In 2013, a toddler was playing under a hollow tree in rural Guinea that happened to be occupied by bats carrying Ebola. The boy contracted the disease and died, becoming patient zero in an epidemic that killed close to 10,000 people. In 2003, a betacoronavirus, SARS, managed to hop from bats in China’s Yunnan Province to the city of Guangzhou. And to the best of our knowledge, sometime in 2019, a bat gave a different betacoronavirus to a wild mammal, which ended up in a market in Wuhan, setting off the COVID-19 pandemic.
Epidemiologists have focused a huge amount of attention on hunting down the moment those viruses made the interspecies leap. Which bats? When? But there’s another, broader question to be asked: Why do certain mammals bump into each other at all? And are there forces that make it more likely that a diseased bat ends up in a place where it can infect people?
In research published last week in the journal Nature, an international team of disease ecologists found that as climate change reshuffles the habitat of mammals, it will make them much more likely to swap viruses. Hotspots for such “first encounters” are disproportionately concentrated in places with lots of people, making it inevitable that some of those viruses will end up in humans.
The research is the latest attempt to link the global process of climate change to the ways it will play out in our daily lives—not just in weather disasters, but in infant mortality, crop health, and disease. “Certainly as we’re starting to get little bits of funding in climate change and health, we’re understanding that the breadth and depth of what we’re facing are even larger than what we’ve been saying,” says Kristie Ebi, who studies climate and human health at the University of Washington.
As the study showed, we’re already living at the peak of that great shuffle; animals are migrating towards cooler temperatures right now. So it’s plausible that viral-spillover events that have occurred in our lifetimes are the result of climate change—we just don’t know which ones.
The conclusions are based on three years of work on a global simulation of the habitats mammals depend on and the viruses they carry. The team began with a map of the habitats of almost 4,000 mammal species, and used it to predict how those ranges would change over time given different climate change scenarios. They then looked for spots where previously isolated species would become neighbors—what they called “first encounters.” Finally, they used a recently developed computer model to predict how likely those animals were to swap viruses.
Although every mammal in the simulation eventually bumped into a new neighbor, there will be hotspots for disease spillover. The bulk will occur in tropical mountains—particularly the highlands of East Africa and Southeast Asia—rather than around the poles. That’s because as species move northward, they tend to do so in lockstep with their current neighbors. But in mountains, animals from every valley and swamp in a region will head up into ever-tighter bands of habitable elevation. That means the already biodiverse tropics will see additional species packing themselves into highlands.
Those migrants will bring pathogens. At least 10,000 zoonotic viruses already thrive in mammalian hosts; as their hosts chase cool temperatures to higher altitudes, they’ll rub shoulders with new neighbors. A species of bat that had once lived in isolated limestone caves in the jungle lowlands might now occupy a mountain slope with other wild refugees. And that could create a fresh network for transmission.
According to the Nature study, bats will spread the majority of interspecies viruses for a simple reason: They fly and can range further in search of comfortable temperatures. Rodents could be a major reservoir as well.
The model shows that the ripest conditions for spillover extend from 2011 to 2040 as animals adjust to current warming. And even under relatively moderate warming scenarios, like the 2 degrees Celsius in the Paris agreement, these spillovers will continue.
So has climate change already caused diseases to leap hosts? The field of climate attribution science, which tries to link real-world events to human-caused global warming, has blossomed over the past decade. But most of its work has focused on the weather—a few days after a “heat dome” melted power lines and killed dozens across the Pacific Northwest last year, climatologists established that atmospheric shifts were almost certainly responsible.
Attributing disease outbreaks to climate change is much more challenging. Most of the existing research has focused on diseases spread by insects, like plague, malaria, or dengue. To do so, epidemiologists need to understand how both changing rainfall and heat shape mosquito colonies, says Ebi. And just because a mosquito exists somewhere doesn’t mean it will transmit disease in that place. “You have to take all this into account when doing these kinds of analyses, which makes them much more complicated than talking about how many people are dying in a heat wave,” Ebi says.
It’s easiest to see the fingerprints of climate change in rare diseases, Ebi says. The spread of ticks and their diseases into southern Canada is a clear example. “There wasn’t Lyme disease in Canada a couple of decades ago,” she explains. “There is today.”
The new Nature paper similarly looks for cases on the fringe that are attributable to climate. In a recent event, a team of veterinary epidemiologists found that a distemper virus spread from Atlantic seals to Pacific sea otters after Arctic sea ice melted and the two mammals were able to mingle. And as flying foxes in Australia moved south over the last century, they apparently passed the zoonotic Hendra virus on to domestic horses.
Other climate-driven epidemics will be harder to spot—maybe animals will cross paths more, rather than meeting for the first time. The team called its simulation “ICEBERG,” as in, these first encounters are just the tip of the problem. And while the authors don’t outright connect the recent Ebola and SARS epidemics to the climate crisis, they do point out that the spillovers are probably the result of a combination of human forces, like deforestation and urbanization, that have put people in closer contact with wild animals.
What makes this kind of attribution even harder is that unlike heat waves, the world is unlikely to see spillovers as they occur. One reason that researchers have focused on insect-borne diseases, says Ebi, is that there’s money available to understand mosquitoes. “That obviously has consequences for what you study, because there’s so little funding available,” she explains. “It doesn’t mean that what we’re researching at the moment isn’t a high priority—but it is where you’re able to get some funding out of federal agencies.”
In a hearing before Congress the day the Nature paper released, one of its lead authors, Colin Carlson, a disease ecologist at Georgetown University, asked lawmakers to invest in systems for monitoring zoonotic pathogens and centralizing the information. “Our field is currently headed into something of a scientific revolution,” he said. “That vision offers renewed hope the COVID-19 pandemic could, in fact, be the last one.”
The study should serve as a wake-up call: We can see exactly how warming and habitat loss will introduce us to thousands of new viruses. Soon, we might be able to tell which outbreaks were caused by climate change itself. But either way, we can get ahead of the problem if we choose a well-informed path.