Potsdam scientists explain puzzling climate changes
Potsdam, 5th Jan. 2001

Embargo: Wednesday 10 January, 1900 London time 

Two researchers from the Potsdam Institute for Climate Impact Research in Germany have found a consistent explanation for the drastic climate swings that plagued the last ice age. With computer simulations, they showed that the Atlantic ocean currents were particularly unstable during this ice age: even tiny perturbations could trigger a major flip in ocean currents, causing a sudden warming of up to 10 degrees centigrade within a decade. The present climate is much more stable according to the model, but it is still vulnerable to global warming.

The last Ice Age started 100,000 years ago and ended 10,000 years ago. The abrupt climate changes which occurred repeatedly within this Ice Age present one of the great puzzles of climatology. Ice cores drilled in Greenland in the 1980's had revealed that more than twenty times, temperatures had risen suddenly by up to 10 degrees centigrade within a decade or less. These climatic anomalies, known as Dansgaard-Oeschger (or D/O) events, warmed at least the entire North Atlantic region and lasted a few hundred up to a few thousand years. From the beginning it was suspected that changes in Atlantic ocean currents must have played a key role in these events. Cores drilled in deep sea sediments confirm this. A specific mechanism that could be verified in model calculations was, however, missing. Although a number of model calculations had looked at a possible collapse of the Atlantic currents, this could only explain a sudden climatic cooling - not the D/O warm events. In addition, such a collapse required a massive outside trigger, such as a big meltwater flood into the Atlantic.

Two scientists from the Potsdam Institute for Climate Impact Research, Andrey Ganopolski and Stefan Rahmstorf, have now for the first time presented model simulations which correctly reproduce the characteristic time evolution and spatial extent of D/O-events (Nature, 11.1.2001). Their calculations show that there were three possible states of the Atlantic during the Ice Age. In the normal cold state that prevailed during most of the glacial time, warm subtropical waters flowed north in the Atlantic only up to the region south of Iceland, where they released their heat to the atmosphere, sank down and returned south as a cold deep ocean current. However, two more circulation states were possible, though not stable: an unusually warm climate state, in which the warm Atlantic waters reached beyond Iceland into the Nordic Seas almost like today, and an exceptionally cold state in which the circulation broke down completely. The two climatologists suggest that D/O-events correspond to a sudden flip from the normal to the warm Atlantic state, that is, an episodic intrusion of warm Atlantic waters into the Nordic Seas which caused the warming found in the Greenland ice cores.

But what triggered this dramatic change in ocean currents? And why did it always come to an end after a few centuries? A systematic analysis by the Potsdam scientists showed that during the Ice Age, the Atlantic was literally "on the edge": a tiny disturbance was enough to push the warm waters into the Nordic Seas. But since this circulation state was not stable, each D/O-event eventually fizzled out by itself. The weak trigger, which was so hugely amplified by the instability of the Atlantic currents at the time, could have been variations in the sun. There is evidence for a solar cycle with a period of 1,500 years - and D/O-events often occur exactly 1,500 years apart.

And what about the present climate? Since the end of the last Ice Age about ten thousand years ago, no comparable climate swings have occurred. The researchers' model gives an explanation for that as well. In a warm climate, the stability of the Atlantic currents is different. In contrast to an Ice Age, the warm circulation state becomes the normal and stable situation, and solar variations cannot disturb it. The Atlantic is simply not on the edge any more. But this is no reason for complacency. Earlier simulations with the same model (Climatic Change 43, 353-367) had already shown that even in the comparatively stable present climate, a sufficiently large disturbance could trigger a collapse of the Atlantic currents. Such a serious disturbance could be caused by the greenhouse effect, if humanity continues to emit large amounts of carbon dioxide and other greenhouse gases to the atmosphere.

Andrey Ganopolski, Potsdam Institute for Climate Impact Research,
Phone: +49 331 288 2594,
E-mail: Andrey.Ganopolski@pik-potsdam.de


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