Western US's stronger storms traced to Asia pollution

Air pollution blowing over the western US from Asia has been a growing environmental concern for several years. Now, it seems, it's giving winter storms added punch as well.

Tiny aerosols and soot from burning wood and coal in winter, especially in China, appear to be seeding clouds in large winter storms that churn thousands of miles east across the northern Pacific, says a team of US scientists. The pollution is turning relatively routine marine rain clouds into towering thunderheads, much like those seen above land.

Previous studies have shown that during the last half of the 20th century, northern Pacific winter storms have become steadily more frequent and stronger, based on tracking wind speeds and atmospheric pressure. The latest results, published last week, indicate that ill winds blowing out of Asia are adding muscle to the types of clouds these stronger winter cyclones carry.

The team posits that the shift toward these types of clouds, with their strong updrafts, could be altering global circulation patterns – perhaps even masking the effects of global warming, because the thicker, taller clouds reflect more sunlight back into space. And they could be contributing to more rapid melting of snow and ice in the Arctic as dark soot is lofted by strong updrafts in these clouds and carried north to fall back to the surface.

Others have noted this effect of smoke and soot on individual thunderstorms in the Amazon. But this is the first time researchers have seen the effect on storm systems hundreds of miles across.

"As you change things in the atmosphere, whether it's particulates or climate change, all of the effects are not obvious" immediately, says Renyi Zhang, an atmospheric scientist at Texas A&M University in College Station, who led the study.

The team has long been interested in the effects soot and tiny particles called aerosols have on climate. The latest report on global-warming science from the Intergovernmental Panel on Climate Change, released last month, shows that researchers have a poor grasp on these effects. Of all the agents affecting climate – sunlight, greenhouse gases, mineral dust blowing around, for example – the "indirect" effect aerosols have on climate through seeding clouds remains the largest source of uncertainty the report lists.

The US and Europe have imposed tough air-pollution laws, and, indeed, no such cloud trends were seen over the north Atlantic during the study period. Asian countries, with their rapidly growing economies, have been slow to follow suit.

For this study, the scientists used satellite photos of winter storm clouds from two sets of satellites. They used images taken throughout each January between 1984 and 2005.

Then they compared cloud information averaged over the first 10 years with cloud data from the second 10 years. The number of so-called deep convective clouds – driven by strong updrafts and typical of intense thunderstorms – rose by 20 to 50 percent over much of the region. Then the team used one of the latest computer models capable of simulating cloud activity to test the notion that aerosols might be driving the change.

The team found a marked increase in deep convective clouds when the aerosols were included. Cloud formation reverted to more normal types when aerosols were left out of the calculations.

Some researchers say the team's explanation for the increase in convective clouds is plausible. But they also note that the physical processes involved challenge the ability of computer models to simulate them.

Still, the results imply a one-two punch, adds Henry Diaz, an atmospheric scientist at the National Oceanic and Atmospheric Administration's Earth System Research Laboratory in Boulder, Colo.

Long-term changes in large-scale wind patterns in the north Pacific, driven by rising ocean temperatures in the tropics, appear to be "forcing more intense cyclones, and those are better able to use the particulates to enhance themselves," he says.

Aerosols' effects, however, may be on the decline. Last year, China adopted a five-year plan than calls for a 20 percent reduction in sulfur-dioxide emissions by 2010.

Last December, researchers at the Argonne National Laboratory in Argonne, Ill., published a study indicating that soot and sulfate aerosols over China have been declining since the mid-1990s. The change, they suggest, came from increased use of cleaner ways to burn coal in power plants and factories and a reduction in household use of solid fuels for heat and cooking.