9.3.6.6 Conclusions
Much of what climate model studies show could happen to weather and climate
extremes in a future climate with increased greenhouse gases is what we would
intuitively expect from our understanding of how the climate system works. For
example, a warming of the surface supplies more water vapour to the atmosphere,
which is a greater source of moisture in storms and thus we would expect an
increase in intense precipitation and more rainfall from a given rainfall event,
both results seen in climate model simulations. There are competing effects
of decreased baroclinicity in some regions due to greater surface warming at
high latitudes, and increasing mid-tropospheric baroclinicity due to greater
mid-tropospheric low latitude warming (Kushner et al., 2001). Additionally,
a number of changes in weather and climate extremes from climate models have
been seen in observations in various parts of the world (decreased diurnal temperature
range, warmer mean temperatures associated with increased extreme warm days
and decreased extreme cold days, increased rainfall intensity, etc.). Though
the climate models can simulate many aspects of climate variability and extremes,
they are still characterised by systematic simulation errors and limitations
in accurately simulating regional climate such that appropriate caveats must
accompany any discussion of future changes in weather and climate extremes.
Recent studies have reproduced previous results in the SAR and this gives us
increased confidence in their credibility (although agreement between models
does not guarantee that those changes will occur in the real climate system):
- An increase in mean temperatures leads to more frequent extreme high temperatures
and less frequent extreme low temperatures.
- Night-time low temperatures in many regions increase more than daytime highs,
thus reducing the diurnal temperature range.
- Decreased daily variability of temperature in winter and increased variability
in summer in Northern Hemisphere mid-latitude areas.
- There is a general drying of the mid-continental areas during summer in
terms of decreases in soil moisture, and this is ascribed to a combination
of increased temperature and potential evaporation not being balanced by precipitation.
- Intensity of precipitation events increases.
Additional results since 1995 include:
- Changes in temperature extremes noted above have been related to an increase
in a heat index (leading to increased discomfort and stress on the human body),
an increase in cooling degree days and a decrease in heating degree days.
- Additional statistics relating to extremes are now being produced. For example,
in one model the greatest increase in the 20-year return values of daily maximum
temperature is found in central and Southeast North America, central and Southeast
Asia and tropical Africa where there is a decrease in soil moisture content,
and also over the dry surface of North Africa. The west coast of North America
is affected by increased precipitation, resulting in moister soil and more
moderate increases in extreme temperature. The increases in the return values
of daily minimum temperature are larger than those of daily maximum temperature
mainly over land areas and where snow and sea ice retreat.
- Precipitation extremes increase more than the mean and that means a decrease
in return period for the extreme precipitation events almost everywhere (e.g.,
20 to 10 years over North America).
Aspects which have been addressed but remain unresolved at this time include:
- There is no general agreement yet among models concerning future changes
in mid-latitude storms (intensity, frequency and variability), though there
are now a number of studies that have looked at such possible changes and
some show fewer weak but greater numbers of deeper mid-latitude lows, meaning
a reduced total number of cyclones.
- Due to the limitations of spatial resolution in current AOGCMs, climate
models do not provide any direct information at present regarding lightning,
hail, and tornadoes. Results derived from earlier models used empirical relationships
to infer a possible future increase in lightning and hail, though there have
been no recent studies to corroborate those results.
- There is some evidence that shows only small changes in the frequency of
tropical cyclones derived from large-scale parameters related to tropical
cyclone genesis, though some measures of intensities show increases, and some
theoretical and modelling studies suggest that upper limit intensities could
increase.
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