However, in some regions where good data are available, there have been some significant increases and decreases in extreme events over time. The impact of such changes can manifest itself in fewer freezing days and late season frosts, such as have been documented in Australia and the United States. Indeed, we expect that the number of days with extremely low temperatures should continue to decrease as global temperatures rise. Widespread, extended periods of extremely high temperatures are also expected to become more frequent with continued global warming, such as the unprecedented high nighttime temperatures during the 1995 heat wave in Chicago, Illinois, and the midwestern United States that caused an estimated 830 deaths. However, the global frequency of such heat waves has not been analyzed at this time.
Higher temperatures lead to higher rates of evaporation and precipitation.
As the Earth warms, we expect more precipitation and it is likely to fall over shorter intervals of time, thereby increasing the frequency of very heavy and extreme precipitation events. Analyses of observed changes in precipitation intensity have been conducted only for a few countries. Perhaps the best evidence of increases in extreme and very heavy precipitation events comes from data in North America as depicted for the United States in Figure 8.1 . In Australia, which is historically prone to heavy precipitation, an increase in rainfall amount from major storms has also been observed. Analyses for South Africa also show increases in extreme precipitation rates.