Abstract
Human populations may be adversely impacted by water stress, a situation which is commonly defined as a per capita water availability of less than 1700 cubic meters of freshwater per person per year. Water stress may result from either overuse of available freshwater resources or a reduction in the amount of available water due to decreases in rainfall and stored water supplies. Analyzing the interrelationship between human populations and water availability is complicated by the uncertainties associated with climate change projections and population projections. We have developed a simple methodology to integrate disparate climate and population data sources and develop first-order per capita water availability projections at the global scale. Simulations from the coupled land-ocean-atmosphere Community Climate System Model version 3 (CCSM3) forced with a range of hypothetical greenhouse gas emissions scenarios have been used to project grid-based changes in precipitation minus evapotranspiration as proxies for changes in runoff, or fresh water supply. Population growth changes, according to Intergovernmental Panel on Climate Change (IPCC) storylines, have been used as proxies for changes in fresh water demand by 2025, 2050 and 2100. These freshwater supply and demand projections have then been combined to yield estimates of per capita water availability aggregated by U.S. watershed. Results suggest that important insights might be extracted from the use of the process developed here, including the identification of potentially vulnerable areas in need of more detailed analysis. This high-level analysis also illustrates the relative importance of population growth versus climate change in in altering future freshwater supplies. However, these are only exemplary insights and, as such, could be considered hypotheses that should be rigorously tested with multiple climate models, multiple observational climate datasets, and more comprehensive population growth projections.
