CHAMPAIGN, Ill. -- While Earth as a whole has warmed during the last half-century, much of the continental United States has grown slightly colder. The trend toward cooler temperatures in the central and eastern United States is due to warmer ocean temperatures, a University of Illinois researcher says.
"Although portions of the U.S. have failed to get warmer, they have gotten cloudier," said Walter Robinson, a UI professor of atmospheric sciences. "Our models suggest there is a strong correlation between this increased cloudiness and higher sea-surface temperatures in the tropical Pacific Ocean."
Conventional wisdom has associated the lack of warming with the influence of anthropogenic aerosols, such as sulfate particles, Robinson said. Unlike greenhouse gases - which contribute to global warming - sulfate particles have a cooling effect. Coal-burning power plants in the Midwest are a major source of such particles.
"The implication has been that if emissions of these particles continue to increase, it could be quite a long time before we experience global warming in the central and eastern U.S.," Robinson said. "If aerosols are not responsible for the cooling, however, we could feel the heat much sooner."
Robinson and his colleagues - James Hansen at the NASA Goddard Institute for Space Studies and Reto Reudy at Science Systems and Applications Inc. - used an atmospheric general circulation model to study the effects of changes in sea-surface temperature on regional cooling in the United States.
The researchers used ocean temperature data from 1950 to 1997. By holding the levels of aerosols, solar irradiance and greenhouse gases constant, the team eliminated them as causes of the cooling.
The model exhibited cooling only when the observed, time-varying tropical ocean temperatures were imposed, Robinson said. Also, the model became cloudier directly over the regions that became cooler, indicating moisture transport was the responsible mechanism. "Warmer sea-surface temperatures in the tropical Pacific Ocean release additional water vapor, which gets swept across Central America and the Gulf of Mexico into North America, where it generates increased cloud cover over the central and eastern U.S.," Robinson said. "By reflecting more solar radiation back into space, this additional cloud cover is directly responsible for the cooling."
While the mystery of the cooling trend may be solved, a deeper question remains: Is the effect a manifestation of global warming, or is it a result of natural variability in the climate system?
"If the tropics are getting warmer because of global warming, then we can expect the cooling trend in the U.S. to continue," Robinson said. "But, if the effect is due to natural variability - and historical records indicate that recent temperature variations in the tropical Pacific are not unusual - then it's only a matter of time before our temperatures 'catch-up' with the rest of the world."
Robinson presented the team's findings at a meeting of the American Meteorological Society, held Jan. 14-19, in Albuquerque, N.M. This work was supported by the National Science Foundation.