A Darkening Sky
A smoky shroud over Asia blocks both sun and rain
By cooling the northern Indian Ocean, the haze reduces evaporation, cutting the water supply for rainfall. On land, the warm air aloft acts as a lid on cloud formation, quashing the convection that feeds thunderstorms. And the aerosols themselves seed the formation of tiny mist particles--so many that they suck water out of the air and choke off the growth of larger drops that would fall as rain. While the haze particles dry out the land, the rain does fall over the sea, where larger, natural sea-salt particles promote droplet growth. "We're shifting rain from the land to the ocean," says Daniel Rosenfeld of the Hebrew University of Jerusalem.
At least that's the theory, and there are signs it may be happening. Some computer climate models predict that the hazes over India should displace the annual monsoon rains, leading to floods in the south and east of the country while drying the north and shrinking the vital Himalayan snowpack. "That's just the pattern we are starting to see emerge," says Surabi Menon of the Goddard Institute for Space Studies in New York City.
In southeastern China, where haze has cut sunlight by 2 percent to 3 percent every 10 years since the 1950s, temperatures are dropping, while rising elsewhere in the country, presumably because of greenhouse gases. The changed temperature patterns have rerouted storm tracks, one recent Chinese study said. The study blamed the shift for severe floods in the nation's south in recent years, coupled with drought in the north. It ranked the new weather pattern as the greatest sustained change in China's climate in more than 1,000 years.
Some scientists also suspect that the pollution cloud could be cooling the sea surface and slowing evaporation in the far western Pacific, off Asia. The effects could ripple across half the globe to the United States, because the western Pacific is the breeding ground for El Niños, the bouts of Pacific warming that change rainfall across the Americas and beyond.
All of this is enough to make Asia's brown cloud, and the sparser hazes elsewhere, into a global climate threat. Fortunately, hazes are far easier to counter than greenhouse gases like carbon dioxide. Clean up industry and smother the fires, and in a few weeks rain would wash the skies clean. Carbon dioxide, in contrast, lingers for centuries, and ordinary pollution controls can't touch it.
Going after hazes. Some scientists, distressed at the reluctance of the U.S. government and many developing nations to tackle greenhouse gases, hope that the relatively easier task of curbing fine particles could kick-start international efforts to address climate change. Going after hazes, particularly those heavy with soot, is "a no-lose situation as far as I'm concerned," says Stanford University atmospheric researcher Mark Jacobson.
The Chinese government, rattled by the data on the country's polluted air, is doing just that. For both health and weather reasons, it has largely replaced home use of coal with cleaner-burning natural gas in big cities and is starting to require catalytic converters on vehicles. China also hopes to restore blue skies to Beijing in time for the 2008 Olympics.
At the same time, some scientists worry that major assaults on aerosols might divert attention from the far tougher problem of carbon dioxide and other culprits in global warming. Jacobson laments that President Bush cited the climate impact of soot as one reason to abandon the Kyoto climate change agreement, which deals with greenhouse gases. "You can't stop with aerosols," Jacobson says. "You definitely have to go after the greenhouse gases, too."
But the lesson of the Asian brown cloud, says Ramanathan, is that there's more to global change than greenhouse warming. "If all you deal with is CO2, then you don't understand climate at all."
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