Threatening levels of cumulative stress due to hydroclimatic extremes in the 21st century

Abstract Hydroclimatic extremes, such as very intense precipitation and drought, are expected to increase with global warming, with their cumulative effects potentially posing severe threats for human and natural systems. We introduce a new metric of potential cumulative stress due to hydroclimatic extremes, the Cumulative Hydroclimatic Stress index (CHS), expressed in “equivalent reference stress years (ERSY)” (i.e., the mean annual stress during a present day reference period). The CHS is calculated for wet and dry extremes in an ensemble of 21st century Global Climate Model projections under the RCP8.5 and RCP2.6 greenhouse gas scenarios. Under the high-end RCP8.5 scenario, by 2100, increases in wet and dry extremes add ~155 ERSY averaged over global land areas (~125 for wet and ~30 for dry extremes), with wet hotspots (>250 added ERSY) throughout regions of Asia, Eastern Africa and the Americas, and dry hotspots (>100 added ERSY) throughout Central and South America, Europe, West Africa, and coastal Australia. Inclusion of population exposure in the stress index definition generates a maximum total (dry + wet) potential stress level exceeding 400 added ERSY over Africa, North America, and Australia, which are thus projected to be extremely vulnerable to increases in hydroclimatic extremes. Under the RCP2.6 scenario, which is close to the 2 °C global warming stabilization target set in the Paris agreement, the total hydroclimatic stress is considerably reduced.