Recent Increases in Missouri River Streamflow Driven by Combined Effects of Climate Variability, Land-Use Change, and Elevated CO2

Missouri River streamflow increased substantially during the 20th century, with multiple large floods occurring since 1990. Using land surface models and water budget simulations, we examined the extent to which increased flow was driven by natural climate variability, anthropogenic climate trends, land-use and land-cover change (LULCC), and ecological effects of elevated atmospheric CO₂. Natural climate variability (arising largely from coupled ocean-atmosphere circulation systems in both the Pacific and North Atlantic) accounted for ∼765 m³ s⁻¹ of the ∼900 m³ s⁻¹ increase in flow since mid-century, while anthropogenic climate trends negatively forced flow by increasing evapotranspiration more than precipitation. LULCC and elevated CO₂ further increased simulated mean streamflow by ∼550 and ∼70 m³ s⁻¹, respectively, relative to pre-Industrial conditions and ∼100 and ∼65 m³ s⁻¹ relative to mid-20th century conditions. The LULCC effect was especially large in wet years, implying that current land cover is ill-suited for buffering against extreme precipitation, likely in large part due to replacement of forest by cropland in the lower basin. Because increases in Missouri River flow over the past century were driven mostly by a recent (and likely transient) pluvial, our results suggest that flow in the basin could revert to a drier mean state when that pluvial ends, likely made worse by increased evaporative demand from anthropogenic warming.