[Surface water gushing into a glacier ‘moulin’: photo by China Crisis reproduced under the terms of the GNU Free Documentation License, Version 1.2 - “... 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.” (Andrew Revkin, ‘A Tempered View of Greenland’s Gushing Drainpipes’, The New York Times, July 4)]

Today, an important report is published in Science [‘Large and Rapid Melt-Induced Velocity Changes in the Ablation Zone of the Greenland Ice Sheet’ by R. S. W. van de Wal, W. Boot, M. R. van den Broeke, C. J. P. P. Smeets, C. H. Reijmer, J. J. A. Donker, J. Oerlemans (Science July 4, 2008: Vol. 321, no. 5885, pp. 111 - 113. DOI: 10.1126/science.1158540)], and with it yet another ‘global warming’ fear drains away.

One of the major worries put forward by ‘global warmers’ is that melting water will lubricate the base of Greenland’s ice sheet, thus accelerating its slide into the oceans. This new Dutch study, led by Roderik S. W. van de Wal of the Institute for Marine and Atmospheric Research, the University of Utrecht, and founded on detailed observations over 17 years, places a fast freeze on such fears, and shows that, far from speeding up, the western edge of Greenland’s ice sheet has actually slowed down by some 10% since 1991.

Mother Nature’s Negative Feedback

The process appears to work as follows. In summer, meltwater gushes down holes in the ice, termed ‘moulins’ [see picture]. Initially, these flows do indeed cause ice velocities to increase rapidly, by a factor of up to 4. Moreover, the process is quick, often being established within days. The pulses of water pour down the ‘moulins’ onto the bedrock, where the sudden mass of water overwhelms the englacial and sub-glacial drainage channels, which have been narrowed during winter, so much so that the increased pressure forces the ice sheet up off the bedrock surface, thus enabling the ice to move faster.

However, this sudden, initial acceleration is short-lived, with the ice velocities falling back to normal within a week or so. After a few days, the narrowed drainage channels are forced open by the weight of water, which then drains away extremely quickly from the glacier base. As a result, the ice falls back onto the bedrock, grinding again to its normal slow pace, any temporary mechanical and lubricant effects being negated.

This is an excellent piece of work. It demonstrates how very careful you have to be in assuming certain feedback mechanisms. ‘Global warmers’, naturally, always opt for cataclysmic, positive feedbacks. Mother Nature, by contrast, so often has more subtle ideas, replacing putative long-term positive feedbacks with classic negative feedbacks that reverse the initial state, sometimes even replacing it over time with an opposite trend, as in this case. The authors conclude:

“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.”

Just so. You should, of course, expect all sorts of souls to melt out of the ice to try to enter urgent caveats. The religious do not like their doom-laden prophecies to be subject to real-world observations. But that is their problem. As we learn all the time, feedbacks do the most surprising things.

Now, I need an urgent negative feedback - an iced coffee to temper all the hot air.

[See also: Andrew Revkin, ‘A Tempered View of Greenland’s Gushing Drainpipes’, The New York Times, July 4; Michael Reilly, ‘Greenland Ice Sheet Slams the Brakes On’, New Scientist, July 3].