Earth's history is replete with wide-ranging climate changes. Analysis of ice core data from Greenland and Antarctica provide evidence that global temperatures have oscillated no fewer than 20 times between glacial and interglacial conditions during the last 110,000 years.
Temperatures may have changed by as much as 15° C. during a single cycle. Furthermore, these changes were abrupt, occurring within a few centuries or even decades.
Thermohaline circulation, or the "oceanic conveyor belt," as some researchers describe it, transfers equatorial heat poleward, where the waters of increasing salinity cool, sink into the abyss, and return at depth. Waters upwell in some regions of the oceans, and downwell (particularly around Greenland and Iceland) in others. A complete cycle of the circulation might take 1,000 years.
During glacial times, great ice sheets retreated under the warming effect of the circulation and discharged increasing flow rates of fresh water. The salinity and density of the Atlantic arctic waters were thus reduced. Downwelling slowed, maybe ceased, and effectively stopped the poleward heat transfer.
Then earth's high latitudes became thermally isolated from the tropics and, as a result, cooled. The ice sheets built up, and the adjacent oceans cooled and eventually became more saline through evaporation.
Ultimately, those waters began to sink, thereby restarting the "conveyor belt." Once again, heat was transferred poleward and the ice sheets retreated under a warming climate. Earth's climate system thus became locked in oscillations on a millennial timescale, shifting repeatedly between glacial and interglacial conditions for 100,000 years.1 2 3
Scientists are attempting to model thermohaline circulation. A trend toward global warming could reduce the circulation and heat transfer to high latitudes. The result would be a cooler climate in regions such as the North Atlantic.
Increased glacial melt in the future, however, would not be sufficient to stop the circulation. Climate oscillations during the glacial epoch required a glacial volume that far exceeded today's volume.
Analysis of ice cores shows that concentrations of atmospheric CO2 changed gradually and remained within a narrow range throughout the epoch of gyrating climate changes. CO2 remained roughly 220-250 ppm during a 40,000-year span, while the earth's climate cycled through several glacial-interglacial oscillations, accompanied by changes of global temperatures of several degrees Celsius.4
The epoch of climate oscillations abruptly ended some 11,500 years ago, when, during a relatively brief 1,000 years, global temperatures warmed by some 15° C., to levels 5° C. warmer than any previous interglacial period. Yet, atmospheric CO2 increased at barely perceptible rates: from 200 rpm to 280 ppm during 7,000 years, ending about 10,000 years ago. CO2 concentrations then remained near 280 ppm until the 19th Century, while temperature variations remained within 2° C.5
Increasing concentrations of CO2 may have contributed to the warming that ended the 100,000-year epoch of glacial cycles, although the timing of the sudden warming does not match the gradual increase of CO2. Since CO2 concentrations changed little during the glacial epoch, CO2 could not have caused the glacial-interglacial oscillations.
References
- Broecker, W., "What Drives Glacial Cycles?" Scientific American, January 1990, p. 49.
- Broecker, W., "Thermohaline Circulation, the Achilles Heel of Our Climate System: Will Man-Made CO2 Upset the Current Balance?" Science, Vol. 278, p. 1,582.
- Broecker, W., "Are We Headed for a Thermohaline Catastrophe?' in Gerhard, L., et al., op. cit., p. 83.
- Houghton, J., "Global Warming-The Complete Briefing," 2nd edition, 1997, Cambridge University Press, Chapter 3, Fig. 4.4, p. 54.
- Sundquist, E., "The Global Carbon Dioxide Budget," Science, Feb. 12, 1993, p. 934.
