Looking at the water resources assessments in more detail we find that the principal cause of these discrepancies is the different estimates for future potential changes in precipitation. This is the weakest part in all climatic scenarios. Indeed, it is changes in precipitation which have the most effect on assessments. The following general conclusions can be made concerning anthropgenic global climate change effects on water resources.
  • Water bodies and water management systems are very sensitive to changes in climatic characteristics; the most noticeable changes are to be expected with a >1ºC warming and >10% precipitation change.
  • The currently available scenarios of potential anthropogenic changes in regional climate are extremely uncertain. There is disagreement among them all, primarily related to precipitation change as the principal factor determining water resources and water use. None of the currently available scenarios for future climate can produce a reliable basis for estimating global changes in water resources for the large natural-economic regions, any of the continents, or for Earth as a whole.
  • When assessing water resources, water use and water availability on a global scale up to 2025, it seems perfectly sensible to neglect possible anthropogenic climate change. This view is based on the data used to compile the diagram on page 29 which shows recent estimates for air temperature rises of just 0.6ºC; it is supported by the great uncertainties encountered in regional precipitation and air temperature changes.
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