The impacts for the availability of water were generated from work from a variety of sources, including the Intergovernmental Panel on Climate Change Working Group 2 report and research by the Walker Institute for Climate System Research, University of Reading.
In addition ,the IPCC WG2 report has collated a great deal of research about the availability of water, and this resource was used to support statements about glacier melt and the contribution of different water sources.
Finally, specific research was done by Prof. Nigel Arnell at the Walker Institute for Climate System Research, University of Reading, looking at the effect of an increase of global average temperature of +4 °C, on the runoff and water availability.
A rise in global temperature of 4 °C would have a substantial effect on river flows and the availability of water resources. Average runoff decreases by up to 70% (compared to present) around the Mediterranean, southern Africa, Central Asia and large areas of North and South America (Figure 1), although the amounts of change are different between different climate models.
In some parts of the world - such as high latitudes and the wet tropics - runoff would likely increase due to climate change. However, this additional runoff may not add to that available for use because it may occur during the wet season and there may be insufficient storage to hold the water through to the dry season; it may also lead to increased flood risk.
The broad patterns of change in runoff are reasonably consistent between different climate models, but there is considerable uncertainty in how runoff may change across south Asia, due largely to differences between climate models in projected changes in the south Asian monsoon.
Climate change will affect not only the volume, but also the timing of river flows in some regions. This may arise because of changes in the timing of the onset or end of a wet season, but is particularly likely to occur where precipitation in winter currently falls as snow. Higher temperatures would mean that winter precipitation falls as rain rather than snow, and runs off rapidly during winter rather than being stored as snow until spring.
At present, approximately 1.5 billion people, out of a total world population of around 6 billion, live in major watersheds where average river runoff is less than 1000 m3/capita/year (a widely used threshold indicating potential exposure to water resources stresses).
By 2080, under a moderate population growth projection, approximately 35% of a larger world population - close to three billion people - could be living in water-stressed areas.
Under a 4 °C rise in global temperature, and the HadCM3 climate scenario, around one billion of these people would see a significant reduction in the amount of water available, and would be exposed to increased scarcity of water resource.
Regions particularly affected - under this scenario - include the Middle East and Central Asia; countries around the Mediterranean; parts of central and north America, and parts of southern and western Africa.
Figure 1. Change in river flow for an increase in global average temperature of +4 °C
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