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On the Use of Near-Surface Air Temperature Data to Assess the Magnitude of Global Warming
Reference
Pielke Sr., R.A., Davey, C.A., Niyogi, D., Fall, S., Steinweg-Woods, J., Hubbard, K., Lin, X., Cai, M., Lim, Y.-K., Li, H., Nielsen-Gammon, J., Gallo, K., Hale, R., Mahmood, R., Foster, S., McNider, R.T. and Blanken, P. 2007. Unresolved issues with the assessment of multidecadal global land surface temperature trends. Journal of Geophysical Research 112: 10.1029/2006JD008229.

What was done
The seventeen researchers analyzed "various unresolved issues in using surface temperature trends as a metric for assessing global and regional climate change," noting that these issues, in their estimation, "are either not recognized at all in the assessments or are understated."

What was learned
Some of the problems Pielke et al. identified were "a warm bias in nighttime minimum temperatures, poor siting of the instrumentation to measure temperatures, the influence of trends in surface air water vapor content on temperature trends, the quantification of uncertainties in the homogenization of surface temperature data, and the influence of land use/land cover change on surface temperature trends."

What it means
One of the major conclusions of their study, in the words of the research team, was that "as a climate metric to diagnose climate system heat changes (i.e., 'global warming'), the surface temperature trend, especially if it includes the trend in nighttime temperature [which it typically does], is not the most suitable climate metric." Instead, they find that the maximum temperature is more appropriate for this purpose. This finding is especially significant to the debate over the degree of global warming that has occurred over the past half-century or so, because, as they note, "minimum temperatures have risen about 50% faster than maximum temperatures in the observed surface data set since 1950 (Vose et al., 2005)," which further suggests that there may well be what Pielke et al. describe as a significant "warm bias" in nearly all major reconstructions of the primary metric used to diagnose the degree of "global warming" experienced by the planet over the period of the observational record.

Reference
Vose, R.S., Easterling, D.R. and Gleason, B. 2005. Maximum and minimum temperature trends for the globe: An update through 2004. Geophysical Research Letters 32: 10.1029/2005GL024379.

Reviewed 12 March 2008