Glaciologists have long cast a worried eye on the waning West Antarctic ice sheet (WAIS). If it were to melt away in a greenhouse-warmed world, the world's oceans would swell 5 meters, swallowing your favorite beach as well as major cities from New Orleans to Bangkok. Now a paper in tomorrow's Science  confirms suspicions that in the recent geologic past, at a temperature perhaps not much warmer than today, the WAIS wasted away to a scrap and flooded the world's coasts.
Eight years ago Reed Scherer at Uppsala University in Sweden analyzed samples from a drill hole in the ice and found evidence of single-celled plants, called diatoms, in mud deep beneath the ice. Those plants appeared in antarctic waters since 1.3 million years ago, so Scherer took their presence as evidence that the ice had retreated at least that long ago. But other researchers pointed out that the diatom fossils might have blown onto the ice sheet and slowly made their way down to the base of the ice.
To rule out that possibility, Scherer and his colleagues have now analyzed sediments from the bottom of nine kilometer-deep holes spread over 10 kilometers of the ice sheet. Four of them had young, marine diatoms. These sediments had none of the Antarctic lake diatoms that would accompany marine diatoms if they were blown onto the ice, which implies that the marine diatoms did not take the high road. The mud also contained significant amounts of the radioactive isotope beryllium-10, which particles can pick up from seawater as they sink to the bottom; the overlying ice had much less beryllium-10, indicating that the diatoms hadn't migrated through the ice. Scherer's analyses make it "highly, highly unlikely there is any windblown" contribution to the samples, says Pennsylvania State University, University Park, glaciologist Richard Alley.
The finding comes on top of some more alarming recent predictions. Staff scientist Michael Oppenheimer of the Environmental Defense Fund in New York City recently reviewed the question of WAIS stability and concluded from the ice sheet's somewhat erratic behavior of late that its most likely fate is disintegration during the next 500 to 700 years. If that scenario comes to pass, it will be small consolation to Florida landowners to know that it has all happened before.