According to a study published Thursday in Nature, over the next 50 years, climate change will prompt thousands of viruses to jump from one species of mammal to another. The shuffling of viruses between animals may increase the risk of jumping into humans and causing another pandemic, the researchers said.
Scientists have long warned that a warming planet could increase the disease burden. Malaria, for example, is expected to spread as the mosquitoes that carry it expand their reach into warming regions. But climate change could also usher in entirely new diseases by allowing pathogens to penetrate new host species.
“We know that species are moving, and when they do, they get an opportunity to share viruses,” said Colin Carlson, a biologist at Georgetown University and co-author of the new study.
To understand what that sharing will look like, Dr. Carlson and his colleagues built a computer model of possible spillovers in a warming world. The researchers began projecting how thousands of mammals could shift their habitat if the climate changes between now and 2070.
As temperatures rise, many species are expected to spread further from the scorching equator to find more comfortable habitats. Others may venture up the slopes of hills and mountains to find cooler heights. When different species first come into contact with each other, the viruses may be able to infect new hosts.
To understand the likelihood of a successful new infection, the researchers started by building a database of viruses and their mammalian hosts. Some viruses have been found in more than one mammalian species, meaning they must have jumped the species barrier sometime in the past.
Using a computer technique called machine learning, the researchers developed a model that could predict whether two host species share a virus.
The more two species overlap geographically, the researchers found, the more likely they would share a virus. That’s because the hosts encountered each other more often, giving their viruses more opportunities to move between them.
dr. Carlson and his colleagues also showed that closely related species were more likely to share a virus than distant relatives. That’s probably because closely related mammals are similar in their biochemistry. A virus adapted to exploit one species is more likely to thrive in a family member. It may also be able to evade an immune system similar to one it’s already adapted to.
These findings enabled Dr. Carlson and his colleagues are able to make predictions about what would happen when mammal species converge for the first time in a warmer world.
Of the 3,139 species studied, the researchers expected more than 4,000 cases of viruses passing from one species to another. In some cases, just one virus will make the jump. But the models also predicted that multiple viruses carried by one species would spread to another.
The researchers couldn’t say exactly which viruses would move between which species. What matters, they argued, is the sheer magnitude of what is to come.
“When you try to predict the weather, you’re not predicting individual raindrops,” said Christopher Trisos, an ecologist at the University of Cape Town and co-author of the new study. “You predict the clouds yourself.”
Rachel Baker, a disease ecologist at Princeton University who was not involved in the study, said the study was an important step forward in understanding how climate change will affect the world’s dangerous viruses. Previous studies have focused on individual viruses as opposed to examining the entire world.
“It’s great progress,” she said. “We want to know as soon as possible whether there is a link between climate change and the spread of pathogens.”
Bats in Southeast Asia, in particular, will be susceptible to these transfers, the researchers found. Many bat species in that region are currently restricted to small ranges and do not interact much. But as the planet warms, these bats will soon fly to suitable climates and meet new species.
These findings could be particularly ominous for humans. As viruses move into new host species, they evolve — and could potentially evolve in ways that make them more likely to infect humans. The coronavirus that caused SARS in 2002 originated in Chinese horseshoe bats and then jumped to another species — possibly raccoon dogs sold in Chinese animal markets — before infecting humans.
In February, scientists released two studies claiming that Covid arose from a similar series of events, with a coronavirus leaping from bats to wild mammals sold in markets in Wuhan before infecting humans.
“We think a lot of this could happen as a result of the interspecific transmission events that we predict,” said Gregory Albery, a disease ecologist at Georgetown University and a co-author on the new study.
When the researchers looked at the places mammals might end up in 2070, they found another reason to expect new human epidemics: They won’t migrate to wildlife sanctuaries. “It turns out that these are all the places where we’ve built cities,” said Dr. Carlson.
A rare rodent that has little contact with humans today can transmit a virus to raccoons, which live comfortably in urban areas. “That opens up a whole new avenue for this virus to spread to humans,” said Dr. albery.
dr. Christine Johnson, an epidemiologist at the University of California, Davis, who was not involved in the study, warned that such a broad model cannot account for details that could have a major impact on individual viruses. “We need locally grounded field studies to understand the effects of climate on species movements and the risk of disease transmission,” she said.
Climate-driven spillovers could start well before 2070. After all, the planet is already 1.1 degrees Celsius warmer than in the 19th century. In their computer model, the researchers found that there has already been enough climate change to confuse viruses, although their model doesn’t let them point to certain viruses that have jumped.
“The amount of warming we’ve had has been enough to set it in motion,” said Dr. Carlson.