In 2013, a toddler was playing under a hollow tree in rural Guinea, which happened to be occupied by Ebola-carrying bats. The boy contracted the disease and died, becoming patient zero in an epidemic that killed nearly 10,000 people. In 2003, a betacoronavirus, SARS, managed to jump to the city of Guangzhou from bats in China’s Yunnan province. And to to the best of my knowledgeSometime in 2019, a bat gave another betacoronavirus to a wild mammal that landed in a market in Wuhan, sparking the COVID-19 pandemic.
Epidemiologists have turned a lot of attention to chasing the moment these viruses made the leap between species. Which bats? When? But there’s another, broader question that needs to be asked: Why do certain mammals bump into each other in the first place? And are there forces that make it more likely that a sick bat will end up in a place where it can infect humans?
In a study published in last week the diary Nature, an international team of disease ecologists found that climate change is reshaping the habitat of mammals and making them more likely to exchange viruses. Hotspots for such “first encounters” are disproportionately concentrated in crowded places, making it inevitable that some of these viruses will end up in humans.
The research is the latest attempt to link the global process of climate change to impacts on our daily lives – not just in weather disasters, but also in child mortality, crop health and disease. “Certainly, as we begin to receive small-scale funding in the areas of climate change and health, we understand that the breadth and depth of what we are facing is even greater than what we have said,” says Kristie Ebi, who studies climate and human health at the University of Washington.
As the study has shown, we are already living at the peak of this great upheaval; Animals are migrating to cooler temperatures. So it’s plausible that viral spillover events that have occurred in our lifetime are the result of climate change – we just don’t know which ones.
[Related: We already know how to keep the next pandemic from catching us off guard]
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 started with a map of the habitats of nearly 4,000 species of mammals and used them to predict how these ranges would change over time under different climate change scenarios. Then they looked for places where previously isolated species would become neighbors – what they called “first encounters”. Finally, they used a recently developed computer model to predict how likely these animals are to exchange viruses.
Though each mammal in the simulation encountered a new neighbor at some point, there will be hotspots for disease spread. The majority will be found in tropical mountains – particularly in the highlands of East Africa and Southeast Asia – and away from the poles. That’s because when species move north, they tend to do so in lockstep with their current neighbors. But in the mountains, animals from every valley and swamp in a region climb to increasingly narrow, habitable heights. This means that in the already species-rich tropics, more species will crowd into the highlands.
These migrants bring pathogens with them. At least 10,000 zoonotic viruses already thrive in mammalian hosts; As their hosts chase cool temperatures to higher elevations, they will meet up with new neighbors. A species of bat that once lived in remote limestone caves in the jungle lowlands may now be occupying a mountainside with other wild escapees. And that could create a new network for transmission.
According to the Nature Study, bats will spread the majority of viruses for one simple reason: They fly and can spread further in search of comfortable temperatures. Rodents could also be an important reservoir.
The model shows that the most mature conditions for spillovers range from 2011 to 2040, as animals adapt 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 skip the host? The field of climate attribution science, which attempts to link real-world events to human-caused global warming, has flourished over the past decade. But most of his work has focused on the weather — days after a “heat dome” melted power lines and killed dozens in the Pacific Northwest last year, established climatologists that atmospheric changes were almost certainly responsible.
Attributing disease outbreaks to climate change is much more difficult. Most of the existing research focuses on insect-borne diseases such as plague, malaria or dengue fever. To do this, epidemiologists need to understand how both changing rainfall and heat shape mosquito colonies, says Ebi. And just because there’s a mosquito somewhere doesn’t mean it carries disease there. “You have to take all of that into account in such analyses, which makes it much more complicated than talking about how many people die in a heat wave,” says Ebi.
It’s easiest to see the fingerprints of climate change in rare diseases, says Ebi. That Spread of ticks and their diseases in southern Canada is a clear example. “A few decades ago, there was no Lyme disease in Canada,” she explains. “Today there are.”
The new Nature Paper similarly searches for edge cases attributable to climate. In a recent event, a team of veterinary epidemiologists found this out a distemper virus that spread from harbor seals in the Atlantic to sea otters in the Pacific Ocean after the Arctic sea ice melted and the two mammals were able to mix. And how Fruit bats in Australia migrated south Over the last century they seem to have died zoonotic hendra virus on domestic horses.
[Related: The animal kingdom is full of coronaviruses. Here’s what that means for COVID’s future.]
Other climate-related epidemics will be harder to detect — perhaps animals’ paths cross more often than they meet for the first time. The team named their simulation “ICEBERG” because these first encounters are only the tip of the problem. And while the authors don’t directly link the recent Ebola and SARS epidemics to the climate crisis, they do point out that the encroachments are likely the result of a combination of human forces, such as deforestation and urbanization, that have brought people into closer contact with wild animals.
What makes this type of attribution even more difficult is the fact that, unlike heat waves, the world is unlikely to see spillovers when they do occur. One reason researchers have focused on insect-borne diseases, Ebi says, is because there is money available to understand mosquitoes. “Of course, that has consequences for my studies, because there is so little money,” she explains. “That doesn’t mean that what we’re researching right now isn’t a high priority — but it’s where you can get some money from federal agencies.”
In a hearing before Congress the day of Nature published, one of the lead authors, Colin Carlson, a disease ecologist at Georgetown University, urged lawmakers to invest in systems to monitor zoonotic pathogens and centralize the information. “Our field is currently on the path to some kind of scientific revolution,” he said. “This vision gives renewed hope that the COVID-19 pandemic may indeed be the last.”
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 may be able to tell which outbreaks were caused by climate change itself. But in any case, if we choose a well-informed path, we can forestall the problem.