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Antarctic history suggests ice sheet ‘danger’ threshold


Assembling a detailed timeline surrounding the Antarctic ice sheet’s inception around 34 million years ago, scientists have identified a carbon dioxide “danger zone” for the ice sheet’s demise.

Based on CO2 levels when the ice sheet formed, the researchers report that Antarctica’s ice will be “dramatically” more vulnerable to melting once CO2 surpasses 600 parts per million in the atmosphere. Concentrations of the greenhouse gas reached 400 ppm last year, well above its 280 ppm preindustrial level.

“With present-day emission rates, it’s expected that we’ll reach 600 ppm before the end of this century,” says study coauthor Simone Galeotti, a paleoclimate scientist at the University of Urbino in Italy. The ice sheet stockpiles enough water to raise sea levels by around 60 meters and reshape Earth’s coastlines.

The new work, published online March 10 in Science, provides the best estimate yet for the CO2 threshold that fostered the Antarctic ice sheet’s appearance, Galeotti says. Scientists previously traced the ice sheet’s beginnings using indirect measurements such as falling sea levels. Those methods aren’t definitive, however.

Galeotti and colleagues studied a roughly 900-meter-long ocean sediment core drilled in 1999 off the coast of East Antarctica. The core provided a detailed record of the Antarctic ice sheet’s size from around 34 million to 31 million years ago. As the ice sheet expanded, sediments piled up along its outermost edge. Using the sediment core, the researchers discovered that the ice sheet formed in two stages.

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