Ice hockey

Our results show that the strong trend in δ18O in West Antarctica in the last 50 years is largely driven by anomalously high δ18O in the most recent two decades, particularly in the 1990s (less so the 2000s). This is evident in the temperature data as well (top panel of the figure). The 1990s were also very anomalous in the tropics — there were several large long-lived El Niño events with a strong central tropical Pacific expression, as well as only very weak La Niña events. As in the tropics, so in West Antarctica: the 1990s were likely the most anomalous decade of the last 200 years.

Our results thus show that, indeed, recent decades in West Antarctica, which have been characterized by very rapid warming, and very rapid loss of ice from the West Antarctic Ice Sheet, are highly unusual. Nevertheless, some caution is in order in interpreting this to mean that current rates of rapid ice loss from West Antarctica represent a long term trend. What we’ve observed is unusual, but it is also dominated by decadal climate variability, and can’t be considered “unprecendeted”. Furthermore, our statistical confidence that recent decades are truly exceptional is low. Our data suggest that there is about a 30% chance the 1940s were just as anomalous as the 1990s, and the 1830s have about a 10% chance of being like the 1990s. Based on the relatively small amount of available evidence from the tropics, both the 1940s and the 1830s were similarly characterized by long-lived El Niños. Looking at the very long term record from the WAIS Divide ice core, it appears that similar conditions could have occurred about once per century over the last 2000 years. Hence our answer to the question, “are the observations of the last few decades a harbinger of continued ice sheet collapse in West Antarctica?”, is tentative: “Probably”.

Anyone expecting a more dramatic result need only turn to the other new ice core paper in Nature Geoscience. Last year, Rob Mulvaney and others from the British Antarctic Survey (BAS), along with French, American, and German colleagues, reached a very similar conclusion to ours, from an ice core from James Ross Island, on the northern Antarctic Peninsula. We discussed that paper at Realclimate last year. With δ18O data alone, it was possible to demonstrate only that recent warming on James Ross Island was “unusual”. The new paper, led by Nerelie Abram, adds a record of melt layers in the ice core to the assessment. The findings: a veritable Antarctic ice hockey stick.

Figure 2. δ18O (scaled to temperature) and melt layer frequency from the James Ross Island ice core.

Abram et al.’s paper is elegant in its simplicity. The key thing that matters to the ice shelves on the Antarctic Peninsula is how much melting occurs in summer, and this is almost exactly what Abram et al. are looking at. I say “almost” because formation of melt layers requires both that melting occurs and that it gets preserved, which depends a bit on the snow structure, the previous winter temperature, etc. But the results are unequivocal: there’s about 5 times the fraction of melt layers in the core as there has been on average over previous decades, and at least twice the maximum of any time before about the 1950s. The amount of melting occurring now is greater than at any time in the past 1000 years. If there has ever been a question about whether the “hockey stick” shape of Northern Hemisphere temperatures extends to at least some areas of the Southern Hemisphere, this record provides a decisive and positive answer.

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