Estimating fresh water availability at regional and decadal scales based on projected changes in climate and population

Human populations are profoundly affected by water stress, or the lack of available freshwater for direct consumption, agriculture, washing, and a host of other necessary uses. “Water stress” can result from an excess number of people taxing the available freshwater resources or by a reduced amount of water due to decreases in rainfall and stored water supplies. Analyzing the complex interrelationship between human populations and available water supplies is made more complicated by climate change projections and an end to “stationarity.” Simulations from the Community Climate System Model version 3 (CCSM3), forced with the IPCC SRES A1FI scenario, are used for projections of climate variables. The IPCC SRES (Special Report on Emissions Scenarios) A1FI used here is fossil-fuel intensive, assumes a convergent world, and considers a future where the world population reaches a maximum around 2050 and then declines till the end of the century. The emissions and hence global-average temperature trajectories from A1FI trend higher than all other IPCC SRES scenarios. However, recent observations show that emissions in the current decade trend slightly higher than the A1FI. While this does not conclusively establish that emissions will trend according to A1FI in the 21st century, this does establish that this so called “worst case” can no longer be ruled out as implausible but needs to be considered as the new “business as usual”. A1FI-forced CCSM3 grid-based changes in P–E (precipitation minus evapotranspiration) are used here as proxies for changes in water supply. Freshwater availability is subsequently estimated by watershed and by political units. Population growth is used as a proxy for changes in water demand. The fact that recent emissions trends towards A1FI do not necessarily make the population growth assumptions correct or even more plausible because the emissions trends are computed based on multiple assumptions. Thus, A1FI projections of P–E are used together with population growth assumptions from both IPCC SRES A2 and A1/B1. While the A1/B1 scenario assumes a “moderate growth” of population with a maxima around 2050 and decline from then till 2100, the A2 scenario, which considers a more divergent world, assumes a “worst case growth” with steady increase of population into the end of the 21st century. A superposition of the freshwater supply versus freshwater demand in each watershed and political unit provides global and regional assessments of the scarcity and deficit of freshwater resources. A comparison with current water usage and with per capita water requirements points to regions most likely to be impacted by water stress. In addition to advances in the scientific understanding of water resources, studies like these can help in the negotiation of water and climate treaties, as well as project the effects of climate and population change on the quality of life for future generations.