July 19, 2000 CHAMPAIGN, Ill. -- The human contribution to global warming is clearly present and must be controlled, say researchers at the University of Illinois. But there is also another, as-yet-unexplained, cyclic contribution that has important implications for monitoring future climate change.
"Appearances can indeed be deceiving," said Michael Schlesinger, a UI atmospheric scientist. "If global warming doesn't persist year after year, we shouldn't be fooled into thinking that human effects are no longer of concern. There is something else at work here that we don't yet fully comprehend."
Using a simple climate/ocean model, Schlesinger and his wife, Natalia Andronova -- also an atmospheric scientist at the UI -- calculated the contributions to the observed changes in global-mean, near-surface temperature caused by human and volcano forcing, putative variations in the irradiance of the sun, and the residual temperature change for the years 1856-1997. The researchers published their results in the July 15 issue of Geophysical Research Letters.
"We found that while the human effect has steadily increased -- and is now the dominant external factor -- there also is a residual factor at work within the climate system," Schlesinger said. "This factor played a significant role in the warmings observed during 1904-1944 and 1976-1990."
During both periods, putative variations in solar output played only a minor role in the observed temperature change, the researchers say. Volcanoes were similarly dismissed as the predominant cause.
"Some scientists have conjectured that since there were more volcanoes in the 19th century than in the 20th century, the observed warming trend was due to a decrease in volcanic activity," Andronova said. "But that is not the explanation we came up with. Although volcanic forcing does contribute during 1904-1944, the residual factor is much larger."
The warming observed during 1976-1990 was nearly equally due to human effects and to the residual factor, with volcanoes contributing a cooling, and the sun at most a small warming, Schlesinger said. "The role of the residual factor is even more dominant during 1944-1976, when the human-induced warming was in opposition to the observed cooling."
One plausible explanation for the residual factor was first proposed by Schlesinger and his graduate student Navin Ramankutty six years ago. In the Feb. 24, 1994, issue of Nature, the researchers identified a temperature oscillation that occurred over the North Atlantic Ocean and its adjacent land areas. The oscillation -- which has a period of 65-70 years -- periodically warms and cools the atmosphere, thus sometimes contributing to and sometimes counteracting the greenhouse effect.
"This means there is a very good chance that the present warming will turn around and we will again experience a protracted period of cooling," Schlesinger said. "If we do see that, we should not conclude that the human effect on climate is small or nonexistent -- or that we have eliminated the problem -- and go back to 'business as usual.' We need to be far more intelligent in our response."
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