Ice Sheets![Figure][1] Increasing carbon dioxide levels could destabilize the Antarctic Ice Sheet PHOTO: © RGB VENTURES/SUPERSTOCK/ALAMY STOCK PHOTO 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][2]; see also p. [34][3] [1]: pending:yes [2]: /lookup/doi/10.1126/science.aab0669 [3]: /lookup/doi/10.1126/science.aad6284