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Less snowmelt in Antarctica

Climatic change in Antarctica is complicated. The northernmost part of the continent, the Antarctic Peninsula, is warming at extreme rates, while elsewhere the pattern is mixed and in some parts there appears to be little or no warming. Up to a point, we glaciologists don’t mind whether Antarctica is warming or not. It is so cold that even an implausible temperature increase wouldn’t come close enough to the melting point to affect the mass balance.

Indeed, there is a plausible argument that warming would make the mass balance more positive. The Antarctic interior is extremely dry because the capacity of the intensely cold atmosphere to deliver water vapour, and therefore snow, is minimal. Warmer air can carry more water vapour, so snowfall should increase in a warmer Antarctica.

The evolving mass balance of Antarctica is most interesting around the edges, though. Warmer ocean water is increasing melting at the bases of ice shelves and pulling grounded ice across the grounding lines at increasingly scary rates. A modest increase in interior snowfall would not make this picture less scary.

Ice-stream dynamics is not the only interesting thing about the periphery of Antarctica. Here, in the least cold latitudes, we observe what little melting does happen. Spread over the continent, it amounts to a few mm of water-equivalent loss per year, against gains by snowfall of about 150 mm/yr. Losses by discharge across the grounding line are much greater. But melting, if negligible in the big picture, is still interesting.

In a recent paper, Tedesco and Monaghan update a standard measure of melt intensity in Antarctica, the so-called melting index. They watch the ice sheet’s emission at microwave wavelengths (8 to 16 mm) and exploit one of the most useful radiative attributes of water. At these wavelengths, the emissivity of frozen water is low, and as conventionally presented in imagery it looks bright, but when it melts its emissivity rises dramatically and it looks black. An intermittently wet snow surface flickers between bright and dark, and we can keep track of melting by noting, in twice-daily overpasses by the imaging satellites, whether the image pixels are bright (cold) or black (warm).

The melting index, summed over a glacierized region for a span of time, is measured in square-kilometre-days, an odd-sounding unit but one that captures what we want to know. For each pixel it is just the number of days on which the pixel was black times the area of the pixel. For the whole region it is the sum of these pixel counts.

The Antarctic melting index has averaged about 35 million km² days per year (October to September, to be sure of keeping the austral summer months together) between 1980 and 2008. Here comes the intriguing feature: in 2009 it was only 17.8 million km² days, which is not only a record low but also continues a trend towards lesser annual indices that began in 2005. The melt extent (the area experiencing at least one day of melting) was the second lowest recorded, reaching only half the average of 1.3 million km².

Tedesco and Monaghan account for this oddity in terms of slow organized variability in how the atmosphere behaves. Two patterns of multi-annual variation in the circulation of the southern atmosphere, the Southern Oscillation and the Southern Annular Mode, together correlate rather well with the melting index. But the authors acknowledge that the correlation breaks down in some Antarctic regions, and that the common variance does not point to a clear-cut physical explanation. (Translation: we don’t understand what is happening.)

Antarctica is a happy hunting ground for climate denialists, but they need to be ignored because they are on a wild goose chase. In the first place, anomalous patterns of temperature change haven’t stopped melting rates from accelerating, and ice shelves from disintegrating, in the warmest part of the continent. Second, global warming is global. Regional non-warming, and even regional cooling, don’t invalidate the main conclusion. The fact that we don’t understand why Antarctica is anomalous doesn’t invalidate it either. Finally, when it comes to Antarctic change it’s the ocean that we need to worry about. From the glaciological standpoint, warmer water is the problem, not warmer air.

Posted by Graham Cogley on February 8, 2010 3:00 PM | Permalink