ScienceDaily (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. 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.”

http://www.sciencedaily.com/releases/2000/07/000719111045.htm