Every 100,000 years or so for the past million years, kilometers-high sheets of ice have ground southward from the Arctic, scouring landscapes and driving all life before them. These episodes seem to be triggered by the rhythmic stretching of Earth's orbit, but exactly how this feeble "orbital variation" could produce such monstrous climate shifts has remained a mystery. Now, scientists say they have found a likely strongman to transmit and enforce the orbital variations' demands: carbon dioxide.
The discovery comes from paleoceanographer Nicholas Shackleton of the University of Cambridge, one of the three scientists who identified orbital variations as the leading candidate for the ultimate driver of the ice ages in 1977. After that insight, some had suggested that feedbacks from the ice sheets themselves amplified the weak orbital signal. In the 15 September issue of Science, however, Shackleton proposes that orbital variations may muster carbon dioxide in and out of the atmosphere, and the resulting waxing and waning of the greenhouse warming may drive the glacial cycle.
Shackleton based his conclusion on a study of two ancient records: the fossil skeletons of microscopic marine animals called foraminifera from muds, and bubbles contained in the 400,000-year-long ice core retrieved from the antarctic ice sheet by Soviet drillers at Vostok station. Comparing these allowed Shackleton to tease out past water temperatures, global ice volumes, and CO2 levels with unprecedented accuracy. To his surprise, Earth's orbital eccentricity, deep-sea temperature, and atmospheric carbon dioxide all varied in step, on the same 100,000-year cycle. Ice volume, however, lagged behind.
Shackleton sees the lockstep as a sign of cause and effect. Changes in eccentricity are still the main driver, because they shift the distribution of sunlight across the globe. This could somehow decrease atmospheric carbon dioxide, which would weaken the greenhouse and cool the ocean and atmosphere. At the end of an ice age, everything reverses. The warming is relatively rapid and so changes in temperature and CO2 would appear to coincide; the sluggish ice volume would lag behind. That delay rules out ice as a prime mover, Shackleton says; it's only a follower.
"It's quite an exciting development," says paleoceanographer John Imbrie, professor emeritus at Brown University. "He's made a major step forward." Even if Shackleton's elimination of ice as a primary factor and support for carbon dioxide stand up, "there's a lot of questions that remain," says geochemist Daniel Schrag of Harvard University. In particular, he says, no one knows how orbital variations would send the carbon dioxide into and out of the atmosphere.