158 The impactsof climate changeFigure 7.7Simulations of
average monthly runoff in the
Sacramento basin of California
comparing (a) current climate
with (b) changed climates
with a 4◦C temperature
increase and a twenty per cent
increase in rainfall and (c) with
the same temperature
increase but with a twenty per
cent decrease of rainfall.
likely that there will be significant increases or decreases. For instance,
precipitation is expected to increase in northern high latitudes in winter
and the monsoon regions of south-east Asia in summer, while other
regions (e.g. southern Europe, Central America, southern Africa and
Australia) can expect significantly drier summers. Further, increase of
temperature will mean that a higher proportion of the water falling on the
Earth’s surface will evaporate. In regions with increased precipitation,
some or all of the loss due to evaporation may be made up. However, in
regions with unchanged or less precipitation, there will be substantially
less water available at the surface. The combined effect of less rainfall
and more evaporation means less soil moisture available for crop growth
and also less runoff – in regions with marginal rainfall this loss of soil
moisture can be critical.
The runoff in rivers and streams is what is left from the precipitation
that falls on the land after some has been taken by evaporation and by
transpiration from plants; it is the major part of what is available for hu-
man use. The amount of runoff is highly sensitive tochanges in climate;
even small changes in the amount of precipitation or in the tempera-
ture (affecting the amount of evaporation) can have a big influence on
it. To illustrate this, Figure 7.7 shows simulations, carried out for the
Sacramento Basin in California, USA (a region where water is stored for
some of the year in mountain snow), of changes in runoff with changed
climate conditions. With a 4◦C regional temperature rise and twenty
per cent decreased rainfall, the runoff in the summer months falls to
between twenty per cent and fifty per cent of its normal value. Even with
twenty per cent increased rainfall and the same temperature increase,