Due to the disappearance of its sunlight-reflecting seasonal snowpack, the Colorado River Basin is losing more water to evaporation than can be replaced by precipitation, researchers report. The study resolves a longstanding disagreement in previous estimates of the river's sensitivity to rising temperatures and identifies a growing potential for severe water shortages in this major basin. The Colorado River, a water source that supplies water to roughly 40 million people and supports more than $1 trillion of economic activity each year, is dwindling. Just as has happened for many rivers feeding water-stressed regions across the globe, increased drought and warming have been shrinking the Colorado River's flow for years. However, the sensitivity and response of river discharge to climate warming remain poorly understood. As a result, critical projections of water availability under future climate-warming scenarios are highly uncertain. Unlike in previous modeling efforts, Christopher Milly and Krista Dunne developed a hydrologic model that accounts for the balance of energy between incoming solar radiation and the albedo of snowy surfaces. Bright snow and ice have high albedo, meaning they reflect solar energy back into space. Combined with historical measurements, Milly and Dunne demonstrate how snow loss in the Colorado River Basin due to climate warming has resulted in the absorption of more solar energy, boosting evaporation of water throughout the basin. The authors estimate that this albedo-loss driven drying has reduced runoff by 9.5% per degree Celsius of warming. The results suggest that this drying will outpace the precipitation increases projected for a warmer future.