Integrated assessment of global water scarcity over the 21st century under multiple climate change mitigation policies

摘要

Water scarcity conditions over the 21st century both globally and regionallyare assessed in the context of climate change and climate mitigationpolicies, by estimating both water availability and water demand within theGlobal Change Assessment Model (GCAM), a leading community-integratedassessment model of energy, agriculture, climate, and water. To quantifychanges in future water availability, a new gridded water-balance globalhydrologic model – namely, the Global Water Availability Model (GWAM) – isdeveloped and evaluated. Global water demands for six major demand sectors(irrigation, livestock, domestic, electricity generation, primary energyproduction, and manufacturing) are modeled in GCAM at the regional scale (14geopolitical regions, 151 sub-regions) and then spatially downscaled to0.5° × 0.5° resolution to match the scale of GWAM.Using a baseline scenario (i.e., no climate change mitigation policy) withradiative forcing reaching 8.8 W m?2 (equivalent to the SRES A1Fiemission scenario) and three climate policy scenarios with increasingmitigation stringency of 7.7, 5.5, and 4.2 W m?2 (equivalent to theSRES A2, B2, and B1 emission scenarios, respectively), we investigate theeffects of emission mitigation policies on water scarcity. Two carbon taxregimes (a universal carbon tax (UCT) which includes land use changeemissions, and a fossil fuel and industrial emissions carbon tax (FFICT)which excludes land use change emissions) are analyzed. The baseline scenarioresults in more than half of the world population living under extreme waterscarcity by the end of the 21st century. Additionally, in years 2050 and2095, 36% (28%) and 44% (39%) of the global population,respectively, is projected to live in grid cells (in basins) that willexperience greater water demands than the amount of available water in a year(i.e., the water scarcity index (WSI) > 1.0). When comparing the climatepolicy scenarios to the baseline scenario while maintaining the same baselinesocioeconomic assumptions, water scarcity declines under a UCT mitigationpolicy but increases with a FFICT mitigation scenario by the year 2095,particularly with more stringent climate mitigation targets. Under the FFICTscenario, water scarcity is projected to increase, driven by higher waterdemands for bio-energy crops.