Trends in extreme weather and climate events: Issues related to modeling extremes in projections of future climate change

Projections of statistical aspects of weather and climate extremes can be d erived from climate models representing possible future climate states. Som e of the recent models have reproduced results previously reported in the I ntergovernmental Panel on Climate Change (IPCC) Second Assessment Report, s uch as a greater frequency of extreme warm days and lower frequency of extr eme cold days associated with a warmer mean climate, a decrease in diurnal temperature range associated with higher nighttime temperatures, increased precipitation intensity, midcontinent summer drying, decreasing daily varia bility of surface temperature in winter, and increasing variability of nort hern midlatitude summer surface temperatures. This reconfirmation of previo us results gives an increased confidence in the credibility of the models, though agreement among models does not guarantee those changes will occur. New results since the IPCC Second Assessment Report indicate a possible inc rease of extreme heat stress events in a warmer climate, an increase of coo ling degree days and decrease in heating degree days, an increase of precip itation extremes such that there is a decrease in return periods for 20-yr extreme precipitation events, and more detailed analyses of possible change s in 20-yr return values for extreme maximum and minimum temperatures. Addi tionally, recent studies are now addressing interannual and synoptic time a nd space scale processes that affect weather and climate extremes, such as tropical cyclones, El Nino effects, and extratropical storms. However, curr ent climate models are not yet in agreement with respect to possible future changes in such features.