West Antarctica: still warming

Another point of complete agreement between our results and O’Donnell et al. is that the most widespread cooling occurs in fall — not summer as discussed in earlier work (e.g. Thompson and Solomon, 2000). This may be something of a problem for the hypothesis that ozone depletion is a major driver of the observed East Antarctic cooling, because the forcing is occurring in spring (when the ozone hole develops). If there is a link between the spring forcing and fall temperature, it is not a simple one, but likely would include a role for sea ice, which offers an obvious source of persistence from season to season (a paper in review by Arnour and others argues exactly this point).

Finally, O’Donnell et al. agree with us on the most basic result of all: there is statistically significant warming in West Antarctica. In this context, it is worth being very clear on what is meant by “West Antarctica”. Reading what has been said about O’Donnell et al. in various places in the blogosphere, one would get the impression that their paper returns the warming of Antarctica to its ‘rightful’ place, the Antarctic Peninsula alone. If that were true, it would certainly be a significant refutation of our work. But in the actual abstract of O’Donnell et al., it is stated that “we find that statistically significant warming extends at least as far as Marie Byrd Land.” Marie Byrd Land is that part of West Antarctica that extends eastward from the Ross Ice Shelf up past Byrd Station and over the central West Antarctic Ice Divide (see the map above). In O’Donnell’s results, there is significant warming all the way from the Peninsula westward past WAIS Divide site, at 112°W, well within Marie Byrd Land and nowhere near the Antarctic Peninsula. Prior to our work, no one had claimed that any area outside the Peninsula was warming significantly. Borehole thermometry at WAIS Divide (Orsi and Severinghaus, 2010) and at the Rutford Ice Stream (closer to the Peninsula; Barrett et al,. 2009) has since provided completely independent validation of these results. O’Donnell et al. is thus merely the latest of several studies to confirm our original finding*: West Antarctica is warming significantly.

To be sure, there is real disagreement between our results and those of O’Donnell et al. For the full fifty year reconstruction of temperature trends, the main reconstruction they discuss in the paper shows cooling in the winter and fall over the Ross Ice Shelf, which contrasts with our finding of significant warming there. As a consequence, their overall warming trends are smaller, by about half. These are the only important differences between our results and those of O’Donnell. Nevertheless, they are significant differences, and certainly may be important for our understanding of Antarctic climate change. In particular both results would tend to suggest a greater role for natural variability than our findings implied. If O’Donnell et al.’s results are correct, this would suggest that the damped response of Antarctica to global radiative forcing (i.e. CO2 increases) that is commonly seen in models (as discussed previously by Spencer Weart, for example) is perhaps more on the mark than our paper would suggest (though note that even the much larger trends we estimated are still significantly damped compared with the Arctic.)

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