Abstract
Greenland's main outlet glaciers have more than doubled their contribution to global sea level rise over the last decade. Recent work has shown that Greenland's mass loss is still increasing. Here we show that the ice loss, which has been well‐documented over southern portions of Greenland, is now spreading up along the northwest coast, with this acceleration likely starting in late 2005. We support this with two lines of evidence. One is based on measurements from the Gravity Recovery and Climate Experiment (GRACE) satellite gravity mission, launched in March 2002. The other comes from continuous Global Positioning System (GPS) measurements from three long‐term sites on bedrock adjacent to the ice sheet. The GRACE results provide a direct measure of mass loss averaged over scales of a few hundred km. The GPS data are used to monitor crustal uplift caused by ice mass loss close to the sites. The GRACE results can be used to predict crustal uplift, which can be compared with the GPS data. In addition to showing that the northwest ice sheet margin is now losing mass, the uplift results from both the GPS measurements and the GRACE predictions show rapid acceleration in southeast Greenland in late 2003, followed by a moderate deceleration in 2006. Because that latter deceleration is weak, southeast Greenland still appears to be losing ice mass at a much higher rate than it was prior to fall 2003. In a more general sense, the analysis described here demonstrates that GPS uplift measurements can be used in combination with GRACE mass estimates to provide a better understanding of ongoing Greenland mass loss; an analysis approach that will become increasingly useful as long time spans of data accumulate from the 51 permanent GPS stations recently deployed around the edge of the ice sheet as part of the Greenland GPS Network (GNET).
Highlights
In a more general sense, the analysis described here demonstrates that Global Positioning System (GPS) uplift measurements can be used in combination with Gravity Recovery and Climate Experiment (GRACE) mass estimates to provide a better understanding of ongoing Greenland mass loss; an analysis approach that will become increasingly useful as long time spans of data accumulate from the 51 permanent GPS stations recently deployed around the edge of the ice sheet as part of the Greenland GPS Network (GNET)
[19] Simulations of the Helheim Glacier, one of the largest glaciers in southeast Greenland and located near KULU, show a rapid speed‐up of ice flow and retreat in 2004 [Howat et al, 2005; Stearns and Hamilton, 2007] followed by a deceleration [Howat et al, 2007] and stabilization in 2006 [Nick et al, 2009]. This behaviour is consistent with our GPS and GRACE measurements at KULU, which suggest a rapid acceleration in uplift and ice mass loss followed by a moderate deceleration
The deceleration is weak, implying that southeast Greenland is still loosing mass at a much higher rate than it was prior to fall 2003. It suggests that even if glaciers in this region have stabilized and are not accelerating further, they are continuing to contribute significantly to sea level rise. This is important for assessing the long‐ term mass balance of the Greenland ice sheet and for projections of future sea level rise
Summary
[2] Many lines of evidence indicate the Greenland ice sheet has been losing mass at a significant rate over the last several years. The mass loss has been especially dramatic along the southeast coast. There is evidence both from GRACE [Velicogna and Wahr, 2006] and from radar interferometry [Rignot et al, 2008] that significant increases in the mass loss rate and in glacial speeds occurred in this region around fall 2003. Recent radar interferometry observations suggest that the increased glacial speeds observed in the south over the last few years, are spreading into regions further north [Rignot et al, 2008]. We use a combination of GRACE and GPS data to confirm that there is an ongoing northward migration of increasing mass loss
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