Stefan Rahmstorf

Abrupt climate change

Many aspects of the climate system are not yet sufficiently understood and are the subject of current research and scientific discussion. An example: the mechanisms of abrupt climate change which have repeatedly occurred throughout earth's history and whose causes are a matter of controversy.

Ice-drilling projects in Greenland have provided scientists with information of hitherto unknown quality on the history of our climate over the past one hundred thousand years. Most notable are the European GRIP and American GISP2 projects on the summit of the Greenland ice sheet, which were concluded in 1992 and 1993 respectively.

They are rightly considered one of the outstanding scientific achievements of the 20th century and have fundamentally altered our understanding of the dynamics of climate. Greenland ice is made up of many thousands of layers of snow which are accumulated year after year and slowly compress the older snow underneath into ice. With the aid of sophisticated analytical methods, the ice cores reveal the history of our climate almost like a book, each layer of snow representing a separate page.

Many climate researchers were shocked by the history revealed by this icy book (Fig. 1). Until then, they had assumed that climate changes in gradual cycles — such as the Milankovich cycles with periods of 23,000, 41,000, and 100,000 years — which are caused by irregularities in the Earth's orbit around the sun and which were already known from cores drilled in deep-sea sediments.

New data provided a resolution in time that had never been achieved before

The new data from Greenland, however, provided a resolution in time that had never been achieved before: individual years could be identified and counted, more or less like the growth rings in trees. For the first time, they clearly and unambiguously revealed abrupt, dramatic changes in climate.

The temperature in Greenland had repeatedly warmed by 8—10°C within just a few years, reverting to normal ice age levels only after several centuries. These climate warmings are known as "Dansgaard-Oeschger events" (DO events) after the men who discovered them, Willi Dansgaard from Copenhagen and Hans Oeschger from Berne.

More than twenty such events have been identified during the last ice age, which lasted for a hundred thousand years. One of the key challenges for climate research ever since has been to unravel the mechanisms for these abrupt climate swings.

First of all, the researchers had to establish beyond all doubt that the spikes in the climate records represented real climate events and were not just spurious data caused, for example, by disruptions in the ice flow. The agreement between the cores obtained by the two teams at locations 30 km apart supported the idea that these were genuine climate events.

Final proof came from the deep ocean, when US researchers were able to drill sediment cores from the bottom of the Atlantic with a resolution rivalling that of the ice cores. Spike for spike, the sediment layers from the subtropical ocean, thousands of kilometres away from Greenland and analysed by totally different methods, revealed exactly the same climate events as the Greenland ice.