Figure SPM-4: Projected
changes in average annual water runoff by the year 2050, relative to average runoff
for the years 1961 to 1990, largely follow projected changes in precipitation.
Changes in runoff are calculated with a hydrologic model using as inputs
climate projections from two versions of the Hadley Centre atmosphere-ocean general
circulation model (AOGCM) for a scenario of 1% per annum increase in effective
CO2 concentration in the atmosphere: (a) HadCM2 ensemble mean and (b)
HadCM3. Projected increases in runoff in high latitudes and southeast Asia and
decreases in central Asia, the area around the Mediterranean, southern Africa,
and Australia are broadly consistent across the Hadley Centre experiments, and
with the precipitation projections of other AOGCM experiments. For other areas
of the world, changes in precipitation and runoff are scenario- and model-dependent.