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."