Although it is widely accepted that future climatic change—if unabated—is likely to have major impacts on biodiversity, few studies have attempted to quantify the number of species whose populations have already been impacted by climate change. Using a systematic review of published literature, we identified mammals and birds for which there is evidence that they have already been impacted by climate change. We modelled the relationships between observed responses and intrinsic (for example, body mass) and spatial traits (for example, temperature seasonality within the geographic range).