Abstract
Within the last decade, the Greenland ice sheet (GrIS) and its surroundings have experienced record high surface temperatures (Mote, 2007; Box et al., 2010), ice sheet melt extent (Fettweis et al., 2011) and record-low summer sea-ice extent (Nghiem et al., 2007). Using three independent data sets, we derive, for the first time, consistent ice-mass trends and temporal variations within seven major drainage basins from gravity fields from the Gravity Recovery and Climate Experiment (GRACE; Tapley et al., 2004), surface-ice velocities from Inteferometric Synthetic Aperture Radar (InSAR; Rignot and Kanagaratnam, 2006) together with output of the regional atmospheric climate modelling (RACMO2/GR; Ettema et al., 2009), and surface-elevation changes from the Ice, cloud and land elevation satellite (ICESat; Sørensen et al., 2011). We show that changing ice discharge (D), surface melting and subsequent run-off (M/R) and precipitation (P) all contribute, in a complex and regionally variable interplay, to the increasingly negative mass balance of the GrIS observed within the last decade. Interannual variability in P along the northwest and west coasts of the GrIS largely explains the apparent regional mass loss increase during 2002–2010, and obscures increasing M/R and D since the 1990s. In winter 2002/2003 and 2008/2009, accumulation anomalies in the east and southeast temporarily outweighed the losses by M/R and D that prevailed during 2003–2008, and after summer 2010. Overall, for all basins of the GrIS, the decadal variability of anomalies in P, M/R and D between 1958 and 2010 (w.r.t. 1961–1990) was significantly exceeded by the regional trends observed during the GRACE period (2002–2011).
Highlights
For the entire Greenland ice sheet (GrIS), our Gravity Recovery and Climate Experiment (GRACE) analysis results in a mass balance of À 238729 GRACE uncert. (Gt)/yr for October 2003–October 2009, the period with ICESat data available (Fig. 1 and Table 1). This value is consistent with our surface mass balance (SMB)–D estimates of À260753 Gt/yr, and our ICESat estimates of À 245728 Gt/yr, as well as with another recent GRACE estimate of À 230733 Gt/yr obtained with a different GRACE inversion strategy
For all regions and the GrIS as a whole, the trends imposed by anomalies in melting and subsequent run-off (M/R) and D after 9-yr (2002–2011) significantly exceed decadal variability of trends in P for 1958–2010, meaning that GRACE and ICESat record long-term changes of the GrIS
Weaker mass loss rates observed with GRACE in the east and to a lesser extent in the southeast after winter 2008/2009, which have previously been attributed to decreasing ice-dynamic flow (Chen et al, 2011), are mainly a consequence of enhanced accumulation
Summary
For the GrIS, abrupt changes in ice motion have locally been linked to penetration of surface meltwater to the bed (Zwally et al, 2002; Bartholomew et al, 2010). GRACE is influenced by all mass changes in the Earth system, ranging from short-term atmospheric pressure variations to the long-term redistribution of mantle material caused by the glacial-isostatic adjustment (GIA). Despite these limitation, van den Broeke et al (2009) and Rignot et al (2011) have demonstrated a promising agreement of SMB–D with GRACE for mass trends of the entire GrIS, which are consistent with ICESat (Sørensen et al, 2011). We find excellent basin-scale agreement of the temporal mass variations of GrIS from GRACE and SMB–D, and trends from ICESat, allowing us, for the first time, to provide a robust regional separation of the relative importance of P, M/R and D in causing recent GrIS mass loss
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