domingo, 21 de abril de 2013

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.
Both our paper and that of Abram et al. add to our understanding of recent climate, glacier, and ice sheet changes in Antarctica by placing them into a longer-term context. Amidst the continuous chatter in the blogosphere about the strengths and limitations about “multiproxy” studies, these studies may be a refreshing return to simpler methods relying on just one type of “proxy”: data from ice cores. While ice core data aren’t perfect proxies of climate, they come pretty close, and aren’t subject to the same kinds of uncertainties that are unavoidable in biological proxies like tree rings.
Our study is the culmination of about a decade of ice core drilling and analysis in West Antarctica, through the ITASE program and the WAIS Divide ice core project. I’m the lead author on the paper but the author list is rightfully long; a lot of people have been involved in drilling and analyzing cores all across Antarctica.
The only “proxy” we use are oxygen isotope ratios. Oxygen isotope ratios (δ18O) in polar snow are well known to be correlated with temperature, and the underlying physics of the relationship is very well understood. In our study, we compile all the available δ18O data from high-resolution well-dated ice cores in West Antarctica and take a look at the average variability through the last 200 years. We also include data from the new WAIS Divide ice core that goes back 2000 years (actually, this core goes back to 68,000 years, and is annually resolved back to at least 30,000 years, but that’s a story for another time).
The average of the records for the last 50 years looks very much like temperature records from the last 50 years, with scaling of about 0.5‰/°C, exactly as expected, providing yet another piece of evidence that recent warming in West Antarctica has been both rapid and widespread (see the figure below). A critical point, though, is that it isn’t necessary to use the δ18O data as a proxy for temperature. Because the physics controlling δ18O is well understood, and we are able to implement δ18O in climate models, we can actually just use δ18O as a proxy for, well, δ18O. This simplifies the problem from “how significant is the recent warming?” to “how significant is the recent rise in δ18O”? We’ve shown previously, and show again in this paper, that δ18O in West Antarctic precipitation reflects the relevant changes in atmospheric circulation just as well (if not better) than temperature or other conventional climate variables do. Putting δ18O into a GCM and using the same experiments that reproduce the observed warming over West Antarctica also produces the observed δ18O increase in the last 50 years.
Figure 1. (a) Comparison of averaged δ18O (blue) across West Antarctica with the recent temperature record of Bromwich et al. (2013) from central West Antarctica (yellow). The light blue background is the decadal smoothed values +/- 1 standard error assuming Gaussian statistics. (b) Number of records used, and probability that the decadal average is as elevated as the 1990s (green).
Data sources: Most of the data for this figure have been available at for some time. There’s a new location (which will link to the old one) where more recent data sets will be placed, but it’s not all up yet:
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.
Why the difference between the Peninsula and the WAIS? After all, both locations are warming at about the same rate. We could speculate that if there were melt layers in the WAIS cores, they would also show a significant increase like the James Ross Island core does. (It’s too cold at all the WAIS sites to have summer melting at all, so such information isn’t available.) I don’t think that is likely though. More important is the specific location of James Ross Island, on the eastern side of the Antarctic Peninsula. On the western Antarctic Peninsula, temperature trends are greatest in winter and spring, just as they are over the WAIS, and we’ve argued elsewhere that the causes are similar: changes in regional circulation forced by anomalous conditions in the tropics (Ding and Steig, in press). But it is on the eastern Peninsula that the most rapid summer warming has occurred, and where the surface-melting has caused ice shelf collapse (indeed, James Ross Island wasn’t really an island until 1995, when the Prince Gustav ice shelf collapsed). Both statistical assessments and modeling results show that the trend in the SAM accounts for this warming trend. As I noted in the introduction to this post, the SAM trend is partly explained by ozone depletion in the stratosphere, and the most clearly anomalous melt in the James Ross Island core occurs after the late 1970s, about the time the ozone hole appeared. But the melt data also show that melting has increased nearly monotonically since the 1930s, well before the advent of the ozone hole. As in West Antarctic δ18O, there was a bit of an increase in melt in the 1830s and the 1940s at James Ross Island, perhaps also ENSO-related, but these little bumps pale in comparison with the amount of melting occurring since the 1950s.
So what does all this mean for the fate of Antarctic Peninsula glaciers and the West Antarctic ice sheet? Both our paper and the Abram et al. paper add substantial new evidence that something rather unusual is occurring in Antarctica. It is not just happenstance that rapid ice sheet, glacier, and ice shelf changes are occurring now, when we have finally begun to observe them closely. Rather, these changes are occurring along with what is happening to the rest of the planet. That said, it appears that we not have yet driven West Antarctic climate (nor West Antarctic glaciers) definitively beyond what might be expected from natural variability alone. In particular, I won’t be surprised if continued decade-to-decade variability in atmospheric circulation results in more, and less, intrusion of circumpolar deep water onto the continental shelf, and to more, and less, rapid thinning of ice shelves in West Antarctica*. On the Peninsula, though, it seems very clear that we have already pushed the system well beyond “normal”, and into conditions reminiscent of the mid-Holocene. I don’t think we’re going to see a return to “normal” conditions any time soon. It’s worth noting that most model projections suggest that the SAM trend may level off for a while as the ozone hole gradually declines, but those same model projections suggest the SAM trend will recover as CO2 continues to rise. See. e.g. Thompson et al. (2011).
The real take home message here is that the ice loss from the WAIS and from the Antarctic Peninsula that have been observed in the last few decades are indeed likely to be harbingers of things to come. The very rapid rate of change in West Antarctica that we’ve seen over the last few decades is clearly overprinted by substantial decadal variability, so caution is in order in projecting that rate forward in time. The magnitude of the century scale trend will depend quite a bit, in my view, on what happens in the tropics over the next century. The sign of the trend, however, is clear. On the Peninsula, it’s crystal clear.
Note: An excellent summary of these two papers by Tas van Ommen will appear in Nature Geoscience in the May issue.

Fonte/Source: RealClimate

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