| Abstract: Using the
results obtained from a coupled ocean-atmosphere-land model with medium
computational resolution, we investigated how the hydrology of the continents
changes in response to the combined increases of greenhouse gases and sulfate
aerosols in the atmosphere, which are determined based upon the IS92a scenario.
In order to extract the forced response from natural, internal variability, the
difference between the mean of an eight-member ensemble of numerical experiments
and a control experiment are used for the present analysis. The global mean
surface air temperature of the coupled model increases by about 2.3°C above the
preindustrial level by the middle of the 21st century. Accompanying the warming,
the global mean rates of both precipitation and evaporation increase by 5.2%,
yielding the average increase in the rate of runoff by approximately 7.3%. The
increase in the rate of runoff simulated by the model is particularly large in
high northern latitudes, where the runoff from some rivers such as the Mackenzie
and Obī may increase by as much as 20%. Runoff from many European rivers
increases by more than 20%. Runoff also increases substantially in some tropical
rivers such as the Amazon and Ganges. However, the percentage changes in
simulated runoff from many other tropical rivers and middle latitude rivers are
smaller with both positive and negative signs. In middle and high latitudes in
the Northern Hemisphere, soil moisture tends to decrease in summer, whereas it
increases in winter. However, in many semi-arid regions in subtropical and
middle latitudes, soil moisture is reduced during most of a year. These
semi-arid regions include the southwestern part of North America, the
northeastern part of China in the Northern Hemisphere, and the region in the
vicinity of the Kalahari Desert and southern part of Australia in the Southern
Hemisphere. Since a semi-arid region usually surrounds a desert, the reduction
of soil moisture in such a region often results in the expansion of the desert.
Soil moisture is also reduced during the dry season in many semi-arid regions.
For example, it is reduced in the savannahs of Africa and South America during
winter and early spring in the Southern Hemisphere. In the Northern Hemisphere,
it is reduced at the Mediterranean coast of Europe in summer. |