Carbon Dioxide and Earth's Future Pursuing the ... - Magazooms

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with which they had to contend in reaching this conclusion were complex enough to make the
effort nearly intractable and render their end result highly questionable.
In ruminating about this confusing situation, Cazenave (2006) described the many knotty
problems that have beset the GRACE technique and led to the disturbingly large scatter in
Greenland ice loss calculations (50 to 250 Gt/year) among the several studies that have
employed it. Almost contemporaneously, however, a new approach to the analysis of GRACE
data was developed by Luthcke et al. (2006); and it would appear to have greatly improved the
fidelity of their findings, which suggested that there has been a mean ice mass loss of only 101
± 16 Gt/year from Greenland over the period 2003 to 2005. Nevertheless, because of the short
time span involved, and the fact that “over Greenland,” as Cazenave describes it, “ice mass
varies widely from year to year,” little could be concluded from the GRACE data that had been
accumulated to date; and she stated that because the different analyses “do not overlap
exactly in time, different trend estimates are to be expected.”
Two years later, Das et al. (2008) established observation sites at two large supraglacial lakes on
the western margin of the Greenland Ice Sheet atop approximately 1000-meter-thick subfreezing
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, which indicated that 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 m 3 /sec) had
little apparent lasting effect on the local ice-sheet velocity.”
But what might be the effect of multiple lake drainages?
In a second study that addressed 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. The data
thereby obtained revealed summer ice-sheet speedups of 50+% in some places. However, the
researchers noted that “the melt-induced speedup averaged over a mix of several tidewater
outlet glaciers is relatively small (