Climate Change 2001: Working Group II: Impacts, Adaptation and Vulnerability

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4.5.4. Impacts of Climate Change on Water Resources: An Overview

This section explores the global-scale implications of climate change for water resources stress and summarizes a few studies into climate change impacts on several real-world water management systems. However, few published studies consider impacts in quantitative terms on real-world systems; most published studies infer changes in water resources from changes in streamflow.

It is very difficult to draw quantitative conclusions about the impacts of climate change, for several reasons. Different studies have used different methodologies and different scenarios, but, most important, different systems respond very differently to climate change. It is possible, however, to make some qualitative generalizations:

• In systems with large reservoir capacity, changes in resource reliability may be proportionately smaller than changes in river flows.
• The potential impacts of climate change must be considered in the context of other changes that affect water management. Few studies haveexplicitly compared climate change with other pressures, but in many environments it is likely that over a time horizon of less than 20 years, climate change impacts will be very small relative to other pressures. This will depend on the system.
• The implications of climate change are likely to be greatest in systems that currently are highly stressed.

By far the majority of studies of the impact of climate change on water resources have concentrated on human aspects of the water environment. Only a very few (e.g., Eheart et al., 1999; Meyer et al., 1999) have considered impacts on the aquatic environment. Some of these studies are considered in Chapter 5, but it must be remembered that water resources systems in many parts of the world increasingly are being managed to maintain instream and wetland ecosystems. This either increases effective water demand or decreases water availability.

Confidence in estimated quantitative impacts of climate change on water resources generally is low, reflecting initial confidence in climate change scenarios and low confidence in estimates of future pressures on water resources (as a result of factors such as changes in demand or legislative requirements). However, techniques for estimating the impacts of a given scenario are now well established.