"Ice, Mud Point to CO2 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. This unknown influence made the isotope mixture in the skeletons an imprecise gauge of the ice volume as it varied over time. Using that gauge, Shackleton saw an apparent correlation between the ice-volume changes and the 100,000-year orbital variations, although the link "was not impressive," Kerr says.

Shackleton then looked at air bubbles in a 400,000-year-long ice core from Antarctica. The oxygen-isotope composition of that air was not affected by ocean temperatures, but was affected by the volume of ice that existed. By comparing this geologic record with the other one, writes Kerr, Shackleton was able "to tease out [the] intimately entangled climatic influences with unprecedented accuracy."

To Shackleton’s surprise, "deep-sea temperature accounted for more variation of oxygen isotopes than ice volume did." Indeed, deep-sea temperature, atmospheric carbon dioxide as recorded in the gas bubbles, and orbital eccentricity "all varied in step, on the same 100,000-year cycle," Kerr reports, while ice volume "lagged behind," apparently ruling out ice as a prime mover.

Shackleton sees the lockstep of the three factors "as a sign of cause and effect," says Kerr. When an ice age began, in his view, "changes in eccentricity—presumably by shifting the distribution of sunlight across the globe—could have decreased atmospheric carbon dioxide, weakening the greenhouse and cooling the ocean and atmosphere." The opposite changes would have occurred at the ice age’s end.

Imbrie and others agree that Shackleton has made "a major step forward." But many questions remain, geochemist Daniel Schrag of Harvard University told Kerr. How, for example, do orbital variations "muster" carbon dioxide into and out of the atmosphere?