Changes in the Greenland Ice Sheet: Responses to Climate Warming?
Until recently, we did not know whether the Greenland ice sheet was growing or shrinking. Now, changes in ice-sheet mass are being derived from satellite measurements of changes of ice surface elevations and sub-satellite gravity fields. Other important measurements include: ice-flow velocities from satellite imaging radars and GPS stations on the surface, surface temperatures from satellite infrared sensors, and surface melting from satellite passive microwave sensors. We now know that during 1992 to 2002 the Greenland ice sheet was thinning at the margins and growing inland, with little overall change in the ice mass. Thinning at the margins from increased melting and inland growth from increased precipitation are both expected responses of the surface balance in a warming climate. Projections of future ice behavior (e.g. IPCC reports) have been based primarily on estimates of changes in these competing surface processes, with the expectation that melting would become dominant in a few decades. Changes in ice flow rates over the next century were expected to be small, because changes in the surface temperature and mass affect the ice dynamics slowly over centuries. Now, ICESat’s laser-altimeter measurements of elevation change and GRACE’s gravity measurements show that during the last 5 years, the Greenland mass balance has changed to a significant net loss. Both the loss at the edges due to increases in melting and the growth inland due to increases in precipitation appear to have accelerated. In addition, summer-time acceleration of the ice flow has been observed, caused by propagation of surface melt to the ice base where it increases the ice sliding across the bedrock. Outlet glaciers that flow directly into the ocean have also accelerated, some of which may be caused by basal melting of floating glacier extensions due to warmer ocean temperatures. As climate warming continues, the Greenland ice sheet will continue to lose mass at an increasing rate, but quantitative predictions of ice sheet changes will require incorporation of new non-linear processes into dynamical ice models.