Researchers from several institutes in Germany presented a new explanation for the enigmatic climate cycle of abrupt climate shifts in the past. The scientists from Heidelberg, Potsdam and Bremerhaven used a computer model to show that small changes in the Sun could have triggered a series of abrupt warmings in the last Ice Age. The eight researchers published their results in the current issue of the journal "Nature".

During the last Ice Age, which began about 120,000 years ago, at least twenty sudden climate shifts occurred. These Dansgaard-Oeschger events, named after their discoverers, started with an increase of the regional temperature in the North Atlantic area by up to 12 degrees centigrade in the course of only a decade. The repeated climate swings have mainly been documented in ice cores from Greenland and in deep-sea sediments from the Atlantic Ocean. But other "climate archives", for example stalagmites in caves, also show these remarkable climate shifts. After the onset of the current interglacial (the warm period that started about 10,000 years ago) such abrupt warmings did not occur again.

"Since the discovery of the Dansgaard-Oeschger events in the 1980s, one of the big challenges for climatologists was to find a plausible explanation for these abrupt warmings in the ice-age" Holger Braun, PhD-student of the Heidelberg Academy of Sciences and first author of the study, points out. His Potsdam colleagues have already presented a physical mechanism a few years ago (Nature, 11 January 2001): an instability in the ocean circulation during the Ice Age could explain the abrupt climate shifts. The regularity of the warmings, however, still remained enigmatic. These occurred, with some exceptions, mostly at intervals of 1470 years (Geophysical Research Letters, 21 May 2003). Some researchers have speculated that variations in the Sun could have triggered the 1470-year cycle. Evidence for periodic variations of the Sun existed for over 150 years, on the basis of sunspot observations. The data in particular show solar cycles with periods of about 87 and 210 years. A cycle of 1470 years, however, has not been found so far.

The eight scientists published results of a study, which was jointly performed by the Heidelberg Academy of Sciences, the University of Heidelberg, the Potsdam Institute for Climate Impact Research and the Alfred Wegener Institute for Polar and Marine Research (Bremerhaven). According to this study, those two solar cycles could be the trigger for the puzzling period of the Dansgaard-Oeschger events: Because they are close to factors of 1470 years (1470/7=210, 1470/17=86.5), the 210-year cycle and the 87-year cycle of the Sun could combine to a period of 1470 years and thus explain the climate cycle of the ice-age.

The model that was used to test this hypothesis also gives a plausible explanation for the disappearance of the 1470-year climate cycle at the end of the last Ice Age: In the Potsdam model, Dansgaard-Oeschger events can only occur during an Ice Age. "After the termination of the ice-age the Atlantic ocean currents became more stable, so that small changes in the Sun could not affect them", Potsdam scientist Stefan Rahmstorf explains. Over the last 10,000 years, a pronounced cycle of 1470 years consequently does not appear in climate data any more.

For further information please contact:
Holger Braun
Forschungsstelle Radiometrie
Heidelberg Academy of Sciences
Phone: +49 (0)6221 546354
Fax: +49 (0)6221 546405
E-Mail: Holger.Braun@iup.uni-heidelberg.de

Prof. Dr. Stefan Rahmstorf
Potsdam-Institute for Climate Impact Research
Phone: +49 (0)331 2882688
Fax: +49 (0)331 2882600
E-Mail: rahmstorf@pik-potsdam.de
Internet: http://www.pik-potsdam.de

Dr. Johannes Schnurr
PR and Press Officer
Heidelberg Academy of Sciences
Phone: +49 (0)6221 543400
Fax: +49 (0)6221 543355
E-Mail: johannes.schnurr@urz.uni-heidelberg.de
Internet: http://www.haw.baden-wuerttemberg.de

Dr. Michael Schwarz
Press Officer of the University of Heidelberg
phone: 06221/542310, fax: 54317
michael.schwarz@rektorat.uni-heidelberg.de
http://www.uni-heidelberg.de/presse/index.html