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Scientists reconstruct Greenland's climate history with the help of Antarctic ice cores

A distance of around 14,000 kilometres separates Greenland from Antarctica. Nevertheless, using climate data from Antarctic ice cores, an international team of researchers succeeded in reconstructing a curve for Greenland temperature changes that goes back 800,000 years into the past, thus enabling completely new insights into the climate history of Greenland and the North Atlantic.

The results have been published in the scientific journal Science under the title "800,000 Years of Abrupt Climate Variability".

The experts used the so-called "seesaw" model of ocean circulation as the basis for their investigations and calculations. This model implies that warm phases in the northern part of the Atlantic Ocean are accompanied by cooling in the south - and vice versa.

"During the last ice age there were abrupt swings in climate on Greenland. They led to temperature fluctuations of up to ten degrees Celsius within a few decades. These jumps were caused by changes in the intensity of the Atlantic overturning circulation that transports heat to high northern latitudes. If this heat pump was suddenly boosted, a more pronounced redistribution of heat from the southern ocean to the North Atlantic took place," says co-author Dr. Gregor Knorr from the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association. The researchers therefore assumed in their theoretical considerations that indications of these rapid climate changes had to exist both in Greenland and in Antarctica.

Today scientists are gaining information on what temperatures prevailed in the two regions at that time from ice cores. These cores are taken from ice sheets and are considered as one of the most informative climate archives because of their fine temporal resolution. The ice sheets are created when snow falls on the top layer, is compacted there and forms a new ice layer in the course of time. This layer then not only contains air bubbles and trace gases from that time, but also archives the water isotope composition of the precipitation, with the help of which scientists can look far back into the history of the climate. "The longest cores from the Greenland ice sheet date back to the last interglacial, i.e. about 120,000 years ago. The ice archive of the Antarctic, by contrast, encompasses the past 800,000 years," states Knorr.

In the first step the researchers compared temperature data from Greenland and Antarctic ice cores of the last 100,000 years. The Greenland core was retrieved from the center of the ice cap. Scientists took the Antarctic cores at so-called Dome C on the East Antarctic plateau in 2004.

Both ice cores confirmed the theoretical assumptions of the scientists for this period. "The data from the first 100,000 years matched each other so convincingly that we decided to look back even further into the past," reports Knorr. On the basis of the actual existing data from the Antarctic, the researchers then calculated an "synthetic" temperature time series for Greenland that goes back as far as 800,000 years.

They subsequently compared the development of these data to climate data from caves in the central Chinese province of Hubei. "Today we know that during the particularly cold phases in the North Atlantic the atmospheric circulation changed so markedly that the summer monsoon in China was weaker. These precipitation patterns are still found nowadays in the stalagmites that grow in these caves and whose climate archive goes back as far as 400,000 years," says Knorr.

The synthetic temperature curve for Greenland not only passed the cave comparison test. It additionally permits us to draw the conclusion that the rapid climate swings on Greenland during the last ice age were not exceptional cases. They apparently occurred in every ice age in the climate history of the last 800,000 years. "With our synthetic time series we can show that sudden climate changes in the course of the last 800,000 years were evidently an integral part of ice ages and of the period when they came to an end. This is a result that definitely gives grounds to conduct further research since rapid climate changes are thus possibly not only a passive parallel phenomenon, but may also have played an active role in the final stages of the ice ages," says Knorr. These new findings could help us better understand how the transition from an ice age to a subsequent interglacial takes place. Up to now this transitional phase has baffled scientists.

First of all, however, the researchers want to find out now what special processes initiate and steer these climate swings. In addition, they face the task of simulating these phenomena with climate models and identifying the determining mechanisms. "The synthetic temperature curve for Greenland could additionally serve as a good comparative framework for future research into abrupt climate changes," says Knorr. With the help of this curve it should be a easier for scientists to refine age of other ice and sediment samples.

Notes for Editors: The complete literature reference for the Science article is: Barker, S. and Knorr, Gregor and Edwards, R. L. and Parrenin, F. and Putnam, A. E. and Skinner, L. C. and Wolff, E. and Ziegler, Martin (2011): 800,000 Years of Abrupt Climate Variability. Science 21 October 2011: Vol. 334 no. 6054 pp. 347-351 doi: 10.1126/science.1203580

You will find printable pictures in the online version of this press release at <www.awi.de.> Dr. Gregor Knorr is available for interviews (tel: +49 (0)471 4831-1769). Your contact in the Communication and Media Department of the Alfred Wegener Institute is Sina Löschke (tel.: +49 (0)471 4831-2008; e-mail: Sina.Loeschke@awi.de).

The Alfred Wegener Institute conducts research in the Arctic, Antarctic and oceans of the high and middle latitudes. It coordinates polar research in Germany and provides major infrastructure to the international scientific community, such as the research icebreaker Polarstern and stations in the Arctic and Antarctica. The Alfred Wegener Institute is one of the seventeen research centres of the Helmholtz Association, the largest scientific organisation in Germany.

idw :: 26.10.2011