Most of this work acknowledges that projections of water scarcity are dependent upon not only scenarios of population change and emissions, but also the number of GCMs used for simulating the future climate. Various studies have explored how both these factors might affect global water scarcity in the future by using population projections and simulated changes in climate from global climate models (GCMs) with water resources models (Alcamo et al. Existing pressures on water resources will be exacerbated by increases in population and also by climate change. Additionally, investigation of the effects of a set of prescribed global mean temperature change scenarios show rapid increases in water scarcity due to climate change across many regions of the globe, up to 2 ☌, followed by stabilisation to 4 ☌. More of the world will see an increase in exposure to water scarcity than a decrease due to climate change but this is not consistent across all climate change patterns. Sensitivity to the WCI and WSI thresholds that define water scarcity can be comparable to the sensitivity to climate change pattern. A substantial proportion of the uncertainty in the global-scale effect of climate change on water scarcity is due to uncertainty in the estimates for South Asia and East Asia. This represents a higher upper-estimate than previous assessments because scenarios are constructed from a wider range of GCMs. Using the WCI, by 2050 under the A1B scenario, 0.5 to 3.1 billion people are exposed to an increase in water scarcity due to climate change (range across 21 GCMs). 1.6 (WCI) and 2.4 (WSI) billion people are estimated to be currently living within watersheds exposed to water scarcity. The Water Crowding Index (WCI) and the Water Stress Index (WSI) are used to calculate exposure to increases and decreases in global water scarcity due to climate change.

Patterns of climate change from 21 Global Climate Models (GCMs) under four SRES scenarios are applied to a global hydrological model to estimate water resources across 1339 watersheds. This paper presents a global scale assessment of the impact of climate change on water scarcity.

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