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A climatic roller coaster - is stochastic resonance to blame?

Potsdam, 18th Jan. 2002

Researchers from the Potsdam Institute for Climate Impact Research in Germany have shown that stochastic resonance may have played a role in triggering abrupt and dramatic climate shifts during the last great Ice Age. This is the result of a sophisticated computer model of the world´s climate, published by Andrey Ganopolski and Stefan Rahmstorf in Physical Review Letters, 21. January 2002.

At least twenty abrupt and dramatic shifts shook the climate during the last great Ice Age, which lasted from about 120,000 to 10,000 years ago. These climate events, dubbed D/O events or Dansgaard-Oeschger events, started with a sudden warming by 6-10 degrees celsius within a decade or so, and lasted for centuries. They are most strongly recorded in cores from the Greenland ice and from North Atlantic deep sea sediments, but repercussions were apparently felt in many parts of the world. Since their discovery in the 1980's, finding an explanation for these astounding events has challenged climatologists.One particularly puzzling clue comes from analysing the frequency of the events: they occur most often about 1,500 years apart, but sometimes also 3,000 or 4,500 years. It is as if some mystery cycle of 1,500 years triggered them, sometimes missing a beat or two. Physicists are familiar with a mechanism that can cause such behaviour: it is called stochastic resonance, and it can happen when three ingredients come together. First, one needs some kind of regular cycle (in this case of 1,500 years duration). Second, one needs "noise", i.e., some random fluctuations. Third, the system needs to have a threshold where it flips from one state to another.

By using a sophisticated computer model of the world's climate, Andrey Ganopolski and Stefan Rahmstorf from the Potsdam Institute for Climate Impact Research (PIK) in Germany have now shown for the first time how this could work for the D/O events. In a series of model simulations, they were able to show that the Atlantic ocean currents were practically "on the edge" during large parts of the Ice Age: small disturbances were able to make them flip between two different states. In one, the warm Gulf Stream waters reached only to mid-latitudes, and consequently Greenland and surrounding areas were very cold. In the second, albeit unstable state, the warm waters penetrated much further north past Iceland and into the Nordic Seas.

The researcher show that under these glacial conditions, stochastic resonance could flip the ocean currents from one state to the other and could indeed explain many of the peculiar features of D/O events. What some would consider an exotic idea of physics could thus be ultimately responsible for triggering some of the most dramatic climate shifts in the Earth's history. It also helps to understand why the Ice Age climate was so much more unstable than the past 10,000 years, the Holocene, during which human civilisation was able to thrive.

While their model shows that an exceedingly weak 1,500-year cycle was all that is needed, one puzzle remains: what is the ultimate cause of this cycle? Many researchers place their bet on the sun, which may fluctuate in intensity with this period.

Andrey Ganopolski and Stefan Rahmstorf, Potsdam Institute for Climate Impact Research,
Phone: ++49 331 288-2594 and -2688

Original article

Related article, explaining Stochastic Resonance in Glacial Climate
(to appear in Eos, Newsletter of the American Geophysical Union)

For further information about D/O events see:

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Margret Boysen
Phone : ++49 331 288-2507 Fax: ++49 331 288-2552

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