Background
Although the Antarctic Peninsula has warmed by several degrees C over the past several decades, Antarctica as a whole has been gradually cooling since the mid-1960s (Doran et al., 2002), apparently as a result of a positive trend in the coupled mode of variability of the Southern Hemisphere Annular Mode (SAM) and the Southern Oscillation (SO), which trend appears to be driven by phenomena of stratospheric origin (Kwok and Comiso, 2002) that Thompson and Soloman (2002) attribute to stratospheric ozone depletion.

What was done
Gillett and Thompson employ a state-of-the-art atmospheric model with high vertical resolution coupled to a mixed-layer ocean model to test this hypothesis.

What was learned
The authors report that "the seasonality, structure, and amplitude of the observed climate trends are simulated in a ? model run ? that is forced solely by prescribed stratospheric ozone depletion," providing evidence that "anthropogenic emissions of ozone-depleting gases have had a distinct impact on climate not only at stratospheric levels but at Earth's surface as well."  They also note that "simulations run with increasing greenhouse gases reveal trends in the SAM that are of the same sign as the observed trends," but they report that "the amplitude of the simulated trends in considerably smaller than that observed."

What it means
For the portion of the planet covered by Antarctica and much of the Southern Ocean, we now have evidence - albeit model-produced - that something other than CO2 and all other anthropogenic greenhouse gases emitted to date is having a predominant effect on earth's near-surface air temperature, which is suggestive of the possibility that many other factors of both natural and anthropogenic origin may also produce superior and oppositely-directed climatic effects that could readily thwart predictions of imminent catastrophic global warming.  We also note that the specific mechanism of climate change highlighted in this study was only conceived within the past couple of years, which is indicative of the likelihood that there may well be many other powerful climatic forces at work in the world of which we are currently totally ignorant.

References
Doran, P.T., Priscu, J.C., Lyons, W.B., Walsh, J.E., Fountain, A.G., McKnight, D.M., Moorhead, D.L., Virginia, R.A., Wall, D.H., Clow, G.D., Fritsen, C.H., McKay, C.P. and Parsons, A.N.  2002.  Antarctic climate cooling and terrestrial ecosystem response.  Nature 415: 517-520.

Kwok, R. and Comiso, J.C.  2002.  Spatial patterns of variability in Antarctic surface temperature: Connections to the South Hemisphere Annular Mode and the Southern Oscillation.  Geophysical Research Letters 29: 10.1029/2002GL015415.

Thompson, D.W.J. and Solomon, S.  2002.  Interpretation of recent Southern Hemisphere climate change.  Science 296: 895-899.