That mystery, which has puzzled meteorologists in recent years, involves long-term climate trends in the southeastern United States and eastern China.

According to experts' predictions, temperatures should warm globally, but should cool in both regions because of the presence of manmade air pollutants called aerosols. But while the Southeast has seen a mild cooling trend, the climate in China has actually warmed slightly over the last 50 years.

Dr. Vinod Saxena, NC State professor of meteorology, says the opposite temperature trends may result from the presence of differing types of aerosols, or airborne particulate matter, in the atmosphere over China and the southeastern United States.

Saxena and his colleagues outline their findings in the Feb. 15 issue of the peer-reviewed journal Geophysical Research Letters.

"The Southeast receives sulfate aerosols blown over the Appalachians from industrial manufacturing in the Ohio River Valley," Saxena explained. "Sulfate aerosols are believed to decrease temperatures by having the same effect as painting a house roof white: They reflect solar radiation back into space.

"The aerosols over eastern China are mostly carbon soot particles from the burning of coal and wood for cooking and heating, and from unregulated industrial emissions," he said. Those aerosols tend to result in higher temperatures by absorbing solar radiation -- especially in the winter, when more coal and wood is burned.

The research by Saxena and his colleagues found that the amount of carbon soot pollution over China increased during the last half century, at the same time that average annual temperatures nudged upward. The temperature increase was most pronounced during the winter months.

That's where Mount Pinatubo enters the picture. Between June 12 and June 16, 1991, the volcano in the Philippines spewed 30 megatons of sulfur dioxide into the atmosphere. The airborne debris became sulfate aerosols that resulted in cooler temperatures around the world, including the southeastern United States and eastern China.

While looking at temperature data from both regions, the meteorologists found that, for all of 1992, the climate was cooler than normal in both regions. After the volcanic sulfate aerosols dissipated in 1993, temperatures increased in China, but not in the Southeast.

That, Sazena explains, underscores the differing roles played by the two types of aerosols in global climate change trends. "The Pinatubo eruption was the best experiment that nature could devise," he said.

Those findings may have implications for negotiations on a global climate change treaty. The 1997 Kyoto Protocol calls on industrialized nations, including the United States, Japan and members of the European Union, to reduce their emissions of greenhouse gases in coming years. Developing nations such as China would be largely exempt from such reductions.

International talks on implementing the treaty broke down last November in part because of disagreements over greenhouse emissions reductions. Scientists believe greenhouse gases are increasing global temperatures by trapping the sun's heat; some scientists predict global warming could lead to rising sea levels and changing weather patterns that result in flooding and drought.

Saxena believes that the Kyoto treaty, as it stands, would not help the world's global warming situation because it doesn't address the unrestricted emissions of carbon aerosols by developing nations such as China and India. Research indicates that those aerosols are the second most important cause of global warming, following the greenhouse gas carbon dioxide.

"The air masses don't know any international boundaries," Saxena said. "Five days after soot is emitted into the atmosphere in China, it shows up on the West Coast of the United States." More research is needed on the sources of carbon soot aerosols and how they are distributed in the atmosphere, he adds.

Saxena's colleagues in the research are Dr. Shaocai Yu, formerly Saxena's student at NC State and now a research associate at Duke University, and Zongei Zhao of China's National Climate Center.

The research was supported by NASA's Mission to Planet Earth, the National Oceanic and Atmospheric Administration's Office of Global Programs, and the Georgia Institute of Technology.

The climate data from China were collected by that nation's National Climate Center from 72 weather stations throughout eastern China between 1951 and 1994. The data from the southeastern United States were taken at 52 comparable weather stations between 1949 and 1994. - By Kevin Potter

(Reference: "A Comparison of Signals of Regional Aerosol-Induced Forcing in Eastern China and the Southeastern United States" published Feb. 15, 2001, in Geophysical Research Letters, vol. 28, no. 4 by Shaocai Yu, formerly of NC State and now at Duke University; V.K. Saxena of NC State University; and Zongei Zhao of the National Climate Center, State Meteorological Administration, Bejing, China.)

[Contact: Dr. Vinod Saxena, Kevin Potter ]

23-Feb-2001