As climate change permanently alters our environment, the world is increasingly opening up to new viruses — with potentially deadly consequences for us humans. A study published Thursday in Nature finds that as climate change is forcing animals to move habitats, they will increasingly come into contact with humans, and with each other, creating more and more opportunities for deadly viruses to mutate and spill over to people.
“Species are going to have to move if they want to track shifting climates,” Colin Carlson, the study’s lead author and an assistant research professor at Georgetown University Medical Centre, said in an email. “When they do, they’ll meet for the first time and share viruses. Our simulations suggest that in the next half-century, this process will completely restructure the global mammal-virus network. That’s bad news for human health.”
While there’s a large body of research on how climate change can shape epidemics, a lot of that work is focused on vector-borne diseases — diseases like malaria, dengue, Zika, and yellow fever that are transferred to humans by blood-feeding insects like ticks and mosquitoes. Hardly any scholarly work has actually looked at how climate could impact how viruses jump from wild animals to humans, also known as zoonotic spillover. Between 60% and 75% of infectious diseases were initially transferred from wild animals to humans; there are currently thousands of virus species with the capacity to sicken humans silently infecting various animals, according to the paper.
The study uses a huge amount of data — on viruses and host mammals as well as on climate change and animal habitats — to create an enormous map of how the habitats of more than 3,100 mammal species might change over the coming decades. As habitats shift, chances increase that different species will cross paths more with each other and with us, and viruses and other pathogens will be along for the ride. In the 2003 SARS outbreak, for instance, research suggests that civet cats, which are eaten in China, may have acted as an intermediary host for the virus, helping it cross from bats to humans. And under a changing climate, bats especially could come into contact more frequently with different animal species, creating new opportunities for viruses to spread.
“Because they can fly, we expect bats will be able to travel the farthest and fastest, and so drive most of this process,” Carlson said.
As a result of these widening habitats, new geographic “hotspots” will emerge: places where potential epidemics and pandemics can be born. For example, ebola outbreaks have traditionally clustered in western African countries, but the study finds that by 2070, ebola outbreaks could be much more common in east Africa. “Climate change is going to create innumerable hotspots of overlap between elevated spillover risk and human populations,” Carlson said.
And we’re facing an uphill battle. The world has already warmed 1.2 degrees Celsius (2.2 degrees Fahrenheit) above preindustrial levels; the process of animals changing habitat and coming into contact with other species, Carlson explained, has already started. What’s more, mitigation, or slowing warming down, might actually exacerbate the problem.
“In extreme warming scenarios, species lose habitat so quickly they go extinct before they have the opportunity to share their viruses in new ecosystems,” Carlson said. “Mitigation slows down the speed at which their habitats move, and gives them a more manageable task — and so it’s easier to get where they’re going, and share viruses when they get there.”
It’s tough to draw a straight line between any given pandemic and climate change, since there are a myriad of factors at play with each outbreak. But this research shows that staying safe will mean keeping a much closer eye on diseases in wildlife.
“We’re committed to a world where climate change might become the dominant driver of pandemic risk (if it’s not already), even with the best-case scenario for climate change,” Carlson said. “It’s urgent that we think about wildlife disease surveillance and outbreak detection as climate change adaptation strategies.”