Regional climatemodels 133evidence from model projections of changes in the frequency of tropi-
cal cyclones or their areas of formation. However, model projections^18
and theoretical studies suggest that, if carbon dioxide concentration is
doubled, the peak wind intensities will tend to increase by perhaps five or
ten per cent and the mean and peak precipitation intensities by twenty to
thirty per cent.
Regarding storms at mid latitudes, the various factors that control
their incidence are complex. Two factors tend to an increased intensity of
storms. The first, as with tropical storms, is that higher temperatures, es-
pecially of the ocean surface, tend to lead to more energy being available.
The second factor is that the larger temperature contrast between land
and sea, especially in the Northern Hemisphere, tends to generate steeper
temperature gradients, which in turn generate stronger flow and greater
likelihood of instability. The region around the Atlantic seaboard of
Europe is one area where such increased storminess might be expected,
a result that some model projections have shown. However, such a picture
may well be too simple; other models suggestchanges in storm tracks
that result in very different changes in some regions and there is little
overall consistency between model projections.
For some other extremes such as very small-scale phenomena (e.g.
tornadoes, thunderstorms, hail and lightning) that cannot be simulated in
global models, although they may have important impacts, there is cur-
rently insufficient information to assess recent trends, and understanding
is inadequate to make firm projections.
Table 6.2 summarises the state of knowledge regarding the likely fu-
ture incidence of extreme events. Although general indications of trends
can be given, there have been few projections with quantitative estimates
of likely changes in the frequency or intensity of extreme events. In many
research centres, work is under way on more detailed studies of the in-
fluence of increased greenhouse gases on extreme events and changes in
climate variability.
Regional climate models
Most of the likely changes that we have presented have been on the scale
of continents. Can more specific information be provided about change
for smaller regions? In Chapter 5 we referred to the limitation of global
circulation models (GCM) in the simulation of changes on the regional
scale^19 arising from the coarse size of their horizontal grid – typically 300
km or more. Also in Chapter 5 we introduced the regional climate model
(RCM) that typically possesses a resolution of 50 km and can be ‘nested’
in a global circulation model. Examples are shown in Figures 6.10 and
7.8 of the improvement achieved by RCMs in the simulation of extremes