The Mass Balance of the Greenland Ice Sheet

Minho · 28 Mar 2007 às 22:48

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Every few weeks a new study of the mass balance of the Greenland Ice Sheet seems to appear in the scientific literature. In the 21 Oct 2005 issue of Science, it was the review of Alley et al. (2005), who claimed "the Greenland Ice Sheet may melt entirely from future global warming," which contention they buttressed with the statement that "recently detected rapid ice-marginal changes contributing to sea-level rise may indicate greater ice-sheet sensitivity to warming than previously considered." Between 1993-94 and 1998-99, for example, they said "the ice sheet was losing 54 ± 14 gigatons per year (Gt/year) of ice, equivalent to a sea-level rise of ~0.15 mm/year," adding that despite excess snowfall in the southeast in 2002 and 2003, "net mass loss over the 1997-to-2003 interval was higher than the loss between 1993 and 1999, averaging 74 ± 11 Gt/year or ~0.21 mm/year sea-level rise."

Just one day before Alley et al.'s paper appeared in print, however, Johannessen et al. (2005), working with satellite-altimeter data from Greenland, reported in a Sciencexpress paper posted online that although below 1500 meters the mean change of the ice sheet height with time was a decline of 2.0 ± 0.9 cm/year over the 11-year period 1992-2003, above 1500 meters there was a positive growth rate of fully 6.4 ± 0.2 cm/year due to snow accumulation; and they determined that averaged over the entire ice sheet the mean result was also positive, at a mean growth rate of 5.4 ± 0.2 cm/year, which when adjusted for an isostatic uplift of about 0.5 cm/year yielded a mean growth rate of approximately 5 cm/year, for a total increase in the mean thickness of the Greenland Ice Sheet of about 55 cm over the 11-year period, a result that was just the opposite of that reported by Alley et al.

Then, like a pendulum changing direction yet again, came the study of Rignot and Kanagaratnam (2005), who using satellite radar interferometry observations of Greenland detected "widespread glacier acceleration." Calculating that this phenomenon had led to a doubling of the ice sheet mass deficit in the last decade and, therefore, to a comparable increase in Greenland's contribution to rising sea levels, they went on to claim that "as more glaciers accelerate ... the contribution of Greenland to sea-level rise will continue to increase." Hard on the heels of their paper, however, came the satellite radar altimetry study of Zwally et al. (2005), which once again sent the pendulum swinging in the opposite direction in response to their finding that "the Greenland ice sheet is thinning at the margins (-42 ± 2 Gt/year below the equilibrium-line altitude) and growing inland (+53 ± 2 Gt/year above the equilibrium-line altitude) with a small overall mass gain (+11 ± 3 Gt/year; -0.03 mm/year sea-level equivalent)."

But the end was not yet, as Chen et al. (2006) subsequently shot the pendulum way back in the other direction with their NASA/DLR Gravity Recovery and Climate Experiment (GRACE) study, wherein they concluded that satellite-measured gravity variations suggested that the Greenland Ice Sheet was currently disappearing at a rate of some 240 cubic kilometers per year. In our review of their paper, however, we noted that the many problems with which they had to contend in reaching this conclusion were so complex as to make the effort nearly intractable and to render their end result highly questionable; and the results of the newest GRACE study of Luthcke et al. (2006), noted below, confirm our concerns.

In ruminating about this same situation, Cazenave (2006) also described the many knotty problems that have beset the GRACE technique and that have led to the disturbingly large scatter in Greenland ice loss calculations (50 to 250 Gt/year) among the several studies that have employed it. Most recently of all, however, a new approach to the analysis of GRACE data was developed by Luthcke et al. (2006); and it would appear to greatly improve the fidelity of the latter's findings, which suggest 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 can be concluded from the GRACE data that have been accumulated to date; and she says that because the different analyses "do not overlap exactly in time, different trend estimates are to be expected [our italics]."

Clearly, the final word on the behavior of the Greenland Ice Sheet, as well as the reason for its behavior, has yet to be written; and we can guarantee you that it will not be written any time soon, for Greenland's thermal history has been incredibly volatile over the past century, with its mean near-surface air temperature rising between 2 and 4°C in less than ten years during the Great Greenland Warming of the 1920s (Chylek et al., 2004), which occurred over a period when the atmosphere's CO2 concentration rose by a grand total of only 3 or 4 ppm. And when Greenland temperatures began to fall in the 1940s, the air's CO2 content significantly accelerated its upward climb. In addition, the most recent warming on Greenland - which climate alarmists describe as being unprecedented and driven by an even more unprecedented increase in the atmosphere's CO2 concentration - has resulted in coastal temperatures that Chylek et al. report are still "about 1°C below [our italics] their 1940 values," which peak temperatures prevailed when the air's CO2 content was far less than it is today.

In light of these several observations, we can only conclude that to truly know what is happening to the Greenland Ice Sheet - and to know why it is happening - will require much more work on the subject than has been conducted to date. Quoting from the concluding remarks of Cazenave, "this goal requires long time series of satellite observations," as well as "continuity of space missions." We agree. And until those "long time series of satellite observations" are in hand, and properly analyzed, all claims about the ongoing rise in the air's CO2 content causing the Greenland Ice Sheet to lose ice mass at a catastrophic rate should be received as what they truly are: little more than unfounded speculation.
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http://www.co2science.org/scripts/CO2ScienceB2C/articles/V10/N2/EDIT.jsp

The Mass Balance of the Greenland Ice Sheet

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