Abrupt climate changes during the last Ice Age – a phenomenon of extreme winters?

Vast seasonal differences in climate history challenge modellers

The severe climate oscillations in the North Atlantic area during the last glacial period were a phenomenon of extreme winters - the summers were only slightly affected. This is the result of the examinations of research teams from the Netherlands, the Alfred Wegener Institute for Polar and Marine Research in the Helmholtz Association, Bremerhaven, and the Leibniz Institute of Marine Sciences, Kiel. It has now  been published in the periodical “Nature Geoscience”.

In climate history, abrupt and severe climate changes between colder and warmer phases  within only a few decades were taking place during and at the end of the last glacial period (80.000 to 100.000 years ago). They were particularly distinctive in the higher latitudes of the North Atlantic region. This is confirmed by temperature reconstructions of marine sediments and Greenland ice cores.

Scientists investigate the causes and consequences of natural climate changes from earth's history for an improved understanding of the global system and to draw conclusions for current and future climate development. Particularly interesting are the climate patterns of the high and low latitudes, their interactions and how the oceans and the atmosphere influence them.

The team of scientists researched these quick and severe climate changes in the lower latitudes and reconstructed - by means of a sediment core from the Gulf of Mexico - the temperature of the sea surface during the summers of the last 300.000 years. The temperature curve of the last glacial period does surprisingly not show any short-term oscillations like the abrupt cooling of up to 15°C registered on Greenland. However, these short-term oscillations were registered by Caribbean climate curves, which primarily depict the winter signal. “The curve of summer temperatures reconstructed by us does not show any abrupt and severe fluctuations,” explains Prof. Dr. Ralf Tiedemann from the Alfred Wegener Institute. “With our result we back up the hypothesis that these abrupt massive cooling phases are rather a reflection of extreme winters.” This extreme seasonal difference is also reflected by the shifting tropical rain belt, the so-called Intertropical Convergence Zone. This follows from comparisons with other climate time series from the Caribbean and South America. During the glacial climate changes, the position of the tropical rain belt was located in the summer relatively stable in the heights of Venezuela. However, it shifted during extreme summers over South America far to the South to about 20°S (Bolivia). This is indicated by altered growth rates on stalagmites and glaciers.

The question, which mechanism caused this shift during the last glacial period to an extreme seasonal bias in the high northern latitudes, is topical time and again. The majority of researchers attribute the quick climate changes during the last glacial period to the fact that the Thermohaline Circulation of the oceans can become unstable: increased precipitation, icebergs and meltwater influx in the North Atlantic decrease the salinity and density of the surface water; this reduces deepwater formation in the North Atlantic and flowing in of warm near-surface water masses via the Gulf Stream. If this is significantly reduced or ceases functioning as a heat pump, massive cooling in the North Atlantic and Northern Europe will occur. This can result in an extensive expansion of sea ice if a threshold is exceeded during the winter months. The reduction of albedo (reflectivity potential of sunlight) linked with it would reinforce the cooling effect – a condition that is apparently compensated during the summer months.

“We can see on the basis of these results seasonal dynamics we can hardly imagine. Contrasts of this kind between summer and winter are not to be expected here,” says Prof. Ralf Tiedemann. “Against the background of prognosticated climate change it is important, however, that we understand seasonal climate changes, their oscillation margin and susceptibility. Dynamics of this kind pose a great challenge for climate modelling and still await clarification.”

Funding of this research was provided by the German Science Foundation (DFG) within the framework of the International Marine Past Global Change Study and the DFG research group ‘Impact of gateways on ocean circulation, climate and evolution’ and by the Netherlands Science Organization (NWO).


Notes for Editors:
Your contact person at the Alfred Wegener Institute is Prof. Ralf Tiedemann (phone: +49 471 4831- 1200; email: Ralf.Tiedemann@awi.de). Your contact person in the public relations department is Margarte Pauls (phone: +49 471 4831 - 1180; email: Margarete.Pauls@awi.de).

The Alfred Wegener Institute carries out research in the Arctic and Antarctic as well as in the high and mid latitude oceans. The institute coordinates German polar research and makes available to international science important infrastructure, e.g. the research icebreaker “Polarstern” and research stations in the Arctic and Antarctic. AWI is one of 15 research centres within the Helmholtz Association, Germany’s largest scientific organization.


Nature Geoscience, Advance Online Publication (AOP); DOI 10.1038/ngeo277 on http://dx.doi.org/
“Persistent summer expansion of the Atlantic Warm Pool during glacial abrupt cold events”
Martin Ziegler1*, Dirk Nürnberg2, Cyrus Karas2, Ralf Tiedemann3 und Lucas J. Lourens1
1Department of Earth Sciences, Utrecht  University, 3508 TA Utrecht, The Netherlands
2Leibniz Institut für Meereswissenschaften, D-24148 Kiel, Germany
3Alfred-Wegener Institut für Polar- und Meeresforschung, D-27568 Bremerhaven, Germany