Did environmental “noise” trigger

A climatic roller coaster during the last Ice Age? Under certain conditions, noise can paradoxically increase a weak signal’s detectability and influence. This phenomenon, called stochastic resonance (SR), has been observed in settings as diverse as chaotic lasers and human reflex systems (see Physics Today, March 1996, page 39 ). Andrey Ganopolski and Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany have shown that SR may have played a role in triggering 20 or so abrupt and dramatic warming events—called Dansgaard-Oeschger (DO) events—during the last Ice Age, which lasted from about 120 000 to 10 000 years before the present. Each DO event started with a roughly 10-year warming of about 10°C over the North Atlantic, and each lasted for up to a few centuries before cooling again. Curiously, the DO events typically were 1500 years apart, but sometimes skipped a beat and occurred after 3000 or 4500 years. The researchers used a global climate model with added environmental “white noise” in the form of random changes in the amount of precipitation and melted ice and snow entering the Nordic seas. Through the SR mechanism, that random influx of fresh water could amplify a weak underlying 1500-year signal of unknown (but perhaps solar) origin. The scientists found that North Atlantic ocean currents, on crossing a salinity threshold, could have flipped between two different states, one in which warm Gulf Stream waters reached only to midlatitudes and another in which warm waters penetrated much farther north. The SR-based model reproduces key features of the DO 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 than that of the past 10 000 years, in which human civilization has thrived. (A. Ganopolski, S. Rahmstorf, Phys. Rev. Lett. 88 , 038501, 2002 http://dx.doi.org/10.1103/PhysRevLett.88.038501 .)