How Much Will Precipitation Increase With Global Warming?

during the 1970s made possible the observation of the seasonally shifting patterns of global precipitation. It was not until recently, however, that the record could be considered long enough to investigate longer-term trends and the relationship between global precipitation and global warming. Using data from the Special Sensor Microwave Imager (SSM/I) instrument, Wentz et al. [2007] reported that global mean precipitation increased at a rate of 7.4 + 2.6% per oC between 1987 and 2006. Meanwhile, general circulation models (GCMs) used to predict climate change simulate twentiethand 21st-century mean precipitation increases of about 1–3% per oC [Held and Soden, 2006]. This difference seems surprising because some GCMs can adequately reproduce the much longer twentiethcentury surface-based land-mean precipitation record [Lambert et al., 2005]. Global precipitation changes are tied to the surface energy budget through evaporation and to the tropospheric energy budget through condensation. Thus, if GCMs do underestimate global precipitation changes, the simulation of other climate variables will be affected. Should GCM results be reevaluated in light of Wentz et al.’s [2007] findings? We find that 20-year trends are not directly comparable to 100-year trends. Hence, observations are not directly comparable to centurylong GCM simulations [see also Previdi and Liepert, this issue]. Proper consideration is necessary of the physical processes potentially behind changes in the water cycle.