21st-Century Evolution of Greenland Outlet Glacier Velocities

Not So Fast 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, Moon et al. (p. 576; 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. A decade-long compilation of velocity data for Greenland’s outlet glaciers shows complex spatial and temporal variability. Earlier observations on several of Greenland’s outlet glaciers, starting near the turn of the 21st century, indicated rapid (annual-scale) and large (>100%) increases in glacier velocity. Combining data from several satellites, we produce a decade-long (2000 to 2010) record documenting the ongoing velocity evolution of nearly all (200+) of Greenland’s major outlet glaciers, revealing complex spatial and temporal patterns. Changes on fast-flow marine-terminating glaciers contrast with steady velocities on ice-shelf–terminating glaciers and slow speeds on land-terminating glaciers. Regionally, glaciers in the northwest accelerated steadily, with more variability in the southeast and relatively steady flow elsewhere. Intraregional variability shows a complex response to regional and local forcing. Observed acceleration indicates that sea level rise from Greenland may fall well below proposed upper bounds.