A multimodel assessment of water scarcity under climate change reveals that climate change is likely to significantly exacerbate regional and global water scarcity. Using a large ensemble of global hydrological models (GHMs) forced by five global climate models and the latest greenhouse-gas concentration scenarios (Representative Concentration Pathways), the study shows that a global warming of 2°C above present (approximately 2.7°C above preindustrial) will affect an additional 15% of the global population with severe water resource decreases and increase the number of people living under absolute water scarcity (less than 500 m³ per capita per year) by another 40% (some models suggest more than 100%). For some indicators of moderate impacts, the steepest increase is seen between the present day and 2°C, whereas indicators of very severe impacts increase beyond 2°C. The study highlights large uncertainties associated with these estimates, with both global climate models and GHMs contributing to the spread. GHM uncertainty is particularly dominant in many regions affected by declining water resources, suggesting a high potential for improved water resource projections through hydrological model development. Water scarcity severely impairs food security and economic prosperity in many countries. Future population growth and economic prosperity will increase water demand, exacerbating these problems. Climate change poses an additional threat to water security due to changes in precipitation and other climatic variables. The effect of climate change on water resources is uncertain due to regional variability in climate model projections and the complexity of hydrological processes. The study uses the Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP) to explore uncertainties and synthesize knowledge about climate change impacts on renewable water resources. The results show that a 2°C warming will affect about 14% of the global population with severe water resource decreases, and a 3°C warming will affect about 17%. The study also finds that climate change can amplify the effects of population growth on water scarcity, with some models suggesting an amplification of up to 100%. The study concludes that the combination of unmitigated climate change and further population growth will expose a significant fraction of the world population to chronic or absolute water scarcity, posing major challenges for societies to adapt their water use and management.A multimodel assessment of water scarcity under climate change reveals that climate change is likely to significantly exacerbate regional and global water scarcity. Using a large ensemble of global hydrological models (GHMs) forced by five global climate models and the latest greenhouse-gas concentration scenarios (Representative Concentration Pathways), the study shows that a global warming of 2°C above present (approximately 2.7°C above preindustrial) will affect an additional 15% of the global population with severe water resource decreases and increase the number of people living under absolute water scarcity (less than 500 m³ per capita per year) by another 40% (some models suggest more than 100%). For some indicators of moderate impacts, the steepest increase is seen between the present day and 2°C, whereas indicators of very severe impacts increase beyond 2°C. The study highlights large uncertainties associated with these estimates, with both global climate models and GHMs contributing to the spread. GHM uncertainty is particularly dominant in many regions affected by declining water resources, suggesting a high potential for improved water resource projections through hydrological model development. Water scarcity severely impairs food security and economic prosperity in many countries. Future population growth and economic prosperity will increase water demand, exacerbating these problems. Climate change poses an additional threat to water security due to changes in precipitation and other climatic variables. The effect of climate change on water resources is uncertain due to regional variability in climate model projections and the complexity of hydrological processes. The study uses the Inter-Sectoral Impact Model Intercomparison Project (ISI-MIP) to explore uncertainties and synthesize knowledge about climate change impacts on renewable water resources. The results show that a 2°C warming will affect about 14% of the global population with severe water resource decreases, and a 3°C warming will affect about 17%. The study also finds that climate change can amplify the effects of population growth on water scarcity, with some models suggesting an amplification of up to 100%. The study concludes that the combination of unmitigated climate change and further population growth will expose a significant fraction of the world population to chronic or absolute water scarcity, posing major challenges for societies to adapt their water use and management.