Volume 9, Number 10: 8 March 2006
Considerable fanfare was recently accorded the study of Rignot and Kanagaratnam (2005), when "using satellite radar interferometry observations of Greenland," they detected "widespread glacier acceleration." Calculating that this phenomenon had led to a doubling of "the ice sheet mass deficit in the last decade" and, therefore, a comparable increase in Greenland's contribution to rising sea levels, they went on to claim that "as more glaciers accelerate … the contribution of Greenland to sea-level rise will continue to increase."
With respect to these contentions, we have no problem with what the two researchers have observed with respect to Greenland's glaciers; but we feel compelled to report that what they have calculated with respect to the mass balance of Greenland's ice sheet and what they say it implies about sea level are diametrically opposed to the story told by other more inclusive real-world data.
A simple introduction to the issue is provided by Dowdeswell (2005), who writes in an accompanying "perspective" piece that "the Greenland Ice Sheet gains mass through snowfall and loses it by surface melting and runoff to the sea, together with the production of icebergs and melting at the base of its floating ice tongues." Hence, it is perfectly clear, as he continues, that "the difference between these gains and losses is the mass balance; a negative balance contributes to global sea-level rise and vice versa."
Where Rignot and Kanagaratnam went wrong was in estimating Greenland's mass gain via snowfall over the vast interior of the ice sheet during the time that coastal glaciers were accelerating. Instead of relying on measurements for this evaluation, they relied on the calculations of Hanna et al. (2005), who used meteorological models "to retrieve annual accumulation, runoff, and surface mass balance."
When actual measurements of the ice sheet via satellite radar altimetry are employed, quite a different perspective is obtained. Zwally et al. (2005), for example, found that although "the Greenland ice sheet is thinning at the margins," it is "growing inland with a small overall mass gain." In fact, for the 11-year period 1992-2003, Johannessen et al. (2005) found that "below 1500 meters, the elevation-change rate is -2.0 ± 0.9 cm/year, in qualitative agreement with reported thinning in the ice-sheet margins," but that "an increase of 6.4 ± 0.2 cm/year is found in the vast interior areas above 1500 meters." Spatially averaged over the bulk of the ice sheet, the net result, according to the latter researchers, was a mean increase of 5.4 ± 0.2 cm/year, "or ~60 cm over 11 years, or ~54 cm when corrected for isostatic uplift."
Consequently, and in direct contradiction of the claim of Rignot and Kanagaratnam, Greenland has experienced no "ice sheet mass deficit in the last decade." Quite to the contrary, it has been host to a net accumulation of ice, which Zwally et al. find to be "contributing -0.03 ± 0.01 mm a-1 to sea-level change." As a result, the net accretion of ice on Greenland over the past decade has actually been ever so slightly lowering global sea level.
As for the future, whereas Rignot and Kanagaratnam contend that "as more glaciers accelerate ... the contribution of Greenland to sea-level rise will continue to grow," Zwally et al. report that "thinning at the margins of the Greenland ice sheet and growth at higher elevations is an expected response to increasing temperatures and precipitation in a warming climate," and this observation suggests that Greenland's accreting-ice trend of the last decade would likely continue in a warming world, which is once again just the opposite of what Rignot and Kanagaratnam contend.
In light of these several observations, it is a sad commentary on the politicization of science that the American Association for the Advancement of Science's press release about the Rignot and Kanagaratnam paper was entitled "Greenland glaciers dumping ice into Atlantic at faster pace." Although technically correct, it failed to convey the far more important knowledge that earth's hydrologic cycle was sucking water out of the ocean and depositing it on Greenland in the form of snow at an even faster pace.
Sherwood, Keith and Craig Idso
Dowdeswell, J.A. 2005. The Greenland Ice Sheet and global sea-level rise. Science 311: 963-964.
Hanna, E., Huybrechts, P., Janssens, I., Cappelin, J., Steffen, K. and Stephens, A. 2005. Journal of Geophysical Research 110: 10.1029/2004JD005641.
Johannessen, O.M., Khvorostovsky, K., Miles, M.W. and Bobylev, L.P. 2005. Recent ice-sheet growth in the interior of Greenland. Science 310: 1013-1016.
Rignot, E. and Kanagaratnam, P. 2005. Changes in the velocity structure of the Greenland Ice Sheet. Science 311: 986-990.
Zwally, H.J., Giovinetto, M.B., Li, J., Cornejo, H.G., Beckley, M.A., Brenner, A.C., Saba, J.L. and Yi, D. 2005. Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992-2002. Journal of Glaciology 51: 509-527.