Headed east.
Sooty haze over China ends up in Pacific clouds.

Clouds, Made in China

As East Asian economies continue to expand, their smokestacks have pumped out ever more of the greenhouse gases responsible for global warming. That may not be their only effect on climate, however. Researchers report that pollution from Asia blown eastward across the Pacific appears to be intensifying cloud cover--and possibly winter storms--there.

Tiny airborne pollutants, known as aerosols, have a big effect on clouds. By acting as "seeds" for water droplets, they influence cloud formation and the duration, amount, and type of precipitation. While this effect is not well understood, previous studies have linked smoke from Amazon forest fires and urban pollution to more violent thunderstorms (ScienceNOW, 27 February 2004). That's worrisome, because satellite measurements have revealed increasing pollutants over East Asia--largely due to coal burning--with sulfate aerosols levels rising 35% each decade.

To investigate whether this was affecting weather, atmospheric scientist Renyi Zhang, of Texas A&M University in College Park, and his colleagues examined satellite data on deep convective clouds, which are associated with winter storms over the Pacific Ocean. The researchers found that winter cloud cover was 20% to 50% higher between 1994 and 2005 than it was between 1984 and 1994. They then used computer models to simulate a storm that occurred in November 2003; a model that incorporated eye-watering levels of Asian aerosols replicated the November storm, while a model that only took natural aerosols into account did not.

Increased cloud cover alone does not guarantee stronger storms, says Zhang. But when combined with plentiful moisture and unstable air--common ingredients in the Pacific in the winter--conditions become perfect for more severe weather, he says. Zhang's team reports its findings online this week in Proceedings of the National Academy of Sciences.

Climatologist V. "Ram" Ramanathan of Scripps Oceanographic Institution, San Diego, California, hails the research for demonstrating for the first time that Pacific storm clouds have intensified over the past 10 years. "We've been looking at the roles of El Niño and other factors in storm activity; this shows we must also consider aerosols," he says. Meteorologist Gary Barnes of the University of Hawaii, Manoa, agrees that Asian aerosols could be enhancing cloud coverage, but he remains skeptical about the link to increased storm activity. "Examining the actual intensity and track of winter storms to support the satellite data would make for a more convincing study," he says.

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