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Antarctic Ice Sheet variability across the Eocene-Oligocene boundary climate transition

Science  01 Apr 2016:
Vol. 352, Issue 6281, pp. 76-80
DOI: 10.1126/science.aab0669

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Sensitive ice sheets

Why did the Antarctic Ice Sheet begin to grow 34 million years ago, and what does that have to do with us? Galeotti et al. studied a marine sediment core recovered from just off the coast of Antarctica (see the Perspective by Lear and Lunt). The ice sheet did not begin to grow until atmospheric CO2 concentrations had dropped to below around 600 parts per million. Indeed, the ice sheet was unstable when CO2 was higher. As modern atmospheric CO2 concentrations continue their rise, a shift back to an unstable Antarctic Ice Sheet could increase harmful rises in sea level.

Science, this issue p. 76; see also p. 34

Abstract

About 34 million years ago, Earth’s climate cooled and an ice sheet formed on Antarctica as atmospheric carbon dioxide (CO2) fell below ~750 parts per million (ppm). Sedimentary cycles from a drillcore in the western Ross Sea provide direct evidence of orbitally controlled glacial cycles between 34 million and 31 million years ago. Initially, under atmospheric CO2 levels of ≥600 ppm, a smaller Antarctic Ice Sheet (AIS), restricted to the terrestrial continent, was highly responsive to local insolation forcing. A more stable, continental-scale ice sheet calving at the coastline did not form until ~32.8 million years ago, coincident with the earliest time that atmospheric CO2 levels fell below ~600 ppm. Our results provide insight into the potential of the AIS for threshold behavior and have implications for its sensitivity to atmospheric CO2 concentrations above present-day levels.

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