The impact of climate change on global water scarcity

We present a global scale assessment of the impact of climate change on water scarcity. The Water Crowding Index (WCI) is used to calculate exposure to increases and decreases in global water scarcity across 1339 watersheds in the year 2050 under four Intergovernmental Panel on Climate Change (IPCC) 4th Assessment Report (AR4) Special Report on Emissions Scenarios (SRES) population change scenarios. To isolate the effect of climate change only, we compare water scarcity in 2050 with and without climate change respectively. Patterns of climate change from 21 Global Climate Models (GCMs) used in the Coupled Model Intercomparison Project Phase 3 multi-model dataset (CMIP3) under four SRES emissions scenarios are used to represent climate change. We find that by 2050 under the SRES 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). This represents a higher upper-estimate than previous assessments because we included more of the CMIP3 GCMs in ours. When considering the range in estimates from using all CMIP3 GCMs, a greater global population experience an increase in water scarcity due to climate change than a decrease, but this is not necessarily the case for individual GCMs. Lastly, we demonstrate that our results are robust across both the older IPCC AR4 SRES emissions scenarios with the CMIP3 GCMs, and the latest scenarios used in the IPCC AR5: the representative concentration pathways (RCPs) with the CMIP5 GCMs.