Mismatch in reindeer resilience to past and future warming signals ongoing declines.

(caribou or reindeer) survived periods of abrupt climatic warming during the last deglaciation but are currently in global decline. Using process-explicit models of likely climate-human- interactions and inferences of demographic change from radiocarbon-dated fossils and ancient DNA, we reconstruct and decipher 21,000 years of population dynamics. These high-resolution population reconstructions pinpoint ecological characteristics and life-history traits that most likely enabled to survive rapid warming events following the Last Glacial Maximum. Projecting these process-driven models into the future reveals that these attributes are unlikely to buffer against wide-scale population declines from expected 21st Century climatic warming. Our findings highlight a need to boost investments in the management and conservation of , particularly in North America, where projected losses are expected to exceed 80%. This will not only support the survival of the species and the vital services it renders in Arctic ecosystems, but also help sustain the socioeconomic, cultural, and emotional well-being of many -dependent communities.