Global Freshwater Availability Below Normal Conditions and Population Impact Under 1.5 and 2 °C Stabilization Scenarios

Based on the large ensembles of the half a degree additional warming, prognosis, and projected impacts historical, +1.5 and +2 °C experiments, we quantify changes in the magnitude of water availability (i.e., precipitation minus actual evapotranspiration; a function of monthly precipitation flux, latent heat flux, and surface air temperature) below normal conditions (less than median, e.g., 20th percentile water availability). We found that, relative to the historical experiment, water availability below normal conditions of the +1.5 and +2 °C experiments would decrease in the midlatitudes and the tropics, indicating that hydrological drought is likely to increase in warmer worlds. These cause more (less) people in East Asia, Central Europe, South Asia, and Southeast Asia (West Africa and Alaska/Northwest Canada) to be exposed to water shortage. Stabilizing warming at 1.5 °C instead of 2 °C would limit population impact in most of the regions, less effective in Alaska/Northwest Canada, Southeast Asia, and Amazon. Globally, this reduced population impact is ~117 million people. This study emerges from the lack of scientific investigations to inform climate policy about differences between two global warming targets (i.e., 1.5 and 2 °C) for the “Intergovernmental Panel on Climate Change Special Report on Global Warming of 1.5°C.” We seek to understand the following: How would water availability below normal conditions (the drier end of hydrological extremes) change at these targets? How would they affect the water shortage of human society? Could we limit the impact by stabilizing the global warming at 1.5 °C instead of 2 °C? To address these questions, we employ the HAPPI (half a degree additional warming, prognosis, and projected impacts) experiments, explicitly designed to differentiate impacts between these targets. Relative to the historical period, future water availability below normal conditions (less than median, e.g., 20th percentile or lower) would decrease in the midlatitudes and the tropics; the globe and most of the regions would endure water shortages. Relative to the 2 °C warming target, stabilizing temperature increase at 1.5 °C would constrain adverse impact on people suffering water shortages in most of the regions (particularly Central Europe, East Africa, East Asia, South Asia, and West Africa) but ineffective in Alaska/Northwest Canada, Southeast Asia, and Amazon. A global sum of this reduced risk is ~117 million people. The first evaluation of global freshwater availability below normal conditions in +1.5 versus +2 °C worlds is conductedRelative to the historical period, future water availability below normal conditions would decrease in the midlatitudes and the tropicsRisk of freshwater shortages in a +1.5 °C world is less than a +2 °C world