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Saturday, March 22, 2014

Sea Levels To Rise More Than Expected Due To Warming-Driven Surge In Greenland Ice Loss

by Joe Romm and Jeff Spross, Climate Progress, March 17, 2014

ice sheet
Greenland’s contribution to global sea level has soared in the past two decades. An important new study finds that the massive northeastern part of the ice sheet, previously thought to be stable, has begun shedding ice. If this trend continues — and researchers say “a self-perpetuating feedback process may have been triggered” — actual sea level rise this century will likely be higher than many current models had projected.
Covering 660,000 square miles — roughly 80% of the country — Greenland’s ice sheet is second only in size to Antarctica’s. Scientists estimate that melting from the ice sheet as a whole has accounted for about 16% of sea level rise every year for the last two decades.
Research had also long suggested the northeastern portion of the ice sheet was stable. As a result, it was largely left out of the models used to anticipate future sea level rise.
But the new study, “Sustained mass loss of the northeast Greenland ice sheet triggered by regional warming,” published in Nature Climate Change (subs. req’d), suggests the northeastern portion began melting rapidly around 2003. And after first jumping from an ice loss rate of zero to about 10 billion metric tons per year, it’s now approaching 15 or 20 billion metric tons per year and may well keep accelerating.
“Most projections of the future behaviour of the ice sheet have no, or little, contribution from this part of Greenland,” said Professor Jeremy Bamber of Bristol University, a co-author of the study. “But these new results suggest that this region is sensitive to changes in climate and has the potential to contribute significantly now and in the future.”
The team arrived at their conclusion using a combination of surface elevation data from airplanes and four different satellites, along with a GPS-linked network of 50 stations located along the coast of Greenland’s ice sheet. The overall collection of data spanned 1978 to 2012 and was used to essentially weigh the ice sheet’s mass.
Specifically, the study suggests a series of particularly warm summers leading up to 2003 — bringing higher temperatures in both the atmosphere and the surrounding ocean — triggered the speed up in melting.
Much of the ice melt was also historically held in check by an ice barrier at the coast, which itself is now melting as well, allowing the entire system to speed up. Coauthor Prof. Kurt Kjær explains that there was “a clear acceleration in loss of mass after 2006.” He warns:
“This acceleration has since continued and even increased. The increased loss of mass can be explained by a combination of warmer air in the summer and a rise in sea temperatures. This has reduced the extent of sea ice, which otherwise helps to stabilize the glaciers. Our results also demonstrate that after 2009 both air and, in particular, sea temperatures have returned to the level from before 2006. At the same time, the sea ice has maintained its previous extent, although the loss in mass has increased even further. This indicates that a self-perpetuating feedback process may have been triggered….”
The loss of Arctic sea ice creates a well-known amplifying feedback whereby white ice is replaced by the dark blue sea, which absorbs far more sunlight and hence far more solar energy. Back in 2011, scientists warned that the Greenland ice sheet “could undergo a self-amplifying cycle of melting and warming“ that is “difficult to halt.” A 2012 study found we may be close to a “tipping point” in which “the summer melt area covers the entire land mass.”
Another cause for concern is Greenland’s ice streams. These are essentially “rivers” of ice that drain ice sheets in much the same way networks of streams and rivers drain water from huge areas on land. And the drainage basin for Greenland’s northeast portion is enormous — covering 16% of the ice sheet and reaching 373 miles inland, all the way to the center of the country.
Regional warming has triggered rapid ice loss in a vast portion of the northeastern Greenland Ice Sheet (NEGIS). Red indicates the fastest melting. (Via Ohio State)
“This implies that changes at the margin can affect the mass balance deep in the center of the ice sheet,” said Shfaqat Abbas Khan, a senior researcher at the National Space Institute at the Technical University of Denmark, and the leader of the research team. “Furthermore, due to the huge size of the northeast Greenland ice stream, it has the potential of significantly changing the total mass balance of the ice sheet in the near future.”
In short, the research adds to the case that actual sea level rise in the future will be on the high end of the projections from the Intergovernmental Panel on Climate Change — which ranged from one to three feet by the end of the century. Other climate scientists have projected even higher numbers.

Most climate scientists and glaciologists that Climate Progress has spoken to in recent years have said humanity should plan for at least three feet of sea level rise this century — and considerably more than that if we stay on our current high-end emissions path.

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