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.
11 Responses to "Antarctic cooling, global warming?"
It is worth pointing out that the myth of bipolar amplification of global warming is remarkably strong and appears in even otherwise sensible places (e.g. 1, at the end) as well as the standard skeptic press.
The IPCC TAR actually says:
and the zonal mean pic (fig 9.8) on that page shows it clearly.
The myth probably arises from the very early days of equilibrium change runs, where the ocean heat sink effect did not apply.
The fact that the southern oceans are absorbing heat may set up another interesting lag effect. Warming oceans will increase rates of evaporation which will pump more heat-trapping large molecules (i.e. gaseous H2O) into the atmosphere. So, while the oceans are a heat sink in the short-term, warmer oceans are a source of climate gases in the long-term. More positive feedback …
See for instance, http://www.gcrio.org/ipcc/qa/09.html
David Risen says
I agree that local events should not be used as evidence for or against global warming. Do you apply the same degree of criticism to authors who use local warming to argue for global warming? Discover Magazine’s #1 story in science for 2004 is that ‘Evidence of global warming became so overwhelming in 2004 that now the question is: What can we do about it?’
Read the article. You will see that the ‘overwhelming evidence’ listed is based on LOCAL conditions. I look forward to your editorial comments debunking this dishonest use of science.
Wouldn’t the real issue about Antarctic cooling be: if the amount of water locked up in the ice cap increasing or decreasing, and at what rate? There is a lot of water down there now, but given the fact that parts of the continent are getting cooler and parts are getting warmers, plus the effects on air currents, etc. this seems like an interesting question to answer. Any studies on the depth of the ice cap?