CR 25:185-190 (2004)  -  doi:10.3354/cr025185

ABSTRACT: 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 Earth¹s 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.

KEY WORDS: Climate change · Model assessment · Vertical temperature structure