Climate Change 2001:
Working Group I: The Scientific Basis
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9.3.6.5 Commentary on changes in extremes of weather and climate

Although changes in weather and climate extremes are important to society, ecosystems, and wildlife, it is only recently that evidence for changes we have observed to date has been able to be compared to similar changes that we see in model simulations for future climate (generally taken to be the end of the 21st century as shown in this chapter). Though several simulations of 20th century climate with various estimates of observed forcings now exist (see Chapter 8), few of these have been analysed for changes in extremes over the 20th century. So far, virtually all studies of simulated changes in extremes have been performed for future climate. A number of studies are now under way for simulated 20th century climate, but are not yet available for assessment. Additionally, in the 20th century climate integrations there is usually a significant signal/noise problem (especially for changes in phenomena like storms). Therefore, here we assess changes in extremes that have been observed during the 20th century (see Chapter 2), and compare these to simulated changes of extremes for the end of the 21st century from AOGCMs run with increases in greenhouse gases and other constituents. Agreement between the observations and model results would suggest that the changes in extremes we have already observed are qualitatively consistent in a very general way with those changes in climate model simulations of future climate, indicating these changes in extremes would be likely to continue into the future.

The assessment of extremes here relies on very large-scale changes that are physically plausible or representative of changes over many areas. There are some regions where the changes of certain extremes may not agree with the larger-scale changes (see Chapters 2 and 10). Therefore, the assessment here is a general one where observed and model changes appear to be representative and physically consistent with a majority of changes globally. Additionally, certain changes in observed extremes may not have been specifically itemised from model simulations, but are physically consistent with changes of related extremes in the future climate experiments and are denoted as such. Also note that the information for tropical cyclones is drawn from Chapter 10, and diurnal temperature range from Chapter 12. A further discussion of the synthesis of observed and modelled changes of extremes, along with results on how extremes can affect human society, ecosystems and wildlife, appears in Easterling et al. (2000).

The qualitative consistency between the observations from the latter half of the 20th century and the models for the end of the 21st century in Table 9.6 suggests that at least some of the changes we have observed to date are likely to be associated with changes in forcing we have already experienced over the 20th century. The implication is that these could continue to increase into the 21st century with the ongoing rise in forcing from ever greater amounts of greenhouse gases in the atmosphere.

Table 9.6: Estimates of confidence in observed and projected changes in extreme weather and climate events.
Confidence in observed changes (latter half of the 20th century) Changes in Phenomenon Confidence in projected changes (during the 21st century)
Likely Higher maximum temperatures and more hot days a over nearly all land areas Very likely
Very likely Higher minimum temperatures, fewer cold days and frost days over nearly all land areas Very likely
Likely, over many areas Increase of heat index b over land areas Very likely, over most areas
Likely, over many Northern Hemisphere mid- to high latitude land areas More intense precipitation events c Very likely, over many areas
Likely, in a few areas Increased summer continental drying and associated risk of drought Likely, over most mid-latitude continental interiors. (Lack of consistent projections in other areas)
Not observed in the few analyses available Increase in tropical cyclone peak wind intensities d Likely, over some areas
Insufficient data for assessment Increase in tropical cyclone mean and peak precipitation intensities d Likely, over some areas
a Hot days refers to a day whose maximum temperature reaches or exceeds some temperature that is considered a critical threshold for impacts on human and natural systems. Actual thresholds vary regionally, but typical values include 32°C, 35°C or 40°C.
b Heat index refers to a combination of temperature and humidity that measures effects on human comfort.
c For other areas, there are either insufficient data or conflicting analyses.
d Past and future changes in tropical cyclone location and frequency are uncertain.

Table 9.6 depicts an assessment of confidence in observed changes in extremes of weather and climate during the latter half of the 20th century (left column) and in projected changes during the 21st century (right column). As noted above, this assessment relies on observational and modelling studies, as well as the physical plausibility of future projections across all commonly used scenarios and is based on expert judgement. For more details, see Chapter 2 (observations) and Chapter 10 (regional projections).

For the projected changes in the right-hand column, "very likely" indicates that a number of models have been analysed for such a change, all those analysed show it in most regions, and it is physically plausible. No models have been analysed to show fewer frost days, but it is physically plausible, since most models show an increase in night-time minimum temperatures, which would result in fewer frost days. The category "likely" indicates that theoretical studies and those models analysed show such a change, but only a few current climate models are configured in such a way as to reasonably represent such changes. "Hot days" refers to a day whose maximum temperature reaches or exceeds some temperature that is considered a critical threshold for impacts on human and natural systems. Actual thresholds vary regionally, but typical values include 32°C, 35°C or 40°C.

For some other extreme phenomena, many of which may have important impacts on the environment and society, there is currently insufficient information to assess recent trends, and climate models currently lack the spatial detail required to make confident projections. For example, very small-scale phenomena, such as thunderstorms, tornadoes, hail and lightning, are not simulated in climate models at present.



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