What Caused the Ice Ages?
"Ice, Mud Point to [CO.sub.2] Role in Glacial Cycle" by Richard A. Kerr, in Science (Sept. 15, 2000), 1200 New York Ave., N.W., Washington, D.C. 20005.
Every 100,000 years or so for the last million years, vast, miles-high glaciers have moved southward from the Arctic, relentlessly driving all life before them. The last ice age ended only about 10,000 years ago, when the ice retreated to its present polar extent. What caused these monstrous ice ages? In recent decades, notes Kerr, a Science staff writer, scientists have come to think that the glacial cycles were somehow linked to slight variations in the shape (or eccentricity) of the Earth's orbit that occur at about the same 100,000-year intervals. John Imbrie, a paleoceanographer at Brown University, has also proposed that the ice sheets themselves amplified the orbital variations' weak effects.
Kerr reports that Nicholas Shackleton, a paleoceanographer at the University of Cambridge (whose original research also appears in this issue of Science), has found a new actor in the drama: carbon dioxide. Shackleton "finds that orbital variations may muster carbon dioxide into and out of the atmosphere, and the resulting waxing and waning of greenhouse warming may drive the glacial cycle."
The mixture of heavy and light oxygen isotopes preserved in skeletons in deep-sea mud and in ancient air bubbles in Antarctic ice provided Shackleton with windows on conditions millennia ago.
The isotope mixture in the fossils of microscopic, bottom-dwelling marine animals depended partly on the mixture of oxygen isotopes in the seawater in which they lived--and that, in turn, depended on the amount of ice trapped on land. But the isotope mixture in the skeletons also partly depended--though to a lesser extent, it was long thought--on the temperature of the seawater. …