130 Climate changein the twenty-firstcenturyand beyondFigure 6.7Changes in the
frequency of occurrence of
different daily rainfall amounts
with doubled carbon dioxide
as estimated by a CSIRO
model in Australia.
the carbon dioxide concentration is shown in Figure 6.7. The number of
days with large rainfall amounts (greater than 25 mm day−^1 ) doubled.
The probability of conditions leading to floods would also at least have
doubled. Similar results (fewer rainy days, higher maximum daily rain-
falls for a given mean rainfall rate) have been obtained from many other
climate models. For instance, a recent modelling study (Figure 6.8) has
shown that if atmospheric carbon dioxide concentration is doubled from
its pre-industrial value, the probability of extreme seasonal precipitation
in winter is likely to increase substantially over large areas of central and
northern Europe and likely to decrease over parts of the Mediterranean
and north Africa. The increase in parts of central Europe is such that
the return period of extreme rainfall events would decrease by about a
factor of five (e.g. from fifty years to ten years). Similar results have been
obtained in a study of major river basins around the world.^17
Note also from Figure 6.7 that the number of days with lighter rain-
fall events (less than 6 mm/day) is expected to decrease in the globally
warmed world. This is because, with the more intense hydrological cy-
cle, a greater proportion of the rainfall will fall in the more intense events
and, furthermore, in regions of convection the areas of downdraught be-
come drier as the areas of updraught become more moist. In many areas
with relatively low rainfall, the rainfall will tend to become less – which
has implications for the likelihood of drought.
Take, for instance, the likelihood of drought in regions where the
average summer rainfall falls by perhaps twenty per cent as is likely to
occur, for instance, in southern Europe (Figure 6.5(b)). The likely result
of such a drop in rainfall is not that the number of rainy days will remain
the same, with less rain falling each time; it is more likely that there
will be substantially fewer rainy days and considerably more chance of