Long before the world had ever heard of Covid-19, Colin J. Carlson and a team of researchers began work on a study that explored how climate change and the destruction of wildlife habitats might affect how diseases are spread from animals to people. Their first draft included a reference to a hypothetical pneumonia outbreak of unknown origin.

“We know that species are on the move—we know that probably has relevance to other viruses,” said Carlson, an assistant professor of biology at Georgetown University. “And for a while, we really wanted to get into what does that mean for human health? What does it mean for pandemics?”

What it meant, Carlson and his co-authors found, was that it may already be too late to limit the spread of zoonotic spillover—when diseases move from animals to humans—because of climate change.

Over the next 50 years, the spread of pathogens between humans and animals in the wild will lead to the transmission of about 4,000 new viruses between species, their research found, and increase the risk of global pandemics.

“We can’t put this one back in the bottle,” Carlson said of his team’s findings.

In recent weeks, the epidemiological world has been focused on a global uptick in cases of monkeypox. Discovered in the 1950s, the virus that causes monkeypox—which was first identified in research primates—is in the same epidemiological family as smallpox. The virus is common in parts of Africa, is spread through close contact and it is treated using antiviral medicines. The smallpox vaccine is also effective in treating monkeypox.

While public health officials say that it has little chance of becoming a pandemic, the world will increasingly have to contend with monkeypox and as-yet undiscovered diseases like it as the destruction of natural habitats—a driver of climate change—brings humans and wild animals in closer contact, according to Carlson’s team. 

The Centers for Disease Control and Prevention is on alert after the emergence of monekypox cases in the United States over the last month. While it may not become the next pandemic — and CDC officials say the risk to the general population is low — public health experts ask that people seek medical attention if they develop a rash, fever or chills. Officials are also asking people to avoid contact with sick people, wear a mask and steer clear of wild animals, dead or alive.

Interactions between people and animals were at the heart of the findings of Carlson’s team, which were published in a peer-reviewed study in the journal Nature in April. It noted the existence of roughly 10,000 viruses with the potential to infect humans—the vast majority of which, researchers said, are already “circulating silently in wild mammals.” Global climate change and evolving land-use patterns will increase the potential for cross-species viral transmission as animals that were once geographically isolated begin to have increased contact with people, the study said. 

One of the most surprising findings of the study, Carlson said, is not just that mitigation doesn’t keep this from happening, but that “a lot of this has probably already happened because we live in a world that’s one degree warmer.”

“We are going to just have to deal with the fact that climate change, as a choice that we’ve already made to some degree, means higher pandemic risk,” he said.

In the study, he and his colleagues wrote: “Whereas most studies agree that climate change mitigation through reducing greenhouse gas emissions will prevent extinctions and minimize harmful ecosystem impacts, our results suggest that mitigation alone cannot reduce the likelihood of climate-driven viral sharing. Instead, the mildest scenarios for global warming appear likely to produce at least as much or even more cross-species viral transmission.”

And there may be more than 4,000 viruses that are shared.

Carlson said in the study they counted the number of times two species that have never met shared viruses for the first time. He said “that could be one virus or it could be all of their viruses.”

“So when we’re saying 4,000, what we mean is there are going to be 4,000 pairs of species sharing viruses for the first time, and that could be 4,000 cross-species transmission. It could be 400,000,” he said. “We just don’t know.”

Carlson said a key takeaway from the research is the importance of monitoring diseases in wildlife and tracking early outbreaks so they don’t evolve into pandemics.

“The goal now is not to change what’s happening in these ecosystems—there’s not a ton we can do about that—but rather to learn to live more safely alongside wildlife,” he said.

Carlson and his co-authors cautioned that the results “should not be interpreted as a justification for inaction, or as a possible upside to unmitigated warming, which will be accompanied by mass defaunation, devastating disease emergence, and unprecedented levels of human displacement and global instability.

“Rather, our results highlight the urgency of better wildlife disease surveillance systems and public health infrastructure as a form of climate change adaptation, even if mitigation efforts are successful,” the study said. 

That notwithstanding, scientists say, it is important to remain vigilant and to continue to monitor how animals migrate to new areas as their existing habitats warm or are razed for development, conditions that create opportunities for zoonotic spillover.

“It’s not entirely surprising that as habitats shrink and the climate warms, you see greater chances for animals to bump into each other—especially animals that have not historically been in contact with each other—and that creates interfaces where pathogens can move from one species to another,” said Aaron Bernstein, the interim director of the Center for Climate, Health, and the Global Environment at Harvard T.H. Chan School of Public Health.

“What it means for us is that we have to really think hard about how we approach emerging infections like Covid-19,” said Bernstein. “And part of that has to take seriously the need to prevent spillover rather than trying to play catch-up.”

Right now the world is focused on what Bernstein says are largely containment strategies for emerging infections—how we can detect them once people have been infected, and how we can deploy vaccines and test drugs quickly, he said. Both are critical because disease emergence is unavoidable, “but we can’t really do as well as we might if we’ve failed to prevent spillover.” 

Bernstein said we know that sharing of habitats, the wildlife trade and  large livestock operations are engines that drive emerging infections risk. There’s value in protecting forests, he said, because the prevention of spillover doesn’t just matter to humans, it is also good wildlife conservation.

“To me, it’s about acting before diseases start,” he said.

Carlson’s research team found that bats, because of their ability to fly long distances during their lifetimes, will likely account for the majority of the disease spillover in decades to come.

Angela Bosco-Lauth, an assistant professor of biomedical sciences at Colorado State University who studies infectious disease, noted that roughly two-thirds of the pathogens that infect humans are zoonotic in nature.

“I think we’re going to see this more and more often,” Bosco-Lauth said. “As a species, we’ve grown to a point that this just has to be—I mean, I hate to use the term ‘the new normal’—but I think it’s going to be the new normal between climate change and population growth and encroachment.

“There’s just no way that humans and wildlife can avoid each other.”