The Asian brown cloud

The Chicago Tribune’s front-pager yesterday was a James P. Miller story about the effect of Chinese air pollution — the “Asian brown cloud” — on U.S. weather. Some of the tidbits:

Add one more item to the long list of things Asia exports to the United States: air pollution. The contaminated air that rides the jet stream to Trinidad is laced with the sulfates and soot from Asia’s industrial smokestacks, and nitrogen oxides that emerge from tailpipes of Asia’s rapidly growing fleet of automobiles. It contains particles from fires set to clear jungles for farming, and from the millions of households that burn coal, wood or animal dung for heating and cooking. Scientists identified the phenomenon five years ago. The Asian brown cloud, researchers now know, routinely climbs high enough into the atmosphere to hitch a ride on the fast-moving jet stream heading east to North America. In April and May, when seasonal winds are strongest, the high-altitude pollution can cross the Pacific in as little as four days…. So far, the increase in ground-level pollution that the Asian brown cloud causes in the United States is “not catastrophic, or even critical,” said David Parrish, a research chemist with the National Oceanic and Atmospheric Administration’s Aeronomy Lab in Boulder, Colorado…. A cloud heavy with particles of dust or pollution is whiter than a non-polluted cloud, because water droplets condense around the particles, explained [scientist V.] Ramanathan. “Double the aerosols, double the droplets,” he said. That means polluted clouds reflect sunlight more efficiently than a clean cloud. And that, in turn, affects the weather. When clouds scatter sunlight, ground-level temperature declines. Such unnaturally high reflectivity also can suppress rainfall, or it can hold rain back so long that when it finally does fall to earth, it comes in the form of a damaging downpour, said Ramanathan. Some researchers, in fact, think the extra-white clouds caused by dirty air are helping to offset the global warming effect. That would offer an explanation for the unsettling fact that “the planet hasn’t warmed as much as the models suggest it should,” given the amount of greenhouse gas that humans have released into the atmosphere, the researcher said. The Asian cloud is only the first and largest of a number of high-atmosphere brown clouds scientists have discovered. This summer, National Oceanic and Atmospheric Administration is funding a major study of a similar blotch found hovering a mile or more above the eastern U.S. (and which sends a plume of dirty air trailing toward Europe.)

It’s not clear if there are any policy implications from this — but I hadn’t seen the phenomenon reported previously.