Saturday, April 14, 2007

Understanding the Earth's Temperature Record and The Polar Ice Cap Effect


Unraveling the Earth's Temperature Record
photo by: Vin MorganPalaeo Environment (Ice Cores) Field Work
Because accumulating layers of glacial ice display annual bands which can be dated, similar to annual rings of a tree, the age of ice core samples can be determined. Continuous ice cores from borings as much as two miles long have been extracted from permanent glaciers in Greenland, Antarctica, and Siberia. Bubbles of entrapped air in the ice cores can be analyzed to determine not only carbon dioxide and methane concentrations, but also atmospheric temperatures can be determined from analysis of entrapped hydrogen and oxygen.

Based on historical air temperatures inferred from ice core analyses from the Antarctic Vostok station in 1987, relative to the average global temperature in 1900 it has been determined that from 160,000 years ago until about 18,000 years ago Earth temperatures were on average about 3° C cooler than today.

Except for two relatively brief interglacial episodes, one peaking about 125,000 years ago (Eemian Interglacial), and the other beginning about 18,000 years ago (Present Interglacial), the Earth has been under siege of ice for the last 160,000 years.

Compiled by R.S. Bradley and J.A. Eddy based on J. Jouzel et al., Nature vol. 329. pp. 403-408, 1987 and published in EarthQuest, vol. 5, no. 1, 1991. Courtesy of Thomas Crowley, Remembrance of Things Past: Greenhouse Lessons from the Geologic Record

As illustrated in this final graph, over the past 800,000 years the Earth has undergone major swings in warming and cooling at approximately 100,000 year intervals, interrupted by minor warming cycles at shorter intervals. This represents periods of glacial expansion, separated by distinct but relatively short-lived periods of glacial retreat.

Temperature data inferred from measurements of the ratio of oxygen isotope ratios in fossil plankton that settled to the sea floor, and assumes that changes in global temperature approximately tracks changes in the global ice volume. Based on data from J. Imbrie, J.D. Hays, D.G. Martinson, A. McIntyre, A.C. Mix, J.J. Morley, N.G. Pisias, W.L. Prell, and N.J. Shackleton, in A. Berger, J. Imbrie, J. Hats, G. Kukla, and B. Saltzman, eds., Milankovitch and Climate, Dordrecht, Reidel, pp. 269-305, 1984.Courtesy of Thomas Crowley, Remembrance of Things Past: Greenhouse Lessons from the Geologic Record

The Polar Ice Cap Effect
As long as the continent of Antarctica exists at the southern pole of our planet we probably will be repeatedly pulled back into glacial ice ages. This occurs because ice caps, which cannot attain great thickness over open ocean, can and do achieve great thickness over a polar continent-- like Antarctica. Antarctica used to be located near the equator, but over geologic time has moved by continental drift to its present location at the south pole. Once established, continental polar ice caps act like huge cold sinks, taking over the climate and growing bigger during periods of reduced solar output. Part of the problem with shaking off the effects of an ice age is once ice caps are established, they cause solar radiation to be reflected back into space, which acts to perpetuate global cooling. This increases the size of ice caps which results in reflection of even more radiation, resulting in more cooling, and so on.

Continental polar ice caps seem to play a particularly important role in ice ages when the arrangement of continental land masses restrict the free global circulation of equatorial ocean currents. This is the case with the continents today, as it was during the Carboniferous Ice Age when the supercontinent Pangea stretched from pole to pole 300 million years ago.

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