Climate change and global water resources: SRES emissions and socio-economic scenarios

In 1995, nearly 1400 million people lived in water-stressed watersheds (runoff less than 1000 m3/capita/year), mostly in south west Asia, the Middle East and around the Mediterranean. This paper describes an assessment of the relative effect of climate change and population growth on future global and regional water resources stresses, using SRES socio-economic scenarios and climate projections made using six climate models driven by SRES emissions scenarios. River runoff was simulated at a spatial resolution of 0.5×0.5° under current and future climates using a macro-scale hydrological model, and aggregated to the watershed scale to estimate current and future water resource availability for 1300 watersheds and small islands under the SRES population projections. The A2 storyline has the largest population, followed by B2, then A1 and B1 (which have the same population). In the absence of climate change, the future population in water-stressed watersheds depends on population scenario and by 2025 ranges from 2.9 to 3.3 billion people (36–40% of the world's population). By 2055 5.6 billion people would live in water-stressed watersheds under the A2 population future, and "only" 3.4 billion under A1/B1.Climate change increases water resources stresses in some parts of the world where runoff decreases, including around the Mediterranean, in parts of Europe, central and southern America, and southern Africa. In other water-stressed parts of the world—particularly in southern and eastern Asia—climate change increases runoff, but this may not be very beneficial in practice because the increases tend to come during the wet season and the extra water may not be available during the dry season. The broad geographic pattern of change is consistent between the six climate models, although there are differences of magnitude and direction of change in southern Asia.By the 2020s there is little clear difference in the magnitude of impact between population or emissions scenarios, but a large difference between different climate models: between 374 and 1661 million people are projected to experience an increase in water stress. By the 2050s there is still little difference between the emissions scenarios, but the different population assumptions have a clear effect. Under the A2 population between 1092 and 2761 million people have an increase in stress; under the B2 population the range is 670–1538 million, respectively. The range in estimates is due to the slightly different patterns of change projected by the different climate models. Sensitivity analysis showed that a 10% variation in the population totals under a storyline could lead to variations in the numbers of people with an increase or decrease in stress of between 15% and 20%. The impact of these changes on actual water stresses will depend on how water resources are managed in the future.