'Asian Brown Cloud' Threatens U.S. - ScienceNOW

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.

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