Hibernation evolved as a means of energy conservation during periods of resource scarcity. By depressing metabolic rate and body temperature, animals reduce energy demand and persist on endogenous and stored reserves. Arctic ground squirrels are extreme hibernators, exhibiting sub-zero body temperatures while hibernating in burrows that may be as cold as -20C. To survive such low ambient temperatures, arctic ground squirrels exhibit thermogenic torpor, which means they defend low torpid body temperature set points through the expenditure of fat reserves. Two reliable signals of climate change in the Arctic are soil warming and changes in the timing of seasonal soil freeze-thaw cycles. These changes have the potential to affect arctic ground squirrels, and other hibernators, by warming burrow temperatures and changing the duration of the sub-zero heterothermic season. Here we examine long-term records of soil temperature and the phenology of hibernation in free-living arctic ground squirrels collected at two field sites on the North Slope of Alaska for signatures of rapid climatic warming. We show that over the last twenty-five years, the duration of seasonally frozen soil has shortened. At the same time, the onset of sub-zero heterothermy is occurring later in hibernating ground squirrels. We discuss the implications of these results for hibernating animals and hibernation as a life history strategy given continued climate change.