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Antarctic Sea Ice Trends: Modelled vs. Measured
Reference
Mahlstein, I., Gent, P.R. and Solomon, S. 2013. Historical Antarctic mean sea ice area, sea ice trends, and winds in CMIP5 simulations. Journal of Geophysical Research: Atmospheres 118: 5105-5110.

Background
The authors state that Lefebvre and Goosse (2008) analyzed the Antarctic sea ice distributions of the CMIP3 climate models and found that "the modelled trends were too negative compared to observations." Likewise, they say that Turner et al. (2012) also reported "a negative sea ice trend for most CMIP5 models." And so it was that they decided that they would also investigate the subject, to see if things were really as bad as what they appeared to be in these prior two model assessments.

What was done
Using historical runs from as many as 25 CMIP5 climate models, Mahlstein et al. compared their hind-casted sea-ice trends for the area around Antarctica against observational data for the period 1980 to 2001, which are archived by the Met Office Hadley Centre (Rayner et al., 2003) and the U.S. National Snow and Ice Data Center (Comiso, 1999, updated 2012).

What was learned
Quoting the three researchers, "the representations of Antarctic sea ice in CMIP5 models have not improved compared to CMIP3," in that "the spread in sea ice area is not reduced compared to the previous models." Most important of all, however, was their finding that whereas most CMIP5 climate models "simulate a decrease in Antarctic sea ice over the recent past," real-world data demonstrate that the "average Antarctic sea ice area is not retreating but has slowly increased since satellite measurements began in 1979."

What it means
It is difficult for a climate model to be more wrong than when it hind-casts just the opposite of what has been observed to be happening over the past three and a half decades in the real world, which is what most of the CMIP5 models apparently do.

References
Comiso, J. 1999, updated 2012. Bootstrap Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS, edited by NSIDC, 2156-2202. Digital Media, Boulder, Colorado, USA.

Lefebvre, W. and Goosse, H. 2008. Analysis of the projected regional sea-ice changes in the Southern Ocean during the twenty-first century. Climate Dynamics 30: 59-76.

Rayner, N.A., Parker, D.E., Horton, E.B., Folland, C.K., Alexander, L.V., Rowell, D.P., Kent, E.C. and Kaplan, A. 2003. Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century. Journal of Geophysical Research 108: 10.1029/2002JD002670.

Turner, J., Bracegirdle, T., Phillips, T., Marshall, G.J. and Hosking, J.S. 2012. An initial assessment of Antarctic sea ice extent in the CMIP5 models. Journal of Climate 26: 1473-1484.

Reviewed 8 January 2014