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How now brown cloud? If emissions of large amounts of dark-colored aerosols from India, China, and southeastern Asia (pollution over northern India and Bangladesh, shown) continue to grow, temperatures over parts of the United States could r

Jesse Allen/Earth Observatory/MODIS/NASA

'Asian Brown Cloud' Threatens U.S.

China and India are some of the world's top polluters, with countless cars, factories, and households belching more than 2 million metric tons of carbon soot and other dark pollutants into the air every year. These pollutants aren't just bad news for the countries themselves. A new study reveals that they can affect climate thousands of kilometers away, warming the United States by up to 0.4°C by 2024, while cooling other countries.

Some forms of pollution—especially light-colored aerosols such as sulfates that spew from power plants and volcanoes—scatter light back into space, cooling Earth. But dark aerosols, such as soot from diesel engines and power plants, absorb more sunlight than they scatter, gaining heat and warming the air around them. Rapidly developing countries, especially China, India, and those in southeastern Asia, are prolific sources of such aerosols. Over the past few decades, the pall hanging over the region has come to be known as "the Asian brown cloud."

Previous studies have shown that even though layers of air polluted with carbon aerosols become substantially warmer, the cloud slightly cools temperatures at ground level, by some estimates reducing the amount of sunlight reaching the surface by between 10% and 15%. The brown cloud also weakens winds during the Asian summer monsoon and changes the timing and location of monsoon rainfall. The cloud has dramatically thickened in recent decades, with some studies showing that dark aerosol emissions from China alone doubled between 2000 and 2006.

To gauge the impact of this thickening, Haiyan Teng, a climate scientist at the National Center for Atmospheric Research in Boulder, Colorado, and colleagues used a detailed climate model that evaluated the interactions among land, sea, atmosphere, and sea ice. In three different scenarios, the team boosted dark, carbon-rich emissions from the equator to 50°N and from 70°E to 150°E, a region that covers India and much of China and southeastern Asia. The scenarios simulated what would happen if dark aerosol emissions doubled between 2005 and 2024, increased to six times current rates, and jumped to 10 times current rates.

Increasing the emissions to six and 10 times current rates over the course of 20 years seems extreme, says Teng. But the team used these values because the climate model somewhat underestimates the atmospheric warming caused by dark aerosols.

Such tweaking of a climate model "is not unusual," says Yi Ming, a climate scientist with the National Oceanic and Atmospheric Administration in Princeton, New Jersey, who was not involved in the study. "It makes sense to increase the [dark aerosol] emissions to produce the right amount of heating, and to better match observations," he says.

The sixfold and 10-fold increases in dark aerosol emissions would cause global average temperatures at ground level to rise 0.1°C by 2024, the researchers report in a forthcoming issue of Geophysical Research Letters. But possibly more important, the thickening brown cloud would trigger significant changes in long-term weather patterns that would affect areas thousands of kilometers away. The effect would be somewhat like a human-made El Niño, the climate phenomenon in which sea-surface warming in the tropical Pacific alters temperature and precipitation in the United States and elsewhere.

In particular, the large increases in dark aerosol emissions would cause as much as 0.4°C warming in the eastern United States during the winter months and a similar temperature increase over most of the United States during the summer months. In contrast, Greenland, much of northern Canada, and significant swaths of Antarctica would cool by 0.25°C or more during summer and winter.

Although scientists have long studied the effects of pollutants on cloud formation and other small-scale phenomena, determining their effects on climate in distant regions is a relatively new field, says Chien Wang, an atmospheric scientist at the Massachusetts Institute of Technology in Cambridge, who wasn't involved in the research. The new findings "are not surprising at all," he notes. However, he adds, the team's new study "is a highly idealized experiment," so the results are probably more accurate in terms of capturing the overall pattern of changes than they are at estimating the precise amount of warming or cooling in a particular locale.