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Covid-19 is not the first disease to migrate from animals to humans. The Black Death of the 14 century was spread by fleas (跳蚤). The H1N1 virus behind the epidemic of 1918 originated in birds. The list goes on.

In the last two decades alone, we’ve been haunted by zoonotic (人畜共患的) outbreaks, including SARS, MERS, Ebola, bird flu, and most recently, COVID-19 and monkey-pox. Some researchers are beginning to call this era a Pandemicene. A study published in the journal Nature in April suggests that climate change may be to blame.

“The study comes from a simple idea,” says Colin Carlson, a global change biologist and a lead author of the paper. Climate change is making many habitats inhabitable. When animals migrate toward cooler climates, they introduce viruses to new environments. “That leads to interesting problems from a viral standpoint,” says Carlson.

To predict displacement patterns, Carlson and his colleagues taught machine-learning models the kinds of habitats used by different mammals. The modeling software then predicted, under a range of possible climate futures, where such conditions might be found in 2070, he explains. Many animals will head to cooler mountains, or to coastal regions with mild climates; Indonesia, the Philippines, India’s Rift Valley, and Sahel region are all expected to have sky-high human population densities and biodiversity levels by the year 2070, making them easy to be affected by outbreaks of zoonotic disease.

Carlson and his colleagues paired these predictions with a model that analyzes why different animals share viruses. The same flight capabilities that make bats excellent mosquito-hunters and seed-spreaders, for example, also make them more likely to transmit diseases. And while it’s easier for viruses to spread between closely related species, even viruses for our distant relatives can find purchase in us, given enough opportunities for infection.

Carlson and his colleagues estimate that by 2070, there will be at least 3,000 instances of novel cross-species infection. Not all will become pandemics, but some will.