Ice hockey

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 http://nsidc.org/data/NSIDC-0425.html 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: http://nsidc.org/data/nsidc-0536.html.

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