Climate change and global water resources

Abstract

By 2025, it is estimated that around 5 billion people, out of a total population of around 8 billion, will be living in countries experiencing water stress (using more than 20% of their available resources). Climate change has the potential to impose additional pressures in some regions. This paper describes an assessment of the implications of climate change for global hydrological regimes and water resources. It uses climate change scenarios developed from Hadley Centre climate simulations (HadCM2 and HadCM3), andsimulatesglobalriver#owsataspatialresolutionof0.5]0.53usinga macro-scalehydrologicalmodel.Changesinnationalwater resources are calculated, including both internally generated runo! and upstream imports, and compared with national water use estimates developed for the United Nations ComprehensiveAssessment of the Freshwater Resources of the World. Although there is variation between scenarios, the results suggest that average annual runo! will increase in high latitudes, in equatorial Africa and Asia,andsoutheastAsia,andwilldecreaseinmid-latitudesandmostsubtropicalregions.TheHadCM3scenarioproduceschangesin runo! which are often similar to those from the HadCM2 scenarios * but there are important regional di!erences. The rise in temperature associated with climate change leads to a general reduction in the proportion of precipitation falling as snow, and a consequent reduction in many areas in the duration of snow cover. This has implications for the timing of stream#ow in such regions,witha shiftfromspringsnowmelttowinterruno!.UndertheHadCM2ensemblemeanscenario,thenumberofpeopleliving in countries with water stress would increase by 53 million by 2025 (relative to those who would be a!ected in the absence of climate change).Underthe HadCM3scenario, the numberof people livingin countrieswith water stress wouldrise by 113 million.However, by 2050 there would be a net reductionin populations in stressed countries under HadCM2 (of around 69 million),but an increaseof 56millionunderHadCM3.Thestudyalsoshowedthatdi!erentindicationsoftheimpactofclimatechangeonwaterresourcestresses couldbeobtainedusing di!erentprojectionsoffuturewater use.The paperemphasisesthelargerangebetweenestimatesof`impacta, and also discusses the problems associated with the scale of analysis and the de"nition of indices of water resource impact. ( 1999 Elsevier Science Ltd. All rights reserved.