Likelihood of rapidly increasing surface temperatures unaccompanied by strong warming in the free troposphere

Recent model simulations of the effects of increasing greenhouse gases combined with other anthropogenic effects predicted larger rates of warming in the mid and upper troposphere than near the Earth's surface. In multiple model comparisons we find that accelerated upper-level warming is simulated in all models for the greenhouse-gas/direct-aerosol forcing representative of 1979-2000. However, in a test of model predictive skill, a comparison with observations shows no warming of the free troposphere over this period. We assessed the likelihood that such a disparity between model projection and observations could be generated by forcing uncertainties or chance model fluctuations, by comparing all possible 22 yr temperature trends in a series of climate simulations. We find that it is extremely unlikely for near-surface air temperatures (surface temperatures) to increase at the magnitude observed since 1979 without a larger warming in the mid-troposphere. Warming of the surface relative to the mid-troposphere was also more likely in control simulations than under anthropogenic forcing. Because errors in the vertical temperature structure would be expected to create errors in water-vapor feedback, cloud cover and moisture content, these results suggest the need for great caution when applying the simulations to future climate predictions and to impact assessments.