The riddle of the ice

During West-Greenland summers, icebergs up to 200 metres high float into the sea while glaciers speed up to 30 metres per day. (Photo: WikiCommons)

Estimates of the current loss of land ice in Greenland vary between 130 and 230 cubic kilometres of water per year. The controversy rages even within the TU’s faculty of Aerospace Engineering.

“The present day climate is too hot for Greenland,” says Dr Ernst Schrama, a remote sensing veteran with a track record at Nasa who recently gave a lecture at the faculty of Civil Engineering Geosciences. Schrama showed results of the Grace satellites orbiting the Earth, mapping variations in gravity and thus in mass. The data required advanced filtering techniques and educated interpretation, but ultimately Schrama presented a mass loss of 200 gigatonnes per year for Greenland. And worse: the ice loss accelerates by 10 percent each year. Together with colleagues from the University of Utrecht and the Royal Netherlands Meteorological Institute, Schrama published these results last year November in Science.

His colleague, Dr Bert Vermeersen (AE), however presented a much lower estimate in a recent article in Nature Geoscience (September 2010). Vermeersen explains that he and Schrama agree on the total mass loss of about 230 gigatonnes per year for the region, but they disagree in their estimate of what share of the total mass loss comes from the present day melting land ice.

“In West-Greenland you can see churches standing in the water,” says Vermeersen, which illustrates the postglacial rebound (PGR), also known as glacial isostatic adjustment (GIA). As the land ice over Canada disappeared after the last ice age, the ground there, liberated from the heavy load, has been rising by about a centimetre per year. And like with an air mattress, if you push down at one point, a bump will appear elsewhere. The postglacial isostatic uplifting in Canada therefore coincides with a sinking in Greenland. Vermeersen and colleagues, including Frank Wu from Nasa/JPL, have developed a new model describing the uplift. This new model yields a land ice loss of ‘only’ 130 gigatonnes per year.

Schrama however thinks that ice loss dominates the figure and that the effect of rebound is relatively small for Greenland. Schrama adds that mass loss is seen to move to the northwest of Greenland, which corresponds well with increased local run-off and other remote sensing data, but the pattern is harder to explain with glacial isostatic adjustment.

How to tease these contributions apart? GPS measurements are used to monitor the movement of the bedrock, but only at the perimeter. A perhaps futuristic approach could be to deploy GPS receivers on top of 3-kilometre long poles resting on the rock under the ice. Odd as it may sound, these pole-rigged GPS receivers might actually be installed, since the Greenland mass loss, with all its implications for the climate debate, is a hot topic at conferences.

Link to article on Delta-site