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Winter Warming of the Antarctic Troposphere
Reference
Turner, J., Lachlan-Cope, T.A., Colwell, S., Marshall, G.J. and Connolley, W.M. 2006. Significant warming of the Antarctic winter troposphere. Science 311: 1914-1917.

What was done
Using radiosonde records that had recently been digitized and "intensively quality controlled," including newly available Russian observations, the authors analyzed tropospheric temperature data from eight coastal Antarctic sites plus the South Pole for the period 1971 to 2003, after which they compared their results with those obtained from similar analyses of (1) 500-hPa ERA-40 reanalysis data over the period 1979 to 2003, (2) 850-300-hPa satellite data over the same time period, and (3) concomitant tropospheric temperature trends derived from several climate model simulations.

What was learned
Using the "new and improved" expanded set of radiosonde records, the British Antarctic Survey team determined that warming had taken place throughout the troposphere, with the maximum increase in temperature occurring in the mid-troposphere (400 to 600 hPa), where they identified a 0.70C per decade warming approximately five kilometers above the surface. The 500-hPa ERA-40 reanalysis data told much the same story, if not a stronger one; but the 850-300-hPa satellite data showed areas of both warming and cooling. Last of all, the climate model simulations showed large variability, with none of them even coming close to matching the observational radiosonde record.

Adding another twist to the story, Turner et al. report that near the surface the radiosonde-derived mean winter temperature trend for the nine stations from 1971 to 2003 was only 0.15C per decade, and that the standard deviation of the data was so large that the result included the possibility of no change or even a slight cooling, which latter possibility is in fact what surface air temperature observations reveal to be the case over the vast bulk of the continent (see, for example, Comiso (2000), Doran et al. (2002) and Turner et al. (2005) in our Subject Index).

What it means
The results of this study raise some intriguing questions. First, why did the surface of Antarctica cool over the past several decades while the troposphere above it warmed? Second, what are the implications of these findings?

No one seems to have a good answer to the first question, as indicated by press reports whose authors consulted "experts" in the field. In a BAS (British Antarctic Survey) press release of 30 March 2006, however, Turner says of the second question that "the warming above the Antarctic could have implications for snowfall across the Antarctic and sea level rise," and well it could, possibly increasing snowfall and leading to greater ice accumulation over the continent, which phenomena would tend to either reduce Antarctica's contribution to sea level rise or lead to an actual lowering of sea level, depending on the sign of the continent's current mass balance of ice, which is by no means well constrained (see, for example, our reviews of the studies of Velicogna and Wahr (2006) and Zwalley et al. (2005), plus our Editorial of 29 Mar 2005).

With respect to the inability of the several climate models they employed to successfully simulate the observed tropospheric warming over Antarctica, Turner states in the same BAS press release that this failure points "to weaknesses in [the models'] ability to represent the Antarctic climate system," and a news item in the Insurance Journal of 31 March 2006 suggests that this fact "raises the possibility that other models being used in climatic research may also be inadequate, and might lead to erroneous conclusions," both of which points are well taken and extremely important. All things considered, therefore, it is no surprise that Turner et al. concluded their paper by stating "we are unable to attribute these changes [in tropospheric temperature] to increasing greenhouse gas levels at this time."

References
Comiso, J.C. 2000. Variability and trends in Antarctic surface temperatures from in situ and satellite infrared measurements. Journal of Climate 13: 1674-1696.

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 advance online publication, 13 January 2002 (DOI 10.1038/nature710).

Turner, J., Colwell, S.R., Marshall, G.J., Lachlan-Cope, T.A., Carleton, A.M., Jones, P.D., Lagun, V., Reid, P.A. and Iagovkina, S. 2005. Antarctic climate change during the last 50 years. International Journal of Climatology 25: 279-294.

Velicogna, I. and Wahr, J. 2006. Measurements of time-variable gravity show mass loss in Antarctica. Sciencexpress: 10.1126science.1123785.

Zwally, H.J., Giovinetto, M.B., Li, J., Cornejo, H.G., Beckley, M.A., Brenner, A.C., Saba, J.L. and Yi, D. 2005. Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992-2002. Journal of Glaciology 51: 509-527.

Reviewed 5 April 2006