3 July 2008The New York Times
I have a story coming tonight in print on a new paper tracking the impact over time of those iconic drainpipes for meltwater forming each summer on the warming flanks of the vast Greenland ice sheet. Here's the nub, with varied reactions coming from glaciologists later:
One of the most vivid symbols of global warming used by scientists and campaigners to spur society to curb climate-warming emissions is photography of gushing rivers of meltwater plunging from the surface of Greenland's ice sheet into the depths.
Recent studies have shown these natural drainpipes, called moulins, can speed up the slow seaward march of the grinding ice by lubricating the interface with bedrock below. The faster that ice flows, the faster seas rise. Now, though, a new Dutch study of 17 years of satellite measurements of ice movement in western Greenland concludes that the speedup of the ice is a transient summertime phenomenon, with the overall yearly movement of the grinding glaciers not changing, and actually dropping slightly in some places, when measured over longer time spans.
The work, the authors and other experts caution, does not mean that more widespread surface melting could not eventually destabilize vast areas of the world's second-largest ice storehouse. But for the moment, the study, which is being published in Friday's edition of the journal Science, throws into question the notion that abrupt ice losses in Greenland are nigh.
"The positive-feedback mechanism between melt rate and ice velocity appears to be a seasonal process that may have only a limited effect on the response of the ice sheet to climate warming over the next decades," said the paper.
The study was led by Roderik S.W. van de Wal of the Institute for Marine and Atmospheric Research of the University of Utrecht.UPDATE 10:15 p.m.: Other measurements show that Greenland is still clearly losing more ice through melting around the edges than it is gaining through the accumulation of snow in the interior.
Richard B. Alley, an expert on Greenland's ice sheet at Penn State, told me it's still possible that flows of meltwater from surface lakes could start large areas of ice moving seaward, particularly if the melt zones continue to expand inland as they have been doing for years now.
Dr. van de Wal said the picture of what makes Greenland's ice stable or unstable is still evolving. "This time we do not predict a disaster, but who knows what the next finding will [teach] us," he said.
Tad Pfeffer of the University of Colorado, Boulder, said the lack of a straight response between flows of water from melting and motion of ice has been well established for decades:
I (and others) have been perplexed for years why so many in the ice sheet dynamics community have resorted to re-inventing the wheel (as a square) rather than avail themselves of this literature - it's not obscure.
In my view increased meltwater input isn't a write-off and it's not an impending catastrophe either. The corresponding reactions to those ideas (ignore it vs. scale sliding velocity directly with melt) are both going to miss reality by a wide margin. The solution is going to come from the sources it has always come from: careful analysis of observations, which in this case are inescapably englacial and subglacial (i.e. work on the ground).
Moulins aren't irrelevant - it used to be supposed that the beds of ice sheets were entirely isolated from surface water, whereas recent work including the moulin studies have revealed a lot of new detail. Moulins are a bit over-sexed, however. We've known, of course, that they exist for years - draining lakes too - and have been putting cameras down holes in glaciers routinely since the late 1970s.
All of this reinforces the idea that the basics remain clear, including that a warming world will have less ice, and that will lead to higher seas. But the pace, and the prospect of sudden surprises, remain locked in complexity. As I wrote in January: Melting Ice = Rising Seas? Easy. How fast? Hard.
But at the current pace of greenhouse-gas emissions, I wouldn't count on having a steady coastline for a very long time to come. One person who isn't counting on that is Jim Titus, the leading expert on sea level at the Environmental Protection Agency.
As I wrote last year, he recently invested about $20,000 to raise his house and yard on the Jersey Shore about five feet.
