Antarctic cooling, global warming?

by Eric Steig and Gavin Schmidt
Long term temperature data from the Southern Hemisphere are hard to find, and by the time you get to the Antarctic continent, the data are extremely sparse. Nonetheless, some patterns do emerge from the limited data available. The Antarctic Peninsula, site of the now-defunct Larsen-B ice shelf, has warmed substantially. On the other hand, the few stations on the continent and in the interior appear to have cooled slightly (Doran et al, 2002; GISTEMP). At first glance this seems to contradict the idea of “global” warming, but one needs to be careful before jumping to this conclusion.

A rise in the global mean temperature does not imply universal warming. Dynamical effects (changes in the winds and ocean circulation) can have just as large an impact, locally as the radiative forcing from greenhouse gases. The temperature change in any particular region will in fact be a combination of radiation-related changes (through greenhouse gases, aerosols, ozone and the like) and dynamical effects. Since the winds tend to only move heat from one place to another, their impact will tend to cancel out in the global mean.

It is important to recognize that the widely-cited “Antarctic cooling” appears, from the limited data available, to be restricted only to the last two decades, and that averaged over the last 40 years, there has been a slight warming (e.g. Bertler et al. 2004. At present, it is not possible to say what the long term change over the entire last century or more has been. The lesson here is that changes observed over very short time intervals do not provide a reliable picture of how the climate is changing.

Furthermore, there are actually good reasons to expect the overall rate of warming in the Southern Hemisphere to be small. It has been recognized for some time that model simulations result in much greater warming in the high latitudes of the Northern Hemisphere than in the South, due to ocean heat uptake by the Southern Ocean. Additionally, there is some observational evidence that atmospheric dynamical changes may explain the recent cooling over parts of Antarctica. .

Thompson and Solomon (2002) showed that the Southern Annular Mode (a pattern of variability that affects the westerly winds around Antarctica) had been in a more positive phase (stronger winds) in recent years, and that this acts as a barrier, preventing warmer air from reaching the continent. There are also some indications from models that this may have been caused by a combination of stratospheric ozone depletion and stratospheric cooling due to CO₂ (Gillett and Thompson, 2002 ; Shindell and Schmidt, 2004). It is important to note, though, that there is evidence from tree-ring based climate reconstructions that the phase of the Southern Annular Mode has changed similarly in the past (Jones and Widman, 2004). We cannot, therefore, ascribe observed recent temperature changes to any one particular cause.

So what does this all of this imply? First, short term observations should be interpreted with caution: we need more data from the Antarctic, over longer time periods, to say with certainly what the long term trend is. Second, regional change is not the same as global mean change. Third, there are very reasonable explanations for the recent observed cooling, that have been recognized for some time from model simulations. However, the models also suggest that, as we go forward in time, the relative importance of increasing radiative effects, compared with atmosphere and ocean dynamic effects, is likely to increase. In short, we fully expect Antarctica to warm up in the future.