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.
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."
Antarctica's ice sheets contain roughly two-thirds of the world's fresh water. A meltdown of the West Antarctic Ice Sheet alone would boost sea levels by an estimated 20 feet, but if the East sheet were to also succumb, along with the Greenland ice sheet, sea levels could rise by more than 200 feet. This would be catastrophic for major population centers near sea level, such as New York City, much of Florida and nearly all of Bangladesh. No one expects the ice sheets to disappear overnight—even the worst timelines span centuries—and uncertainty about their fate remains, but radar altimetry from NASA satellites indicates that melting is under way in some parts of the East sheet, as well as in much of the West sheet. Researchers say the effects of melting ice sheets could be apparent within a lifetime as undersea currents are disrupted and weather patterns shift.
DeConto's collaborator, climate modeler David Pollard of Pennsylvania State University, says the answer to the puzzling disparity between model predictions and the core data could lie in an erroneous assumption about Antarctica itself. For example, Pollard says, some parts of the land underlying the East ice sheet might be much lower than currently believed. In that case, if warming oceans strip away the surrounding ice shelves, significant chunks of the ice sheet could slide into the ocean. Subglacial lakes, which form as glaciers slide over depressions, may have an underappreciated role, he added. DeConto says polar stratospheric clouds also need further study. There are indications, he says, that they act as infrared reflectors, which might contribute to ice sheet melting in ways not yet accounted for in models.
Whatever the cause, the key evidence for ice sheet dynamism in the Antarctic comes from the core's lithographic record. Sedimentologists have studied its facies, the visible characteristics that distinguish each stratum, for indications of how warm or cold the surrounding environment was. The McMurdo Sound facies repeatedly vary from "ice proximal," where fractures, scraping and larger grain size indicate a glacier rumbling by, to "ice distal," where laminated sediments and marine fossils speak of lapping waves in an ice-free marine environment.
Numerous other lines of evidence from the core support the idea of wide climate swings in Antarctica's past. There are spikes in the amount of pollen found within the core, for example, indicating flowering plants nearby. Levels of various organic molecules repeatedly rise and fall in the column, showing that microorganisms and shellfish flourished in the warmer periods and receded during cold times. The presence or absence of clasts—chunks of sedimentary rock carried along and deposited by glaciers—also indicates a fluctuating Antarctic climate. Co-chief scientist Fabio Florindo of Italy's National Institute of Geophysics and Volcanology notes that surges of magnetic mineral in the core may also mark warming trends. Reviewing the record, sedimentologist Christopher Fielding of the University of Nebraska-Lincoln says that in its warmest periods Antarctica must have resembled Patagonia today, where winters average a few degrees below freezing and summertime highs occasionally reach 80 degrees Fahrenheit.
However, Fielding, DeConto and others agree the core data is not conclusive. The McMurdo Sound core offers a pinhole view of East and West Antarctic Ice Sheet behavior, but at the price of extreme difficulty of interpretation. Scientists expect that firmer answers will emerge from other Antarctic research, including a hoped-for third ANDRILL project planned for Coulman High, also in the vicinity of Ross Island.
Harwood, a University of Nebraska geoscientist, says there is already evidence enough for policymakers to take action against global warming in hopes of preventing a dramatic Antarctic meltdown. "This core is going to be studied for the next 20 to 30 years," he notes, but already, he adds, the Miocene-age evidence it contains strongly suggests that it would be a mistake to count on ice-sheet stability in the Antarctic. "We see two or three periods of ice-sheet collapse, including one that looks abrupt, with very rapid deglaciation."