14Between 10,000 and 600,000 species of mammal virus are estimated to have the 15 potential to spread in human populations, but the vast majority are currently cir-16 culating in wildlife, largely undescribed and undetected by disease outbreak surveil-17 lance 1,2,3 . In addition, changing climate and land use drive geographic range shifts 18 in wildlife, producing novel species assemblages and opportunities for viral sharing 19 between previously isolated species 4,5 . In some cases, this will inevitably facilitate 20 spillover into humans 6,7 -a possible mechanistic link between global environmental 21 change and emerging zoonotic disease 8 . Here, we map potential hotspots of viral 22 sharing, using a phylogeographic model of the mammal-virus network, and projec-23 tions of geographic range shifts for 3,870 mammal species under climate change and 24 land use scenarios for the year 2070. Shifting mammal species are predicted to ag-25 gregate at high elevations, in biodiversity hotspots, and in areas of high human pop-26 ulation density in Asia and Africa, sharing novel viruses between 3,000 and 13,000 27 times. Counter to expectations, holding warming under 2°C within the century 28 does not reduce new viral sharing, due to greater range expansions-highlighting 29 the need to invest in surveillance even in a low-warming future. Most projected vi-30 ral sharing is driven by diverse hyperreservoirs (rodents and bats) and large-bodied 31 predators (carnivores). Because of their unique dispersal capacity, bats account for 32 the majority of novel viral sharing, and are likely to share viruses along evolutionary 33 pathways that could facilitate future emergence in humans. Our findings highlight 34 the urgent need to pair viral surveillance and discovery efforts with biodiversity 35 surveys tracking range shifts, especially in tropical countries that harbor the most 36 emerging zoonoses. 37 2 Main Text 38In the face of rapid environmental change, survival for many species depends on moving 39 to track shifting climates. Even in a best case scenario, many species are projected 40 to shift a hundred kilometers or more in the next century 9,10 . In the process, many 41 animals will bring their parasites and pathogens into new environments 4,11 , creating new 42 evolutionary opportunities for host jumps 8 . Most conceptual frameworks for cross-species 43 transmission revolve around how these host jumps facilitate the spillover of new zoonotic 44 pathogens into humans 12,13,14 , but viral evolution is an undirected process 15 , in which 45 humans are only one of over 5,000 mammal species with over 12 million possible pairwise 46 combinations 16 . Despite their indisputable significance, zoonotic emergence events are 47 just the tip of the iceberg; almost all cross-species transmission events will occur among 48 wild mammals, largely undetected and mostly inconsequential for public health.
49Of the millions of possible pairwise viral exchanges, the vast majority are biologically 50 implausible, as host specie...
