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Modeler and geologist Robert DeConto of the University of Massachusetts, Amherst, says the policy implications are grim. “Our models may be dramatically underestimating how much worse it’s going to get,” he says, noting that many population centers worldwide are within a few meters of sea level. Looking at signs of meltwater in the early Miocene, DeConto says, “we’re seeing ice retreat faster and more dramatically than any model predicts.”

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New sediment cores from an Antarctic research drilling program suggest that the southernmost continent has had a more dynamic history than previously suspected

ERICE, Italy—If you think of Earth’s poles as fraternal twins, the Arctic has been the wild one in recent years, while the Antarctic has been a steady plodder. Withered by summer heat, Arctic sea ice has shrunk to record low coverage several times since 2005, only to rebound to within 95 percent of its long-term average extent this winter. By comparison, Antarctica, with some 90 percent of the world’s glacial reserves, has generally shed ice in more stately fashion.

However, emerging evidence from an Antarctic geological research drilling program known as ANDRILL suggests that the southernmost continent has had a much more dynamic history than previously suspected—one that could signal an abrupt shrinkage of its ice sheets at some unknown greenhouse gas threshold, possibly starting in this century. Especially troubling, scientists see evidence in the geological data that could mean the vast East Antarctic Ice Sheet, which holds at least four-fifths of the continent’s ice, is less resistant to melting than previously thought.

ANDRILL, a collaboration among scientists from Germany, Italy, New Zealand and the U.S., obtained the evidence from a 3,734-foot-long core extracted in 2007 from the seafloor on the southern McMurdo Sound, near Antarctica’s Ross Island.

A prior core, extracted from the McMurdo Ice Shelf between October 2006 and January 2007, indicated that the West Antarctic Ice Sheet has frequently advanced and retreated. As ANDRILL scientists met here April 6-11 to integrate core results, the geologists and climate modelers pondered the hints of dynamism observed in the much larger East Antarctic Ice Sheet.

Contrary to what climate simulations suggest, David Harwood, the program’s co-chief scientist, says, “nature seems to give us a record that the ice sheets are coming and going.”

The southern McMurdo Sound core yielded clear evidence of some 74 cycles of ice sheet buildup and retreat during a 6-million-year stretch starting in the Miocene Epoch some 20 million years ago. The unexpected ice-sheet dynamism has ANDRILL climate modelers considering what input or software adjustments would make the simulation produce the kind of dynamism seen in the geological record. Their model currently indicates that even if the imperiled West Antarctic Ice Sheet succumbs to current warming trends, the much larger East sheet should stubbornly resist melting. According to the simulation, the East ice sheet melts only when atmospheric carbon dioxide levels are at least eight times higher than preindustrial levels. The ice sheet’s so-called hysteresis, or resistance to change, is now in doubt.

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– research thanks to Tony H.