Volume 11, Number 19: 7 May 2008
In his book An Inconvenient Truth, Al Gore states that if Greenland melted or broke up and slipped into the sea, "sea levels worldwide would increase by between 18 and 20 feet," and he presents a host of speculative maps of how many of the world's coastlines would have to be redrawn if such were to occur (p. 196-209). Likewise, in his testimony before the U.S. House of Representatives' Select Committee of Energy Independence and Global Warming on 26 April 2007, NASA's James Hansen stated "there is increasing realization that sea level rise this century may be measured in meters if we follow business-as-usual fossil fuel emissions."
What is the source of these dire contentions?
A phenomenon that both Al Gore and James Hansen promote involves the thousands of meltwater pools or supraglacial lakes that form on the surface of the Greenland Ice Sheet each summer, and the conduits or moulins that can form in fractures beneath them and rapidly drain their water to the bedrock at the bottom of the ice sheet, enhance the ice sheet's basal lubrication, and thereby cause portions of it to flow faster towards the sea. Two papers published in Sciencexpress on 17 April 2008 bring a much needed dose of reality to the discussion of the role that is likely to be played by this phenomenon in Greenland's potential contribution to future sea level rise; and their findings are radically different from what Gore and Hansen contend.
In the first study, Das et al. (2008) established observation sites at two large supraglacial lakes on the Greenland Ice Sheet's western margin atop approximately 1000-meter-thick sub-freezing ice. One of the lakes rapidly drained on 29 July 2006 in a dramatic event that was monitored by local GPS, seismic and water-level sensors. These data indicated the entire lake drained in approximately 1.4 hours, with a mean drainage rate exceeding the average rate of water flow over Niagara Falls. One consequence of this event was a westward surface displacement of 0.5 meter in excess of the average daily displacement of 0.25 meter. However, pre- and post-drainage lateral speeds did not differ appreciably, leading the researchers to conclude that "the opening of a new moulin draining a large daily melt volume (24 m3/sec) had little apparent lasting effect on the local ice-sheet velocity."
But what might be the effect of multiple lake drainages?
In the second study, which addresses this question, Joughin et al. (2008) assembled a comprehensive set of interferometric synthetic aperture radar (InSAR) and GPS observations over the period September 2004 to August 2007. These data allowed the construction of 71 InSAR velocity maps along two partially overlapping RADARSAT tracks that included Jakobshavn Isbrae (western Greenland's largest outlet glacier), several smaller marine-terminating outlet glaciers, and a several-hundred-kilometer-long stretch of the surrounding ice sheet. These data indicated summer ice-sheet speedups of 50+% in some places. However, the researchers note that "the melt-induced speedup averaged over a mix of several tidewater outlet glaciers is relatively small (< 10 to 15%)." And when factoring in the short melt-season duration, they say that "the total additional annual displacement attributable to surface melt amounts to a few percent on glaciers moving at several hundred meters per year." In addition, they report that "the limited seasonal observations elsewhere in Greenland suggest a low sensitivity to summer melt similar to that which we observe."
In concluding, Joughin et al. write that "surface-melt-enhanced basal lubrication has been invoked previously as a feedback that would hasten the Greenland Ice Sheet's demise in a warming climate," specifically citing, in this regard, Al Gore's An Inconvenient Truth. However, their real-world observations of this phenomenon show that "several fast-flowing outlet glaciers, including Jakobshavn Isbrae, are relatively insensitive to this process."
To the south of Jakobshavn Isbrae, however, Joughin et al. note that the ice sheet's western flank is relatively free of outlet glaciers and that ice loss there is primarily due to melt; and they say that "numerical models appropriate to this type of sheet flow and that include a parameterization of surface-melt-induced speedup predict 10-to-25% more ice loss in the 21st Century than models without this feedback." This estimate, of course, is based on a model parameterization of surface-melt-induced speedup that may or may not be an adequate representation of reality. Nevertheless, we can probably safely conclude, as Joughin et al. do, that the phenomenon of surface-melt-enhanced basal lubrication likely will not have a "catastrophic" effect on the Greenland Ice Sheet's future evolution, the strident claims of Al Gore and James Hansen not withstanding.
Sherwood, Keith and Craig Idso
References
Das, S.B., Joughin, I., Behn, M.D., Howat, I.M., King, M.A., Lizarralde, D. and Bhatia, M.P. 2008. Sciencexpress 10.1126/science.1153360.
Joughin, I., Das, S.B., King, M.A., Smith, B.E., Howat, I.M. and Moonb, T. 2008. Seasonal speedup along the western flank of the Greenland Ice Sheet. Sciencexpress 10.1126/science.1153288.