River flow in the near future: a global perspective in the context of a high-emission climate change scenario

Abstract. There is high confidence that global warming intensifies all components of the global water cycle. Our goal is to investigate the possible effects of the global warming on river flows worldwide in the coming decades. We conducted 18 global hydrological simulations to assess how the river flows are expected to change in the near future (2015–2050) compared to the recent past (1950–2014). The simulations are forced by runoff from HighResMIP-CMIP6 GCMs, which assume a high-emission scenario for the projections. The assessment includes estimating the signal-to-noise (S/N) ratio and the time of emergence (ToE) of all the rivers in the world, with further evaluation of those presenting significant departures from their historic mean flow. Consistent with the water cycle intensification, the hydrological simulations project a clear positive global river discharge trend from ~2000, that emerges beyond the levels of natural variability and becomes 'unfamiliar' by 2017 and 'unusual' by 2033. This climate change signal is dominated by strong increases in flows of rivers originating in central Africa, east Russia, Alaska and Greenland. African rivers project most future annual cycles above the climatological annual cycle, with the largest differences occurring during peak flows. Recent unprecedent floods in the Republic of Congo, D.R.C., Nigeria, and Chad highlight the potential catastrophic consequences of these changes in metropolitan areas. However, the positive trend of Lake Chad tributaries may aid its recovery from the strong reduction observed since the 1970s. Lastly, the projected Nile streamflow rise reinforces the need for collaboration in dam management. The simulated and observed extra release of freshwater into the Arctic Ocean produces a freshening of the ocean, potentially impacting the global ocean overturning circulation. It is concerning that several important rivers are projected to exceed their natural variability. However, the hydrological predictions assume a very high baseline emission scenario and should be interpreted as an upper limit for decision-making.