Global River Discharge Projections From a Large Multi‐Model Ensemble of CMIP6 and ISIMIP3b Simulations

Abstract

Rivers are central to the global hydrological cycle, supporting ecosystems and human water use. River discharge, one of the best observed hydrological variables, is expected to change under anthropogenic warming with potentially devastating consequences. We assess global changes in river flow by routing grid‐cell runoff from 18 CMIP6 and 25 ISIMIP3b simulations along the river network using the routing model mRM, combining online (fully‐coupled CMIP6) and offline (bias‐adjusted ISIMIP3b) approaches. Annual river flow projections, climatological seasonal cycles, and discharge trends are comparable between the two approaches and biases with respect to observations are consistent in magnitude with the literature. Therefore, we pool all 43 simulations into one large multi‐model ensemble covering 250 years (1850–2099). The projections show changes in river discharge in response to warming which are largely consistent with previous studies. In particular, at 3.0°C global warming, annual mean and extreme river discharge are projected to decrease in the Mediterranean, Central America, large parts of South America, and Southern Africa; and increase in Central Africa, South Asia, and high northern latitudes. Seasonal mean flow changes mirror annual projections, except for drying in high northern latitudes during boreal summer. Regarding changes to the continental water balance, the simulations indicate that while total global drainage from rivers to the ocean remains roughly constant with increasing warming, regional differences intensify which may affect ocean freshening. Notably, in many world regions at least 66% of simulations agree on the direction of change relative to the pre‐industrial baseline already at current global warming levels.