Christine Ann Parisek, Jonathan A. Walter, Steven Sadro, Andrew Rypel

Freshwater ecosystems in mountain landscapes are increasingly threatened by climate change. Accumulated heat in ecosystems can result in lethal short-term heat exposure, while the velocity of change governs severity and rates of long-term heat exposure. Here, we novelly integrate heat accumulation and velocity of change approaches to classify climate-vulnerable USA mountain lake watersheds. Our results broadly demonstrate how rates of heat accumulation are increasing across mountain landscapes, and that this rise is most pronounced at lower elevations. We estimate 19% of mountain watersheds are currently at greatest vulnerability, and this value is set to jump to 33% by end-of-century. Further, mean killing degree days (i.e., mean number of days above 90th percentile) will increase 215 – 254% (mean = 236%) over this same timeframe. Taken together, results indicate heat accumulation will increase substantially over the next 75 years; changes will be experienced most severely in lower elevation landscapes and those with the greatest historical velocity of change. This degree of climate change will likely restructure species’ distributions. Decision-makers can utilize these classifications to understand landscapes likely to support desired species and ecosystem services into the future, thereby enabling more effective allocation of limited conservation resources.