Mass balance estimates

Combining ice thickness data, ice velocity measured with satellite imaging radars on nearly all the glaciers in Greenland, and a melt and accumulation model for snow/ice referenced to the 1960s, we calculated the ice sheet mass balance and its change through time caused by glacier acceleration. The result, which is the contribution of changes in ice dynamics to the mass balance, showed that the ice sheet lost 56±30 km3 of ice per year in 1996, increasing to 167±40 km3 of ice per year in 2005. Added to that, there were changes in snow/ice melt and snowfall, or surface mass balance of the glaciers. Changes in the surface balance calculated by Hanna et al (2005) added to the loss from ice dynamics because ice melt has increased faster and removed more ice than the increase in snowfall in the interior was able to supply. This increased the estimated mass deficit to 91±31 km3 in 1996 and 224±41 km3 of ice per year in 2005 (i.e. nearly 50 cubic miles of ice per year).

Other techniques employed to survey the ice sheet have confirmed these estimates or yielded lower values. Lower values are obtained by airborne surveys because these surveys do not cover all glaciers and only include one or two laser profiles on each one (Krabill et al, 2004; Thomas et al, 2006). The observed trend in mass loss, however, agrees quite well with the mass flux technique. The airborne surveys were also able to reveal the enormous rates of ice thinning that are affecting the glaciers, up to a 50 m drop in ice elevation on Kangerdluqssuaq Glacier in 2003-2004. Another technique, satellite radar altimetry, gives lower numbers and even ice sheet growth (Zwally et al, 2005). But this instrument was designed to sample the flat surface of oceans and is not well adapted to survey the steep margins of an ice sheet, and especially narrow glaciers surrounded by mountains (Thomas et al, 2005). The resulting estimates are therefore mostly characteristic of the vast interior, but not of the entire ice sheet.

Gravity surveys from the GRACE satellite mission are providing another set of independent estimates of the mass balance, with a spatial resolution of about 400 km, but without having to make an assumption about snow/firn density because this technique directly samples changes in mass of the ice sheet. Many corrections must be applied to the data, however, before analyzing the results in order to account for the influences of ocean variability, atmospheric variability and postglacial rebound. The results also vary with time and the GRACE satellites were only launched in 2002, at a time when the Greenland Ice Sheet was changing rapidly. Despite these limitations, the GRACE data confirmed a large mass loss from Greenland and an acceleration of the loss over the last few years (Chen et al, 2006; Luthcke et al, 2006; Velicogna and Wahr, 2006).

Overall, observations suggest that the ice sheet is contributing to an increase in global sea level of about 0.3-0.6 mm/yr since the 1990s. This represents a significant fraction of the observed global sea level rise of 3 mm/yr that has been observed since about 1993 with satellites. At this rate, sea level will increase by 6 cm (over 2 inches) by 2100 due to the loss of ice from Greenland alone.

There are, however, no physical reasons why the present rate should not continue to increase. As warming continues and propagates along the northern shores, the mass deficit will increase, with estimates that it could double in the next decade. The central question is no longer to determine whether the ice sheet is losing mass to the ocean or not, but how fast it will lose ice over the next hundred years and beyond.

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