Comparison of Modeled and Observed Trends in Indices of Daily Climate Extremes

Gridded trends of annual values of various climate extreme indices were estimated for 1950 to 1995, presenting a clearer picture of the patterns of trends in climate extremes than has been seen with raw station data. The gridding also allows one, for the first time, to compare these observed trends with those simulated by a suite of climate model runs forced by observed changes in sea surface temperatures, sea ice extent, and various combinations of human-induced forcings. Bootstrapping techniques are used to assess the uncertainty in the gridded trend estimates and the field significance of the patterns of observed trends. The findings mainly confirm earlier, less objectively derived, results based on station data. There have been significant decreases in the number of frost days and increases in the number of very warm nights over much of the Northern Hemisphere. Regions of significant increases in rainfall extremes and decreases in the number of consecutive dry days are smaller in extent. However, patterns of trends in annual maximum 5-day rainfall totals were not significant. Comparisons of the observed trend estimates with those simulated by the climate model indicate that the inclusion of anthropogenic effects in the model integrations, in particular increasing greenhouse gases, significantly improves the simulation of changing extremes in temperatures. This analysis provides good evidence that human-induced forcing has recently played an important role in extreme climate. The model shows little skill in simulating changing precipitation extremes.