoves, they boiled kettles and even cooked lunch. They concluded that smoke emissions from India's domestic cooking fires produce between i and 2 million tons of aerosols a year, including a quarter of a million tons of soot. That makes them responsible for some 40 percent of India's aerosol emissions.
Discussion about the climatic impact of the Asian brown haze has become a statistical minefield. The "headline figure," widely quoted, is that in winter the haze reduces the amount of solar radiation reaching the ground in India by an average of about 22 watts per 10.8 square feet. That is a reduction of about a tenth, and would be enough to cause massive cooling. The statistic is literally true, but only part of the story. For only about 7 watts of that radiation is lost entirely, "backscattered" into space. The other 15 watts is absorbed by the soot in the aerosols and re-radiated, heating the atmosphere. Thus, though the radiation budget is much altered, the cooling effect is much less than it might otherwise be. Even so, in winter it is sufficient both to counteract global warming and to cool the air across much of India by an average of about o.9°F. In summer, when the pollution is rained out in the monsoon and the skies are clearer, temperatures have risen in recent decades by about the same amount, in line with the global average.
The consequences don't end there, says Veerabhadran Ramanathan, the Indian scientist who, with Crutzen, masterminded INDOEX from the Scripps Institution of Oceanography. In particular, the cooling impact of the haze over the Indian land surface delays the heating of the land that stimulates the monsoon winds. It thus threatens the lifeblood of India: the monsoon rains.
There seems to be some confusion among scientists about the Indian monsoon. Scientists investigating the brown haze all claim that the monsoon has weakened in recent decades, and they see this as a likely effect of the haze. But researchers investigating global warming are equally certain that it has increased in intensity. What undisputed evidence there is suggests that the monsoon rains have become more intense in the traditionally wetter south of India, where the haze is thinner, but have diminished in the north, where the haze is thickest. How those trends develop is obviously of vast importance for a country entirely dependent on just a hundred days of monsoon rains to water the crops that feed a billion people. A wider collapse of the monsoon in South Asia would be a global calamity of immense proportions. It could happen.
East Asia could be in the same boat—a situation that would threaten food production for the world's most populous nation, China. North of the Himalayas, there is a similar intense brown haze in winter, though it is composed less of the smoke from burning cow dung and more of the sulfur dioxide and other fumes from burning coal. And it is interrupting the sun's rays. When Yun Qian and Dale Kaiser, of the U.S. government's Northwest National Laboratory, in Richmond, Washington, studied the records of Chinese meteorological sunshine recorders over the past fifty years, they found a decline in sunshine since 1980 of 5 or 6 percent in the most polluted south and east of the country.
And this decline is lowering temperatures. While global warming is evident across much of China, daytime temperatures in the most polluted regions have fallen by about 1°F. That, in turn, is altering rainfall patterns. In the south of the country, the monsoon rains are becoming stronger, with flooding in the great southern river, the Yangtze; whereas farther north, in the catchment of the Yellow River, there is now less rainfall. Chinese records, which are among the most meticulous in the world, suggest that this shift is the biggest alteration in the country's rainfall patterns in a thousand years. To some extent, links between the rainfall trends and the increasing brown haze are conjecture. But when climate models are programmed to include a strong Asian brown haze, many of them produce extra rainfall in southern
China, coupled with near-permanent droughts in the north. So if the models are right, while the haze lingers, these major calamities are set to continue.
Meinrat Andreae estimates that about 8 billion tons of biomass is burned in the tropics each year—approaching 1 ton for every inhabitant of Earth. All of it produces aerosols that billow into the
Asian countries, with their huge populations, have the worst smog. But parts of Africa and the Brazilian Amazon are also shrouded when farmers clear land for crops by burning grasslands and forests. Hundreds of thousands of fires burn across the Brazilian Amazon each year, covering the area with billowing dense smoke. During the weeks of burning, the amount of sunshine reaching the ground typically drops by 16 percent. In Zambia, studies have found a 22 percent drop in sunlight as the savannah is burned.
The changes are "causing all sorts of havoc with the atmospheric circulation," says Dale Kaiser, of the Northwest National Laboratory, who is the author of the Amazon study. Over the Amazon, he says, the smoke causes cooling and suppresses the formation of raindrops. That both reduces rainfall and keeps the aerosols in the air longer. Meanwhile, the buildup of water vapor results in the upper atmosphere's becoming wetter, according to Daniel Rosenfeld, of the Hebrew University, who flew research planes through the smoke over the Amazon. It eventually forms a few extremely intense thunderstorms, known in the trade as "hot towers," which cause hailstorms. Hail falls in the Amazon only when fires have been burning.
Some of these changes could have impacts far beyond the regions where the smoke forms. Condensation in Amazon hot towers releases very large amounts of heat into the upper atmosphere, influencing jet streams and other wind patterns across the tropics and beyond. And more water vapor may reach the stratosphere, where it could increase ozone destruction. Meanwhile, modeling studies supervised by Jim Hansen suggest that soot emissions over India and China may trigger drought in the African Sahel and even warming in western Canada—though exactly how remains unclear.
These impacts are, of course, only the predictions of climate models. It is hard to prove whether they reflect events in the real world. But the models are based on real physical processes in the atmosphere. So at the least, they suggest the potential for a worldwide climatic change from the effect of aerosol emissions in the tropics. Cooking stoves in India, it seems, could have global consequences.
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