Weather and climatic extremes can have serious and damaging effects on human society and infrastructure as well as on ecosystems and wildlife. Thus, they are usually the main focus of attention of the news media in reports on climate. There are some indications from observations concerning how climatic extremes may have changed in the past. Climate models show how they could change in the future either due to natural climate fluctuations or under conditions of greenhouse gas-induced warming. These observed and modeled changes relate directly to the understanding of socioeconomic and ecological impacts related to extremes. This is the first of five papers in the "Understanding Changes in Weather and Climate Extremes" series. The following series of five articles was motivated by a need to develop a more comprehensive assessment of changes in weather and extreme climate events. We were interested not only in the impact of extreme weather and climate events, but whether these events were changing in frequency or intensity along with their impacts. Impacts were viewed in terms of loosely managed ecosystems where wildlife flourishes, as well as socioeconomic systems and more heavily managed ecosystems such as agriculture. From a climate perspective , this included a focus both on the historical record and projections for future change. During the summer of 1998 a group of nearly 30 climate scientists, social scientists, and biologists met for 10 days at the Aspen Global Change Institute to discuss what we now know, and how we could reduce some of our major uncertainties. These articles summarize much of the work during that meeting and new information since the meeting.

Projections of statistical aspects of weather and climate extremes can be derived from climate models representing possible future climate states. Some of the recent models have reproduced results previously reported in the Intergovernmental Panel on Climate Change (IPCC) Second Assessment Report, such as a greater frequency of extreme warm days and lower frequency of extreme 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 variability of surface temperature in winter, and increasing variability of northern midlatitude summer surface temperatures. This reconfirmation of previous 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 increase of extreme heat stress events in a warmer climate, an increase of cooling degree days and decrease in heating degree days, an increase of precipitation extremes such that there is a decrease in return periods for 20-yr extreme precipitation events, and more detailed analyses of possible changes in 20-yr return values for extreme maximum and minimum temperatures. Additionally, recent studies are now addressing interannual and synoptic time and space scale processes that affect weather and climate extremes, such as tropical cyclones, El Nino effects, and extratropical storms. However, current climate models are not yet in agreement with respect to possible future changes in such features. This is the third of five papers in the "Understanding Changes in Weather and Climate Extremes" series.

Statistical downscaling of general circulation models (GCMs) and limited area models (LAMs) has been promoted as a method for simulating regional-to point-scale precipitation under changed climate conditions. However, several studies have shown that downscaled precipitation is either insensitive to changes in climatic forcing, or inconsistent with the broad-scale changes indicated by the host GCM(s). This has been recently attributed to the omission of the effect that changes in atmospheric moisture content have on precipitation. We describe validation of a nonhomogeneous hidden Markov model (NHMM) for changed climate conditions and apply it to a network of 30 daily precipitation stations in southwestern Australia. NHMMs fitted to 1 × CO 2 LAM data were validated by assessing their performance in predicting 2 × CO 2 LAM precipitation. The inclusion of 850 hPa dew point temperature depression, a predictor reflecting relative (rather than absolute) atmospheric moisture content, was found to be crucial to successful performance of the NHMM under 2 × CO 2 conditions. The NHMM validated for the LAM data was fitted to the historical 30 station network and then used to downscale the 2 × CO 2 LAM atmospheric data, producing plausible predictions of station precipitation under 2 × CO 2 conditions. Our results highlight that the validation of a statistical downscaling technique for present day conditions does not necessarily imply legitimacy for changed climate conditions. Thus statistical downscaling studies that have not attempted to determine the plausibility of their predictions for the changed climate conditions should be viewed with caution.

Recent results from greenhouse warming experiments, most of which follow the Intergovernmental Panel on Climate Change (IPCC) IS92a scenario, have shown that under increasing atmospheric CO 2 concentration, the Antarctic Oscillation (AAO) exhibits a positive trend. However, its response during the subsequent CO 2 stabilization period has not been explored. In this study, it is shown that the upward trend of the AAO reverses during such a stabilization period. This evolution of an upward trend and a subsequent reversal is present in each ensemble of three greenhouse simulations using three versions of the CSIRO Mark 2 coupled climate model. The evolution is shown to be linked with that of surface temperature, which also displays a corresponding trend and reversal, incorporating the well-known feature of interhemispheric warming asymmetry with smaller warming in the Southern Hemisphere (smaller as latitude increases) than that in the Northern Hemisphere during the transient period, and vice versa during the stabilization period. These results indicate that once CO 2 concentration stabilizes, reversal of the AAO trend established during the transient period is likely to be a robust feature, as it is underpinned by the likelihood that latitudinal warming differences will reduce or disappear. The implication is that climatic impacts associated with the AAO trend during the transient period may be reversible if CO 2 stabilization is achieved.

Abstract.We analyse temperature and precipitation changes for the late decades of the 21st century (with respect to present day conditions) over 23 land regions of the world from 18 recent transient climate change experiments with coupled atmosphere-ocean General Circulation Models (AOGCMs). The analysis involves two different forcing scenarios and nine models, and it focuses on model agreement in the simulated regional changes for the summer and winter seasons. While to date very few conclusions have been presented on regional climatic changes, mostly limited to some broad latitudinal bands, our analysis shows that a number of consistent patterns of regional change across models and scenarios are now emerging. For temperature, in addition to maximum winter warming in northern high latitudes, warming much greater than the global average is found over Cen-