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Much Ado About Practically Nothing: Mass Balance of the Greenland Ice Sheet
Reference
Krabill, W., Abdalati, W., Frederick, E., Manizade, S., Martin, C., Sonntag, J., Swift, R., Thomas, R., Wright, W. and Yungel, J. 2000. Greenland ice sheet: High-elevation balance and peripheral thinning. Science 289: 428-430.

What was done
The authors used data obtained from aircraft laser-altimeter surveys over northern Greenland in 1994 and 1999, together with previously reported data from southern Greenland, to evaluate the mass balance of the Greenland Ice Sheet.

What was learned
Above an elevation of 2000 meters, there were areas of both thinning and thickening of the ice sheet. These two phenomena nearly balanced each other, so that in the south there was a net thinning of 11 7 mm/year and in the north there was a net thickening of 14 7 mm/year. Altogether, the entire region exhibited a net thickening of 5 5 mm/year. But in compensating for bedrock uplift, which averaged 4 mm/year in the south and 5 mm/year in the north, the average thickening rate decreased to practically nothing. In fact, the word used by the authors to describe the balance was "zero."

At lower elevations, thinning was said to predominate along approximately 70% of the coast. Here, however, flight lines were few and far between, so few and far between, in fact, that, in the words of the authors, "in order to extend our estimates to the edge of the ice sheet in areas not bounded by our surveys, we calculated a hypothetical [our italics] thinning rate on the basis of the coastal positive degree day anomalies." They then interpolated between this calculated coastal thinning rate and the nearest observed elevation changes to obtain their final answer: a total net reduction in ice volume "associated with the interpolated values" of 51 km3/year.

What it means
It is hard to know what estimates derived from interpolations based on calculations of a hypothetical thinning rate mean. Hence, we question their reliability; and, in fact, the commentary of the authors themselves tends to do the same. They note, for example, that they do not have a "satisfactory explanation" for the "widespread thinning at elevations below 2000 m," which suggests to us that the reason this phenomenon is unexplainable is that it is not real. Furthermore, they note that even if the thinning was real, it could not be due to global, or even regional, warming; for they state that Greenland temperature records indicate that "the 1980s and early 1990s were about half a degree cooler than the 96-year mean."

After discussing some other factors that could possibly be involved, the authors state that they are left with changes in ice dynamics as the most likely cause of the hypothetical ice sheet thinning. But they admit in their final sentence that "we have no evidence for such changes, and we cannot explain why they should apply to many glaciers in different parts of Greenland."

It would seem, therefore, that the logical thing to do is admit that this study resolves almost nothing about the mass balance of the coastal regions of the Greenland Ice Sheet, and that it resolves absolutely nothing about the subject of global warming and its effect or non-effect upon this hypothetical phenomenon. What we have here is a wonderful example of the French pastry syndrome: something that looks provocatively enticing - over which the media truly salivate (see our Editorial Media Mania Over Purported Greenland Meltdown: Fueled by Fear of Frying) - but which, once bitten, is revealed to consist of little more than a mass of hot air.


Reviewed 26 July 2000