Ice hockey

Eric Steig

It is well known that ice shelves on the Antarctic Peninsula have collapsed on several occasions in the last couple of decades, that ice shelves in West Antarctica are thinning rapidly, and that the large outlet glaciers that drain the West Antarctic ice sheet (WAIS) are accelerating. The rapid drainage of the WAIS into the ocean is a major contributor to sea level rise (around 10% of the total, at the moment).

All of these observations match the response, predicted in the late 1970s by glaciologist John Mercer, of the Antarctic to anthropogenic global warming. As such, they are frequently taken as harbingers of greater future sea level rise to come. Are they?

Two papers published this week in Nature Geoscience provide new information that helps to address this question. One of the studies (led by me) says “probably”, while another (Abram et al.) gives a more definitive “yes”.

The somewhat different details of the two papers appear to have hopelessly confused many journalists (though the Christian Science Monitor has an excellent article, despite a somewhat misleading headline), but both are really just telling different aspects of the same story.

There is already strong evidence that anthropogenic forcing has played a significant role in the collapse of ice shelves on the Antarctic Peninsula, cause by significant melting at the surface during summer. The warm summer air temperatures have been related to an increase in the “Southern Annular Mode” (SAM), essentially the strength of the circumpolar westerlies. Increased CO2 is clearly part of the forcing of the observed positive trend in the SAM, though a larger player is likely to be ozone depletion in the stratosphere. Nevertheless, the short length of the observations – of both the ice sheet and climate – make it difficult to assess to what extent these changes are unusual. There is evidence for one ice shelf that a collapse like that observed in the 1990s has not occurred since at least the mid-Holocene, but comparable evidence is lacking elsewhere.

The connection between climate change and glacier response is more complex for the West Antarctic Ice Sheet than the Peninsula. As on the Peninsula, temperatures over the WAIS have risen significantly in the last few decades, but this is a symptom, rather than a cause. For WAIS, the culprit for the rapid thinning of ice shelves is increased delivery of warm ocean water to the base of the ice shelves. This isn’t due to a warming ocean (though the deep water off the Antarctic coast line is indeed warming), but to changes in the winds that have forced more circumpolar deep water onto the continental shelf. Circumpolar deep water, at about +2°C, is very hot compared with the in situ melting point of glacier ice. In a series of papers, we’ve shown that the warmer temperatures observed over the WAIS are the result of those same atmospheric circulation changes, which are not related to the SAM, but rather to the remote forcing from changes in the tropical Pacific: changes in the character of ENSO (Steig et al., 2012; Ding et al., 2011; 2012).

As on the Peninsula, there is evidence of anthropogenic forcing for the WAIS too: anomalous conditions since the 1980s in the tropical Pacific are characteristic of the expected fingerprint of global warming (e.g. Trenberth and Hoar, 1997; Collins et al., 2010). Still, as on the Peninsula, the short length of the instrumental observations make it difficult to say anything very definitive about long term trends.

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