Recent High Missouri River Basin Runoff Was Unlikely Due to Climate Change

October-September runoff increased 6% and 17% in the Upper (UMRB) and Lower (LMRB) Missouri River Basin, respectively, in a recent (1990-2019) compared to past (1960-1989) climate. The runoff increases were unanticipated, given various projections for semi-permanent drought and/or aridification in the North American Great Plains. Here, five transient coupled climate model ensembles are used to diagnose the effects of natural internal variability and anthropogenic climate change on the observed runoff increases, and to project UMRB and LMRB runoff to the mid-21st century. The runoff increases observed in the recent compared to the past climate were not due to anthropogenic climate change, but resulted mostly from an extreme occurrence of internal multi-decadal variability. High runoff resulted from large, mostly internally-generated, precipitation increases (6% in the UMRB and 5% in the LMRB) that exceeded simulated increases due to climate change forcing alone (0-2% inter-model range). The precipitation elasticity of runoff, which relates runoff sensitivity to precipitation differences in the recent compared to the past climate, led to 1-3-fold and 2-4-fold amplifications of runoff versus precipitation in the UMRB and LMRB, respectively. Without the observed precipitation increases in the recent compared to the past climate, effects of human-induced warming of about 1°C would alone have most likely induced runoff declines of 7% and 13% in the UMRB and LMRB, respectively. Ensemble model simulations overwhelmingly project lower UMRB and LRMB runoff by 2050 compared to 1990-2019, a change found to be insensitive to whether individual realizations experienced high flows in the recent climate.