Under a high-emissions scenario, we find that 35% of mammals and 29% of birds are projected to have over half of their 2070 climatic niche in countries in which they are not currently found.

We map these transboundary range shifts globally, identifying borders across which international coordination might most benefit conservation and where physical border barriers, such as walls and fences, may be an overlooked obstacle to climate adaptation.

The construction of such barriers can disturb or destroy habitats, fragment populations, prevent dispersal and migration, and directly or indirectly increase mortality via entanglement, poaching, and predation.

For example, border security fencing in Central Asia is likely to be impeding ungulate migrations (11, 41), while recently erected razor-wire security fencing along the Slovenia–Croatia border has increased mortality in herons and ungulates.

Barriers along stretches of the United States–Mexico border were found to de- crease the abundance of puma (Puma concolor) and coati (Nasua narica) (43), and the planned extension of this barrier is likely to prevent the re-establishment of dwindling or recently extirpated populations of endangered species in the United States, such as the Mexican gray wolf (Canis lupus baileyi) and Sonoran pronghorn (Antilocapra americana sonoriensis).

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Finally, maintaining and enhancing habitat connectivity across borders between area-based conservation measures will be critical to enable range shifts under climate change, and this effort should be targeted to the regions in which it will have the most impact. We have shown that this is likely to be where borders cut across broad climatic gradients in species- rich areas, such as around the tropical Andes and Amazon, the Himalayas, and east–west borders such as the United States– Mexico border. Where border security barriers are a threat to this ecological connectivity, particularly along the United States–Mexico border and parts of Asia, we must implement appropriate measures.