Greenland ice sheet mass balance: distribution of increased mass loss with climate warming; 2003-07 versus 1992-2002

We derive mass changes of the Greenland ice sheet (GIS) for 2003-07 from ICESat laser altimetry and compare them with results for 1992-2002 from ERS radar and airborne laser altimetry" The GIS continued to grow inland and thin at the margins during 2003-07, but surface melting and accelerated flow significantly increased the marginal thinning compared with the 1990s. The net balance changed from a small loss of 7 +/- 3 Gt a(-1) in the 1990s to 171 +/- 4 Gt a(-1) for 2003-07, contributing 0.5 mm a(-1) to recent global sea-level rise. We divide the derived mass changes into two components: (1) from changes in melting and ice dynamics and (2) from changes in precipitation and accumulation rate. We use our firn compaction model to calculate the elevation changes driven by changes in both temperature and accumulation rate and to calculate the appropriate density to convert the accumulation-driven changes to mass changes. Increased losses from melting and ice dynamics (17-206 Gt a(-1)) are over seven times larger than increased gains from precipitation (10-35 Gt a(-1)) during a warming period of similar to 2 K (10 a)(-1) over the GIS. Above 2000 m elevation, the rate of gain decreased from 44 to 28 Gt a(-1), while below 2000m the rate of loss increased from 51 to 198 Gt a(-1) Enhanced thinning below the equilibrium line on outlet glaciers indicates that increased melting has a significant impact on outlet glaciers, as well as accelerating ice flow. Increased thinning at higher elevations appears to be induced by dynamic coupling to thinning at the margins on decadal timescales.