Observational uncertainties in snow mass have made the detection and attribution of human-forced snow losses elusive. This study shows that human-caused warming has caused declines in Northern Hemisphere-scale March snowpack (1981-2020). Using an ensemble of snowpack reconstructions, robust snow trends were identified in 82 out of 169 major Northern Hemisphere river basins, with 31 confidently attributed to human influence. A highly nonlinear temperature sensitivity of snowpack was revealed, where snow becomes more sensitive to warming above -8°C. This nonlinearity explains the lack of widespread snow loss so far and predicts sharper declines and water security risks in populous basins. Human-forced snow losses and their water consequences will accelerate with near-term warming, posing significant risks to water resources.

Alexander R. Gottlieb, Justin S. Mankin