Mike Tidwell comments -In terms of rainfall, little can be said with certainty. And certainly not all aerosols increase cloud cover. Aerosols are given a slight cooling effect in modern climate models with good results when compared to the real world.

Research suggests pollution over the Pacific will make Oregon wetter, cooler.

Tuesday, March 06, 2007RICHARD L. HILL

Severe air pollution from Asia is expanding the winter cloud cover over the north Pacific Ocean and could bring cooler, wetter weather to the Northwest, a new study suggests.

The booming industrial age in China and India has fired up factories, cook stoves, diesel engines and coal plants, sending gritty particles blowing over the Pacific Ocean toward North America. Those particles have expanded clouds in the storm track that sweeps west to east across the Pacific each winter, researchers say. Those changes ultimately could alter weather patterns in the U.S. and even globally.

“The bottom line from our study is that if you change the Pacific storm system, then you’re going to change the weather in some places,” said Renyi Zhang, a professor of atmospheric chemistry at Texas A&M. “This is definitely going to change the weather system over the United States.”

The pollutants sweep into the Northwest with storms and prevailing wind, bringing dust, salt, soot and fine particles of mercury, arsenic, copper, lead and zinc across the Pacific in a few days. The particles can alter the size of the water droplets that form clouds. The larger, deeper clouds that result would reflect more sunlight back into space, cooling the surface, as well as boosting the chance of precipitation, Zhang said.

The researchers looked only at the cloud changes — mostly brought on by soot and sulfate particles from such activities as coal burning — not what specific weather changes resulted. “But this is the first study that demonstrates that pollution can change a part of the weather system, which in turn can change the climate,” Zhang said.

The Texas A&M researchers used a combination of satellite measurements and computer models to study the pollutants. They compared the deep clouds in January between two decades: 1984-94 and 1995-2005.

They found that the average amount of deep clouds in the north Pacific had increased by 20 percent to 50 percent during the most recent decade compared with the previous 10 years.

The study findings are reported today in the Proceedings of the National Academy of Sciences.

Though the study measured only cloud cover and not the changes in temperature and rainfall that might result, some towns along the Oregon Coast recorded an impressive increase in rainfall during the last years of the study. For example, January rainfall in Newport totaled 89 inches in 1984-1994 but hit 128 inches for 1995-2005.

Some scientists with expertise in clouds and climate are skeptical about the findings. They say that the change in winter clouds may be connected to varying regional climate cycles and other dynamic weather processes rather than Asian pollutants.

They say the cloud changes may be linked to the Pacific Decadal Oscillation, a little-understood phenomenon in which the climate flip-flops between wet-cool and dry-warm phases about every 10 to 30 years. A shift to a wetter, cooler phase in the Northwest occurred in the mid-1990s, which coincides with the most recent years studied.

James A. Coakley, an atmospheric science professor and cloud expert at Oregon State University, said the study authors didn’t show a clear link between pollutants and clouds. He said other factors may be triggering the change, perhaps global warming or the Pacific Decadal Oscillation. “What they’ve found may have little to do with increases in pollution from Asia.”

David Covert, a research professor and atmospheric chemist at the University of Washington, agrees. Pollution sources “are a minor player in the clouds, storms and storm track,” he said. “This is not to say that the effects are insignificant but that they are very hard to detect” above typical variations in weather and climate.

Zhang emphasized that the study looked only at the changing storm track over the Pacific Ocean. “But I think other regions are going to be affected. That needs to be further studied.”

The Arctic especially may be susceptible to the pollutants and their effects on the weather, Zhang said, with black soot absorbing more heat from the sun and increasing the melting of ice.

“There’s a lot of unanswered questions about the changes to the large weather systems that these pollutants may be bringing,” Zhang said. “The scientific community needs to look at this a lot closer.”

Richard L. Hill: 503-221-8238; richardhill@news.oregonian.com