Global Environmental Change 9 (1999) S31}S49Climate change and global water resourcesNigel W. ArnellDepartment of Geography, University of Southampton, Southampton S017 1BJ, UKReceived 3 June 1999AbstractBy 2025, it is estimated that around 5 billion people, out of a total population of around 8 billion, will be living in countriesexperiencing water stress (using more than 20% of their available resources). Climate change has the potential to impose additionalpressures in some regions. This paper describes an assessment of the implications of climate change for global hydrological regimesand water resources. It uses climate change scenarios developed from Hadley Centre climate simulations (HadCM2 and HadCM3),and simulates global river #ows at a spatial resolution of 0.5;0.53 using a macro-scale hydrological model. Changes in national waterresources are calculated, including both internally generated runo! and upstream imports, and compared with national water useestimates developed for the United Nations Comprehensive Assessment of the Freshwater Resources of the World. Although there isvariation between scenarios, the results suggest that average annual runo! will increase in high latitudes, in equatorial Africa andAsia, and southeast Asia, and will decrease in mid-latitudes and most subtropical regions. The HadCM3 scenario produces changes inruno! which are often similar to those from the HadCM2 scenarios * but there are important regional di!erences. The rise intemperature associated with climate change leads to a general reduction in the proportion of precipitation falling as snow, anda consequent reduction in many areas in the duration of snow cover. This has implications for the timing of stream#ow in suchregions, with a shift from spring snow melt to winter runo!. Under the HadCM2 ensemble mean scenario, the number of people livingin countries with water stress would increase by 53 million by 2025 (relative to those who would be a!ected in the absence of climatechange). Under the HadCM3 scenario, the number of people living in countries with water stress would rise by 113 million. However,by 2050 there would be a net reduction in populations in stressed countries under HadCM2 (of around 69 million), but an increase of56 million under HadCM3. The study also showed that di!erent indications of the impact of climate change on water resource stressescould be obtained using di!erent projections of future water use. The paper emphasises the large range between estimates of `impacta,and also discusses the problems associated with the scale of analysis and the de"nition of indices of water resource impact.  1999Elsevier Science Ltd. All rights reserved.Keywords: Climate change; Global water resources; Global runo!; Hydrological impacts of climate change1. IntroductionGlobal warming, due to the enhanced greenhousee!ect, is likely to have signi"cant e!ects on the hydro-logical cycle (IPCC, 1996). The hydrological cycle will beintensi"ed, with more evaporation and more precipita-tion, but the extra precipitation will be unequally distrib-uted around the globe. Some parts of the world may seesigni"cant reductions in precipitation, or major alter-ations in the timing of wet and dry seasons. The SecondAssessment Report of the Intergovernmental Panel onClimate Change (IPCC) warned that global warmingwould lead to increases in both #oods and droughts.Many aspects of the environment, economy and so-ciety are dependent upon water resources, and changes inthe hydrological resource base have the potential toseverely impact upon environmental quality, economicdevelopment and social well-being. There have, so far,been few assessments (e.g. Alcamo et al., 1997) at theglobal scale of the potential impacts of climate change onwater resources.Climate change, however, is just one of the pressuresfacing water resources and their management over thenext few years and decades (see Gleick, 1998). In the mostgeneral terms, there are both supply-side and demand-side pressures. The supply-side pressures include climatechange (reducing or increasing the amount of wateravailable), but also include environmental degradation,where for example pollution reduces the amount of wateravailable for use. Demand-side pressures include popula-tion growth and concentration, leading to increaseddemands for domestic, industrial and agricultural0959-3780/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved.PII: S 0 9 5 9 - 3 7 8 0 ( 9 9 ) 0 0 0 1 7 - 5(particularly irrigation) water, increased environmentaldemands, and the e!ects of changes in the way demandsfor water are managed. Climate change may a!ect thedemand side of the balance as well as the supply side.In 1997, the United Nations published a Comprehens-ive Review of the Freshwater Resources of the World(WMO, 1997). The assessment included four components:collation of up-to-date national-level data on water re-sources and their use, the development of projections offuture use (to 2025 and 2050), description of present andfuture pressures, and the assessment of strategies andoptions for the sustainable development of world waterresources. The assessment highlighted the e!ects of in-creasing population and economic development on waterresource availability. It estimated that approximatelyone-third of the world's population currently lives incountries experiencing moderate to high water stress, andforecast that by 2025 as much as two-thirds of a muchlarger world population could be under stress conditionssimply due to the rise in population and water use.This paper describes an assessment of the e!ects ofclimate change on water resources stresses, over andabove the e!ects of population and economic change.The study represents one component of a `Fast Trackaassessment of the impacts of climate change across sev-eral sectors at the global scale (Parry et al., 1999), basedon climate change experiments conducted by the HadleyCentre. The paper "rst outlines the methodology used,then discusses the indices of water resources stress em-ployed. The rest of the paper "rst describes changes inhydrological characteristics at the global scale, and thenconsiders e!ects on water resource stresses.2. Methods and data sources2.1. IntroductionThe project methodology essentially follows the stan-dard climate change impact assessment approach (Parryand Carter, 1998). The study uses scenarios based on theHadCM2 and HadCM3 climate change experiments(Hulme et al.,