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 |
|
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.
