During the Medieval Climate Anomaly, Western North America experienced episodes of intense aridity that exceeded multiple decades, “megadroughts” more persistent than any event over the last century. These droughts were caused by natural variations of the ocean-land-atmosphere system, and there is concern regarding how likely they are to occur again in the future. In my talk, I will demonstrate that climate change will dramatically increase the risk of a megadrought occurring during the latter half of the 21st century. Importantly, this increase in drought risk will not be forced by relatively uncertain precipitation changes, but primarily through increases in evapotranspiration from the robust warming expected in response to increased greenhouse gas concentrations. This represents a fundamental shift in the major controls on drought dynamics and trends from a situation where variability is supply (precipitation) dominated to one where demand (evapotranspiration) is the main driver. Further, I will highlight ongoing drought events where these mechanisms are beginning to emerge.