Researchers analyzing recent data from the SPOT 5 and ASTER satellites say that previous studies have largely overestimated mass loss from Alaskan glaciers over the past 40 years. Writing in a recent issue of Nature Geoscience, the team suggests that mass loss in these glaciers contributed 0.12 mm/year to sea-level rise between 1962 and 2006, rather than 0.17 mm/year as previously estimated.

The new estimate was obtained by comparing recent topographies, derived from Spot 5-HRS (SPIRIT project with maps from the 1950-60s, which enabled loss from three quarters of the Alaskan glaciers to be measured.

In 1995, and then again in 2001, researchers from the Geophysical Institute of the University of Alaska estimated that the contribution of Alaskan glaciers to sea-level rise was 0.17 mm/year using an airborne laser to measure the surface elevation of 67 glaciers along longitudinal profiles.

These elevations were then compared with those mapped in the 1950s and 1960s. From this, the researchers inferred elevation changes and then extrapolated this to other glaciers. Their results pointed to a major contribution to sea-level rise for the 1950-1995 period (0.14 mm/year sea-level rise), which then doubled in the recent period (after 1995).

But authors of the new study say the University of Alaska team overestimated ice loss from these glaciers by 50% because the impact of rock debris that protects certain glacier tongues from solar radiation (and thus from melting) was not taken into account. Moreover, their sampling was limited to longitudinal profiles along the center of a few glaciers, which geometrically led to overestimation of ice loss.

The new study confirms that the thinning of Alaskan glaciers is very uneven, and shows that it is difficult to sample such complex spatial variability on the basis of a few field measurements or altimetry profiles. Thanks to their regional coverage, satellite data make it possible to improve observations of glacial response to climate change and to specify the contribution of glaciers to sea-level rise.

Ice loss from Alaskan glaciers since 1962 is evidently smaller than previously thought. However, thinning (sometimes over 10 m/year, as in the Columbia glacier) and glacial retreat remain considerable. Moreover, the spectacular acceleration in mass loss since the mid 1990s, corresponding to a contribution of 0.25 to 0.30 mm/year to sea-level rise, is not in question and proves to be a worrying indication of future sea-level rise.

Citation: Berthier E., Schiefer E., Clarke G.K.C., Menounos B., Remy, F., 'Contribution of Alaskan glaciers to sea-level rise derived from satellite imagery', Nature Geoscience, 2010, 3 (2), 92; doi: 10.1038/ngeo737