Ice sheets are often assumed to respond slowly to climate change, with dynamic response times measured in centuries to millennia. On page 1756 of this issue, however, Ekstrom et al. (1) describe a dramatic increase in glacial seis-micity over the pastseveral years, which coincides with the acceleration of many of Greenland's major outlet glaciers (2-4). The rapidity of these changes counters the view of a sluggishly responding ice sheet and indicates that outlet glacier dynamics can respond swiftlyto climate change with consequent increases in sea level. Over the past decade alone, glacier acceleration has increased Greenland's contribution to sealevel rise by more than 0.3 mm year~(-1) (2).
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