Abrupt climate changes revisited: How serious and how likely?

Widespread climate changes in the distant past were larger and more rapid than those experienced during more recent historical times. For example, the cooling of the climate leading into the last "ice age", the peak of which occurred roughly 21,000 years ago, and the subsequent climate transition to a warmer, more modern world were punctuated by abrupt climate changes that were one-third to one-half as large as the change from an "ice age" to a warm climate (i.e., the roughly 11-13 degrees F [6-7 degrees C] transition from an "ice-age" to a warm climate, globally). Paleoclimate records further indicate that during these abrupt shifts many aspects of the climate in many regions changed precipitously in the timespan of a few years to as little as a single year.

Moreover, the current warm period since the peak of the last ice age (21,000 years ago) was previously thought to be very stable with none of the large climate shifts that so characterized "ice ages". Contrary to this once widely held notion, new evidence from deep-sea sediments and ice-cores shows that this warm period was interrupted by a series of abrupt cooling events, each lasting several hundred years. One of the most prominent of these events occurred roughly 12,800 years ago, after Greenland had warmed to near present conditions. Another smaller but significant abrupt cooling event occurred roughly 8,200 years ago when temperatures in Greenland were slightly above present-day temperatures. These and other recent, abrupt cooling events have been detected from Scandinavia to Africa, some of which occurred within a human lifetime. One such notable event 4,200 years ago (2,200 BC), is shown to be synchronous with the collapse of the world's first human empire in Mesopotamia.