Melting of the Greenland and Antarctic ice sheets is the largest potential contributor to sea level rise, but calculating the mass balance over time of such large and topographically diverse areas is difficult. Many individual measurements must be combined to create a composite picture of the whole, and techniques that track surface elevation accurately in the larger, more uniform interior sections are not as accurate when applied to the relatively narrow, high-relief coastal margins. Moreover, mass change estimates based only on elevation data do not take into account the height variations caused by compaction of the snow that covers the ice. Recent studies have documented mass loss along the margins and concurrent mass gain in the interiors, but the net effect of these compensatory processes is unclear.
Zwally et al. used satellite-based radar altimeters to track elevation changes for nearly all of Greenland and Antarctica over a decade. In addition to applying improved methods of data analysis, the authors incorporated estimates of density variation due to firn compaction. Their integrated assessments suggest that although Greenland is gaining mass, Antarctica is melting at a comparatively faster rate, resulting in a net rise in sea level. These conclusions differ both in sign and in magnitude from those of several other studies (for instance, see Rignot and Kanagaratnam, Reports, 17 February 2006, p. 986), leaving open the question of how to reconcile the findings. — HJS
J. Glaciol. 51, 509 (2005).