21st-Century Evolution of Greenland Outlet Glacier Velocities

<jats:title>Not So Fast</jats:title> <jats:p> Recent observations of some of Greenland's outlet glaciers have shown large and rapid increases in the speeds at which their ice has streamed to the sea. Simple projections of ice loss and sea level rise, based only on these increases, result in alarmingly high values and correspondingly great public concern. In order to provide a more comprehensive and detailed picture of this type of ice sheet mass loss, <jats:bold> Moon <jats:italic>et al.</jats:italic> </jats:bold> (p. <jats:related-article xmlns:xlink="http://www.w3.org/1999/xlink" ext-link-type="doi" page="576" related-article-type="in-this-issue" vol="336" xlink:href="10.1126/science.1219985">576</jats:related-article> ; see the cover) compiled a decade-long record of ice stream velocity measurements for nearly all of Greenland's major outlet glaciers. The pattern of flow variability around the ice sheet was both spatially and temporally complex, with clear differences between marine- and land-terminating types, as well as between regions. Furthermore, the integrated velocity of all of the outlet glaciers measured was considerably less than the upper bounds that have been proposed on the basis of a few rapidly accelerating locations, implying that sea level rise over the next century may be less than the 2 meters that have been suggested. </jats:p>