Climate-Driven Changes to Suspended-Sediment Yields by the End of the Century

Anticipated changes in climate by the end of this century are likely to modify suspended-sediment yields (S_y) in diverse ways. Past work has shown how hydrological non-stationarity may alter water discharges and hence S_y, but less attention has been given to the impact of likely future changes in upland sediment-detachment rates on downstream S_y. In certain environments, climatically driven changes in vegetation cover on upland hillslopes may more than counteract the effects of changing runoff on S_y. Changes in precipitation, temperature, and vegetation may, therefore, interact in nonlinear ways to produce unexpected changes. In this work, we simulated future changes to background S_y (i.e., changes unrelated to land-use changes and dams) with climatological and vegetative data output from an ensemble of CMIP6 Earth System Model (ESM) simulations. Depending on the future scenario, the cumulative annual sediment flux of 780 globally distributed rivers increases by between 2.3% and 8.4%. Significant deviations from historical S_y are projected at high latitudes in response to each forcing variable, while low-latitude responses are regionally varied. In regions where ensemble members agree on future changes in forcing variables, large S_y changes are forecast with high confidence (e.g., >200% S_y increase for several northeastern U.S. rivers at the 95% level). In contrast, ensemble variability in vegetation projections results in considerable uncertainty in the projected S_y of rivers in other regions. Further improvements to the vegetation components of ESMs will help to reduce regional uncertainties in projected changes to S_y.