Evidence of very rapid climate changes in the past

In recent months there has been a flurry of research articles describing evidence of abrupt changes in the earth's climate - particularly during and at the end of the last glacial period.

Most of this type of work is based on meticulous sampling and analysis of ice cores from Greenland and Antarctica, and of sediment cores from oceans and lakes. In all cases the analysis includes determination of isotopes of elements such as oxygen, nitrogen and argon (in the ice itself, or in air bubbles trapped in the ice). The ratios of these stable isotopes (such as ¹⁸O/¹⁶O) are sensitive indicators of ambient temperature during ice or sediment formation. In some of these studies the levels of other important climate indicators, such as carbon dioxide and methane, have also been measured.

The recent research shows remarkably rapid global-scale climate warming at specific times. For example, work carried out at the University of Bern (Lang and others) suggests a 16° C change in temperate-zone temperature over approximately 100 years from 69,830 to 69,730 years b.p. (ie. before 1950). This change does not actually correspond with the end of a glacial period, but rather a relatively short-lived warm interval during the last glacial period.

Similar work conducted at the Scripps Institute in California (Severinghouse and Brook) shows a 10° C change in temperature over an even shorter time period - the 50 years from 14,650 to 14,600 years b.p. This interval represents the beginning of the end of the last glaciation, and while the vast sheets of continental and alpine ice did not melt over such a short time, there is strong evidence that the warming was global in extent. It is argued, for example, that within 10 years (by 14,590 b.p.) the methane content of the atmosphere had increased dramatically. This change would have resulted from rapid melting in more tropical regions, producing extensive flooding of continental areas, and an increased production of methane from decaying vegetation.

It is not precisely known what triggered these rapid climate changes, although one of the preferred mechanisms is related to abrupt changes in thermal stratification, convection and the patterns of currents within the oceans - factors which might have resulted in more effective transfer of heat from equatorial to polar regions.

What is extremely important, however, is the implication that the earth's climate is not very stable, and that highly significant temperature fluctuations can take place over very short periods. In other words, the earth's climate is "tippy", and certain factors - which we don't understand at all well - can cause it to change rapidly. This should give us even greater incentive than ever to think about what we are now doing to the levels of atmospheric greenhouse gases.

References:

Lang, C., Leuenberger, M., Schwander, J. and Johnsen, S., 16° C rapid temperature change variation in central Greenland 70000 years ago, Science, Vol. 286, p. 934-937, October 29^th 1999.

Severinghaus, J. and Brook E., Abrupt climate change at the end of the last glacial period inferred from trapped air in polar ice, Science, Vol. 286, p. 930-934, October 29^th 1999.

Allen, J. and others, Rapid environmental changes in southern Europe during the last glacial period, Nature, V. 400, p. 740-743, August 19th, 1999.

Haug, G., Sigman, D., Tiedemann, Pedersen, T. and Sarnthein, M., Onset of permanent stratification in the subarctic Pacific Ocean, Nature, V. 401, p. 779-782, August 21st, 1999.

Dokken, T. and Eystein, J., Rapid changes in the mechanism of ocean convection during the last glacial period, Nature, V. 401, p. 458-461, September 30th, 1999.