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Global Hydrology and Water Resources: Projections
Climate change is expected to significantly reduce available water in many of the water-scarce areas of the world. It will increase water supply in some other areas. Water supply is reflected by projected runoff. (see Figure 1).
There could be more floods with greater intensity in many regions because of increased frequency of heavy precipitation events.
This would increase runoff in most areas, as well as increase groundwater recharge in some floodplains. Land-use change could intensify these effects. The water flow in rivers and streams normally decreases in summer. It will decrease even further in many areas, due to greater evaporation.
Climate change may affect irrigation withdrawals. This water demand will depend on how increases in evaporation are balanced or worsened by changes in precipitation. Higher temperatures mean higher evaporation from crops. This will increase irrigation requirements.
Scientists estimate that several hundred million to a few billion people will suffer a supply reduction of 10% or more by the year 2050 for climate change projections corresponding to 1% per year increase in CO2 emissions (see Table 1).
Adding to the effect of reduced supply is the fact that higher water temperatures will likely harm freshwater quality.
The cumulative effects of climate changes on water scarcity, water quality, and the frequency and intensity of floods and droughts, will make water and flood management more difficult.
Table 1: Water resource effects of climate change if no climate policy interventions are made. Judgments of confidence use the following scale:
- very high (95% or greater)
- high (67-95%), medium (33-67%)
- low (5-33%)
- very low (5% or less)
Source: IPCC Climate Change 2001 Synthesis Report p. 72
Figure 1. Projected changes in average annual water runoff by the year 2050, relative to average runoff for the years 1961 to 1990
Source: IPCC Climate Change 2001 Synthesis Report p. 13 (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.)