Recent climate change research has uncovered a disturbing feature of the earth's climate system: it is capable of sudden, violent shifts. This is a critically important realization. Climate change will not necessarily be gradual, as assumed in most climate change projections, but may instead involve sudden jumps between very different states.

No such shift has occurred over the duration of human civilization, yet records of prehistoric shifts are clear. A mounting body of evidence suggests that continued greenhouse gas emissions may push the oceans past a critical threshold and into a drastically different future. Abrupt, hard-to-predict climate change will present an enormous challenge to human societies and the global environment as a whole.

Ice cores, drilled from ancient ice sheets, yield detailed records of past climate. Photo Lonnie Thompson, Ohio State University.

Paleoclimatology is the study of past changes in the climate system. By studying a range of long term natural records - including tree rings, cave deposits, ice cores and deep-sea sediments - scientists have begun to assemble an almanac of global change spanning hundreds of thousands of years.

One of the most important findings to come from these records is that the climate system has undergone many sudden shifts.

Computer models and paleoclimate evidence both suggest that increases in freshwater supply to the North Atlantic periodically make the surface waters too buoyant to sink. As a result, the northward transport of heat stops abruptly, and temperatures around the North Atlantic suddenly drop by as much as 10 degrees Celsius.

The fact that abrupt collapses of the formation of North Atlantic Deepwater puncuated the last Ice Age is a worrisome hallmark of the potential instability of earth's climate. Researchers believe global oceanic circulation may have more than one stable configuration - and once the configuration has changed, it's difficult to change it back.

As an analogy, consider a topheavy bookshelf. If you push it lightly, nothing happens. If you push it somewhat harder, nothing happens. But if you push it beyond a certain point, it will suddenly fall over. Once the bookshelf has fallen over, it's extremely difficult to put it upright again (and there are books all over the floor).

Recent climate modeling suggests that accumulating greenhouse gases could trigger a future collapse of the NADW and bring a rapid, severe cooling -a little Ice Age - to the North Atlantic and surrounding land masses.

Next: The great ocean conveyor