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Climate kick from the Southern Ocean

July 29, 2003

This much was already known: in the closing phase of the last ice age the Southern Hemisphere began warming first. As a result, the Antarctic sea ice melted. It was at least a thousand years later - as evidenced by investigations of Greenland ice cores - that the high northern latitudes began to get warmer. Sea ice in the North Atlantic retreated and the great continental ice sheets in Scandinavia, Greenland, and North America melted.

In the Nature article, the two investigators document for the first time how the climate shift was initiated by the interplay between the southern and northern Atlantic regions at the end of the last ice age about 15,000 years ago. Feeding their ocean computer model with known climate data, provided by investigations of marine sediment deposits, their analyses resulted in the following scenario:

The melting sea ice in the southern polar region had a twofold effect. For one, the density relationships of the water masses in the Southern Ocean were altered. This triggered an ocean current that flowed northward along the coast of South America into the Caribbean, accumulating significant amounts of heat as it passed through the equatorial regions. A second effect was the strengthening of the warm and salty Agulhas Current, which flows out of the Indian Ocean around the Cape of Good Hope into the South Atlantic, then continues toward Brazil and the Caribbean. During the cold period this path had been blocked by the northward-reaching Antarctic sea ice. The heat- and salt-rich waters of the two marine currents eventually reached the high northern region.

A critical point was reached in the North Atlantic after 1,000 years. The now salty North Atlantic water, giving up its stored heat to the atmosphere and thereby cooling down, becomes so heavy that it sinks rapidly in the Greenland Sea and flows southward at a depth of about 2,000 meters. In response, at the surface, more warm water is pulled in from the south to replace the heavy sinking water. The heat pump of the Gulf Stream triggers the onset of the warm period in the Northern Hemisphere.

"Our calculations show that the Gulf Stream was abruptly turned on again about 15,000 years ago. This led to a temperature increase in the North Atlantic of about 6 degrees Celsius," says Dr. Gerrit Lohmann. "This, in turn, resulted in the partial melting of the continental ice, and an influx of melt water into the ocean." Nevertheless, the model calculations presented indicate that this light, non-saline water didn't significantly affect the heat pump in the North Atlantic, which only functions with heavy, saline water. "Many climate researchers are now concerned with the question of whether the Gulf Stream will begin to falter as a heat pump for our latitude due to the human-induced climate shift and the resulting increase of fresh-water input from melting ice," offers Dr. Lohmann. "It is also important to investigate where the flywheel for the oceanic conveyor belt is located and how it functions." And here the southern polar sea plays a key role.

The current Nature article was made possible through the research program DEKLIM. It was encouraged by the Federal Research Ministry in order to fill gaps in our knowledge in the field of climate research as well as to promote work by young new scientists.

Research Center Ocean Margins (University of Bremen)