... In an article published on Nature's website today, Jacob and colleagues3 describe a study based on satellite data for Earth's changing gravity field that tackles this problem. Their results have surprising implications for both the global contribution of glaciers to sea level and the changes occurring in the mountain regions of Asia. ... Nonetheless, until recently there was little alternative to some form of extrapolation of the terrestrial observations to large regions and numbers of glaciers. One such high-profile assessment5 concluded that, during the period 1996–2006, the mass loss from glaciers and ice caps (GICs) increased steadily, contributing a sea-level rise of 1.1 ± 0.24 millimetres per year by 2006. In this study5, the authors concluded that GICs had been the dominant mass contributor to sea-level rise over the study period, and they extrapolated their results forward to argue that this would also be the case in the future.
Then along came the Gravity Recovery and Climate Experiment (GRACE), which consists of a pair of satellites that have been making global observations of changes in Earth's gravity field since their launch in 2002. They have been used in various studies to examine the changing mass of the great ice sheets of Antarctica and Greenland6 and several other large glaciated regions7. But, so far, the data have not been analysed simultaneously and consistently for all areas. ...
To isolate the GIC signal from others at the surface, Jacob and colleagues defined units of mass change — called mass concentrations, or mascons — within each of their 18 GIC regions (including the European Alps; Fig. 1). Each region might have many tens of mascons defining the geographic extent of significant ice volume within the sector3. Combined with global models of land hydrology and atmospheric-moisture content, the authors were able to isolate the GIC mass trends over the eight-year (2003–10) period of the observations. What they found was unexpected.
First, the contribution of GICs (excluding the Antarctica and Greenland peripheral GICs) to sea-level rise was less than half the value of the most recent, comprehensive estimate8 obtained from extrapolation of in situ measurements for 2001–05 (0.41 ± 0.08 compared with 1.1 mm yr−1). Second, losses for the High Mountain Asia region — comprising the Himalayas, Karakoram, Tianshan, Pamirs and Tibet — were insignificant. Here, the mass-loss rate was just 4 ± 20 gigatonnes per year (corresponding to 0.01 mm yr−1 of sea-level rise), compared with previous estimates that were well over ten times larger. By a careful analysis, the authors discounted a possible tectonic origin for the huge discrepancy, and it seems that this region is more stable than previously believed. ... (*)