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Out-of-the-way glaciers

In glacier monitoring, one of the things we worry about is undersampling. The measurements are sparse, and we have to interpolate, that is, make plausible guesses about the glaciers we can’t measure. Gaps in coverage mean that there is always a chance that new measurements in remote areas will change the picture. One of these areas is the Subantarctic islands, scattered across the Southern Ocean and holding about 8,000 km² of glacier ice in all. Our knowledge of this ice has been fragmentary until recently. Could the Subantarctic be an exception to the global rule of glacier shrinkage?

The knowledge base is beginning to improve, and we can now say that the answer is “No”. For example, with Étienne Berthier, of the Laboratoire d’Études en Géophysique et Océanographie Spatiales in Toulouse, I am writing a chapter on the Subantarctic for a book about GLIMS, the Global Land Ice Measurements from Space initiative. Étienne kindly sent me an April 2009 ASTER satellite image of the west coast of Kerguelen, in the southern Indian Ocean.

The small protruding glacier tongue in the lower part of the picture belongs to Glacier Pierre Curie, which now ends a kilometre from the sea but in 1963 had a calving front about 600 m wide. The two stubby tongues in the upper part are Glacier Pasteur, whose calving front was 1700 m wide in 1963 but is now only barely in contact with tidewater. In another few years, it will have retreated away from the shoreline of Anse des Glaçons (the cove of ice floes, a place name which will provoke nostalgia one day).

The retreat of these two adjacent outlets of the Cook Ice Cap doesn’t count, nowadays, as startling news. Berthier and his co-authors recently reported that Cook Ice Cap shrank at 2.4 km²/yr, half a percent per year, between 1963 and 2001. At this link you can watch an animation of the shrinkage of Glacier Ampère, on the opposite side of the ice cap from Pasteur and Pierre Curie. But if you picked any two neighbouring glaciers almost anywhere in the world, the odds are that they would have shrunk at something like that rate, or perhaps a bit less. So now we know that Kerguelen was not one of the out-of-the-way places where a surprise was awaiting us. The Kerguelen glaciers even follow the widely-observed tendency of accelerating shrinkage (that is, faster recently than earlier).

Another out-of-the-way place about which we now know a lot more is Heard Island, in the Indian Ocean southeast of Kerguelen at 53° South. It has tidewater glaciers mainly because it rises to 2,755 m above sea level. My other co-author, Shavawn Donoghue of the University of Tasmania in Hobart, finds that Heard Island’s 30 glaciers are dwindling just as are those of Kerguelen. Six have parted company with the sea during the decades since the first air photos in 1947, leaving a dozen still delivering icebergs to the ocean. Gotley Glacier, which drains the summit crater of Big Ben, is still standing in the sea as it has done for as far back as we have information.

Calving glaciers are more challenging than ordinary ones when it comes to documenting change. Those that manage to advance far down a fiord can misbehave spectacularly. More often, however, as with Gotley and its 11 neighbours, the icebergs break off as soon as the ice reaches sea level, so the calving front doesn’t change much.

Change in a previously unknown region that turns out to be globally typical – “Subantarctic Glaciers Not Surprising” – is difficult to sell as a motive for political action. What we are after here, apart from a conversation piece for your next cocktail party or trivia game, is something that will inject the necessary urgency into the deliberations of the politicians and policymakers. They will assemble in Copenhagen this December for the most important negotiating session in the history of the human race, the fifteenth Conference of the Parties to the UN Framework Convention on Climate Change. Faced with the evidence, they seem to have got the scientific message, but it hasn’t really clicked yet. Graphs that fall off the bottom of the page haven’t done the trick. Neither Gotley’s continued stillstand nor Pasteur’s impending loss of tidewater status are likely to make the communications breakthrough, but you never know.

Posted by Graham Cogley on October 19, 2009 3:00 PM | Permalink