The recent decrease in the amount of mountain snow, or snowpack, in the western United States is due to a combination of natural variability and warming induced by human (anthropogenic) activities. This finding is reported in Nature Communications this week. Future projections under a business-as-usual climate scenario suggest a further loss of around 30% on average in mountain snowpack in the next three decades.

Mountain snowpack in the western United States has decreased in the past few decades and this has been linked to warming over the North American continent. However, the exact contribution of human activities and natural variability to this decrease is poorly understood.

John Fyfe and colleagues simulate historical snowpack with a climate model in two parallel configurations: one with only contributions from natural variability and a second which includes both contributions from natural variability and anthropogenic activities. The models accurately reproduce observations of snowpack from 1982-2010 when both natural and anthropogenic influences on the climate are considered, but not when only natural variability is applied. Future projections with both natural and anthropogenic forcing under a business-as-usual scenario show an average snowpack loss of 30% by 2040, although estimates of changes in snowpack ranged from a 3% gain to losses of up to 60%.

Mountain snow cover in the western United States is a critical water resource. Understanding changes in snowpack will help policymakers plan effectively and minimize impacts on hydropower, municipal and agricultural sectors in the region.