Abstract: Subarctic mountain environments in northwestern North America are experiencing rapid changes in climate. For example, over the past 30 years winter temperatures have increased by up to 6º C, resulting in less predictable winter and spring snowpack conditions. However, the impacts of climate variability and change on northern alpine ecosystems are poorly understood. I will outline the emerging results of a long-term study in the southwest Yukon that is providing new insight into the responses of alpine plants and animals to climate change, with emphasis on three mammalian herbivores. Populations of collared pikas (Ochotona collaris) declined dramatically during uncharacteristically warm winter conditions in 1998/99, and have remained at very low numbers. Hoary marmots (Marmota caligata) numbers have remained relatively constant over several years, but respond to interannual climatic variability by reduced female reproductive success and juvenile survival in years with delayed snowmelt. For Dall sheep (Ovis dallii), both reproduction and horn growth are highly correlated with variation in climate conditions that likely influence availability and quality of summer forage. Horn growth in particular shows regular periodic variation consistent with known ‘11-year cycles’ in precipitation. Experimental studies have shown that plant productivity is correlated with the timing of snow melt, but much less with summer warming. Thus, although mammals living in northern alpine environments appear to be well adapted for cold conditions, climate change may create problems for some species. A warming climate is apparently lethal for pikas and juvenile marmots, and sublethal for adult marmots and Dall sheep in the Yukon.