THE East Antarctic ice sheet looks unlikely to release its frozen grip any time soon. A new model suggests that prehistoric sea-level rise long thought to have been caused by the ice sheet melting was actually the result of local subsidence.
About 400,000 years ago, Earth went through a warm interglacial period similar to today’s. The geological record shows traces of beaches and marine fossils in areas of Bermuda and the Bahamas far from the coast, suggesting that sea level was 20 metres higher than now.
Global sea level could have been that high only if the East Antarctic ice sheet melted at the time, according to climate models. And that is odd: this ice sheet doesn’t seem to have melted at any other point in its long history.
Others have looked for more exotic explanations – for instance, that a mega tsunami hit Bermuda and the Bahamas and formed the confounding shoreline.
Maureen Raymo of Columbia University in Palisades, New York, thinks there is a simpler explanation. Ice sheets are heavy and weigh down the crust, while the ice-free areas just beyond the sheet bulge upwards as a consequence. When the ice melts, the crust beneath the former sheet rebounds, but the crust in the peripheral bulge subsides, making the land there dip and sea-level rise appear more extreme.
Bermuda and the Bahamas lie where the peripheral bulge of the North American ice sheet would have been 400,000 years ago. When Raymo and colleagues corrected the climate model to include Earth’s elastic response to losing a heavy ice sheet, it added the missing metres to the lower sea level calculated for the Caribbean at the time. It means that the apparently extreme sea-level rise was only a local phenomenon (Nature, DOI: 10.1038/nature10891).
The results, Raymo says, mean “it is extremely unlikely the East Antarctic ice sheet can melt catastrophically in a slightly warmer climate”.