In a warmer world, the hydrological cycle will change in intensity and in its geographic behaviour. This, in turn, will change patterns of river flood and the risk associated with them. The Last Interglacial (LIG; 125,000 years ago) is the most recent instance of climate warmer than today - especially in the high northern latitudes-, sea level was higher, ice sheets were smaller and monsoons were stronger. We use daily output from multi-century LIG simulations of an ensemble of paleoclimate models, and study how global precipitation patterns and extremes de­viate from the preindustrial climate. We validate these results by comparing them with the first compilation, to our knowledge, of global LIG precipitation patterns. Successively, we use the daily temperature and precipitation from the paleoclimate models to drive two global hydrological models (PCR-GLOBWB and CWATM), and simulate river discharges at 5-30’ resolution. With this, we force a hydrodynamic model, CaMa-Flood, and produce floods maps for different return periods. At the end of this model cascade, we look into what would happen if a climate similar to the LIG were to materialize in the coming decades: we combine the flood maps with maps of exposure through vulnerability relationships, and to calculate the risk that floods may pose to future people and assets.

We investigate hydrology during a past climate slightly warmer than the present: the Last Interglacial (LIG). With daily output of pre-industrial and LIG simulations from eight new climate models we force hydrological model PCR-GLOBWB, and in turn hydrodynamic model CaMa-Flood. Compared to pre-industrial, annual mean LIG runoff, discharge, and 100-year flood volume are considerably larger in the Northern Hemisphere, by 14%, 25% and 82%, respectively. Anomalies are negative in the Southern Hemisphere. In some boreal regions, LIG runoff and discharge are lower despite higher precipitation, due the higher temperatures and evaporation. LIG discharge is much higher for the Niger, Congo, Nile, Ganges, Irrawaddy, Pearl, and lower for the Mississippi, Saint Lawrence, Amazon, Paraná, Orange, Zambesi, Danube, Ob. Discharge is seasonally postponed in tropical rivers affected by monsoon changes. Results agree with published proxies on the sign of discharge anomaly in 15 of 23 sites where comparison is possible.