Significance Climate change is altering the seasonal environmental conditions to which animals have adapted, but the outcome may differ between seasons for a particular species. Demographic responses therefore need disentangling on a seasonal basis to make accurate forecasts. Our study shows that climate change is causing seasonally divergent demographic responses in a hibernating mammal. Continued climate change will likely have a positive effect on summer survival but a negative effect on winter survival. This potentially has wide-ranging consequences across other species occupying temperate to more extreme arctic and alpine habitats, which are also where the most rapid changes in climate are observed.