Random Noise and An Ice-Age Roller Coaster

Random noise could have triggered a climatic roller coaster during the last Ice Age, new research suggests. Under certain conditions, random noise, such as electrical static, can paradoxically increase a weak signal's detectability, and in general amplify the signal's influence on its surroundings. This phenomenon, called "stochastic resonance" (SR), has been observed in settings as diverse as chaotic lasers and human reflex systems (Updates 121, 293, 509).

Interestingly, the very concept of SR was proposed in 1982, to explain how random climate events may have helped generate a regularly repeating interval of approximately 100,000 years between Ice Ages. However, subsequent evidence did not support this idea.

Now, SR is coming back home to climate: Researchers (Andrey Ganopolski and Stefan Rahmstorf, Potsdam Institute for Climate Impact Research in Germany, andrey@pik-potsdam.de) have shown that stochastic resonance may have played a role in triggering Dansgaard-Oeschger (D/O) events, abrupt and dramatic climate shifts during the last great Ice Age, which lasted from about 120,000 to 10,000 years ago. These events started with sudden warmings of at least 10 degrees Celsius over the north part of the Northern Atlantic, taking place over approximately a decade and lasting for centuries.

Curiously, the D/O events most often occurred 1,500 years apart, but sometimes they "missed a beat" and occurred after 3,000 or 4,500 years. This suggests they were caused, at least in part, by a weak underlying cycle, such as a periodic, but slight, fluctuation in the sun's intensity.

Furthermore, using a sophisticated computer model of the world's climate, the researchers found that North Atlantic ocean currents during the Ice Age could flip between two different states, one in which warm Gulf Stream waters reached only to mid-latitudes and another in which warm waters penetrated much farther north. As the researchers explain, these climate-altering circulation patterns might have switched from one state to another through the influence of a weak 1,500 year cycle, whose effects were amplified by environmental noise, such as random changes in the amount of precipitation and meltwater (melted ice and snow) entering the Nordic Seas.

While the exact source of the regular cycle remains unspecified, a SR-based explanation reproduces key features of the D/O events and North Atlantic ocean circulation during the last Ice Age. If confirmed, this mechanism may help to explain why the Ice Age climate was so much less stable compared to that of the past 10,000 years, in which human civilization was able to thrive. (Ganopolski and Rahmstorf, Physical Review Letters, 21 January 2002.)