Multimodel assessment of water scarcity under climate change

Abstract

Water scarcity severely impairs food security and economic prosperity in many countries today. Expected future population changes will, in many countries as well as globally, increase the pressure on available water resources. On the supply side, renewable water resources will be affected by projected changes in precipitation patterns, temperature, and other climate variables. Here we use a large ensemble of global hydrological models (GHMs) forced by five global climate models and the latest greenhouse-gas concentration scenarios (Representative Concentration Pathways) to synthesize the current knowledge about climate change impacts on water resources. We show that climate change is likely to exacerbate regional and global water scarcity considerably. In particular, the ensemble average projects that a global warming of 2 °C above present (approximately 2.7 °C above preindustrial) will confront an additional approximate 15% of the global population with a severe decrease in water resources and will increase the number of people living under absolute water scarcity (<500 m(3) per capita per year) by another 40% (according to some models, more than 100%) compared with the effect of population growth alone. 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 unabated beyond 2 °C. At the same time, 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.

Keywords: Inter-Sectoral Impact Model Intercomparison Project; climate impacts; hydrological modeling.

Fig. 1.

Relative change in annual discharge at 2 °C compared with present day, under RCP8.5. (Upper) Color hues show the multimodel mean change, and saturation shows the agreement on the sign of change across all GHM–GCM combinations (percentage of model runs agreeing on the sign; color scheme following ref. 58). (Lower) Ratio of GCM variance to total variance; in red (blue) areas, GHM (GCM) variance predominates. GHM variance was computed across all GHMs for each GCM individually, and then averaged over all GCMs; vice versa for GCM variance. Greenland has been masked.

Fig. 2.

Adverse impact of climate change on renewable water resources at different levels of global warming. Markers show the percentage of the world population living in 0.5° × 0.5° grid cells where the 31-y average of annual discharge falls short of the 1980–2010 average by more than 1σ (SD of annual discharge during 1980–2010), or by more than 20%, under the RCP8.5 climate scenario and SSP2 population scenario. The five GCMs are displayed in separate vertical columns (in the order in which they are listed in Materials and Methods; note that only four GCMs have sufficient coverage of the 3 °C warming level), and the 11 GHMs are displayed in unique colors. The black boxes give the interquartile range, and the horizontal black lines the median, across all GCMs and GHMs.

Fig. 3.

Percentage of world population living in countries with annual mean BW availability (Materials and Methods) below 500 m³ per capita (Left) and below 1,000 m³ per capita (Right). Symbols as in Fig. 2. (A and B) RCP8.5 climate scenario, population change according to SSP2. (C and D) Amplification by climate change of the level of water scarcity that is expected from population change alone; computed as the difference between a constant-climate scenario (SI Appendix, Fig. S7) and the full scenario shown above, divided by the constant-climate scenario, and expressed as percentage (so that the population-only case equals 100%). For example, in C, the MMM indicates that at 2 °C global warming, climate change amplifies the level of absolute water scarcity (number of people below 500 m³ per capita) expected from population change alone by about 36%.