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For the world’s viruses, this is a time of unprecedented opportunity. An estimated 40,000 viruses lurk in the bodies of mammals, of which a quarter could conceivably infect humans. Most do not, because they have few chances to leap into our bodies. But those chances are growing. Earth’s changing climate is forcing animals to relocate to new habitats, in a bid to track their preferred environmental conditions. Species that have never coexisted will become neighbors, creating thousands of infectious meet-cutes in which viruses can spill over into unfamiliar hosts—and, eventually, into us. Many scientists have argued that climate change will make pandemics more likely, but a groundbreaking new analysis shows that this worrying future is already here, and will be difficult to address. The planetary network of viruses and wildlife “is rewiring itself right now,” Colin Carlson, a global-change biologist at Georgetown University, told me. And “while we thought we understood the rules of the game, again and again, reality sat us down and taught us: That’s not how biology works.”

In 2019, Carlson and his colleague Greg Albery began creating a massive simulation that maps the past, present, and future ranges of 3,100 mammal species, and predicts the likelihood of viral spillovers if those ranges overlap. The simulation strained a lot of computing power; “every time we turn it on, an angel dies,” Carlson told me. And the results, which have finally been published today, are disturbing. Even under the most optimistic climate scenarios, the coming decades will see roughly 300,000 first encounters between species that normally don’t interact, leading to about 15,000 spillovers wherein viruses enter naive hosts.

“It’s a little harrowing,” says Vineet Menachery, a virologist at the University of Texas Medical Branch. The study suggests that the alarming pace at which new or reemergent viruses have caused outbreaks in recent decades “is not some abnormal situation,” Menachery told me, “but what we should be expecting, maybe even with an acceleration.”

Carlson and Albery drolly nicknamed their study “Iceberg,” denoting a huge and mostly hidden threat that we unwittingly collide with. Indeed, their simulation revealed that mammalian viruses have already been dramatically reshuffled, to a degree that likely can’t be undone even if all carbon emissions cease tomorrow. The Anthropocene, an era defined by humanity’s power over Earth, is also an era defined by viruses’ power over us—a Pandemicene. “The moment to stop climate change from increasing viral transmission was 15 years ago,” Carlson said. “We’re in a world that’s 1.2 degrees warmer [than preindustrial levels], and there is no backpedaling. We have to prepare for more pandemics because of it.”

The Iceberg study suggests that new spillovers will follow surprising rules. For example, the team assumed that these events would be concentrated in the Arctic because warming temperatures nudge animals toward higher, cooler latitudes. But if two species move northward in parallel, nothing changes. The real drama occurs, for instance, when animals seek higher, cooler altitudes, and when those living on opposite sides of a mountain meet in the middle. This means that spillovers will be concentrated not in the poles, but in the mountainous and species-rich parts of tropical Africa and southeast Asia.

Southeast Asia will also be especially spillover-prone because it’s home to a wide range of bats. Flight gives bats flexibility, allowing them to react to changing climates more quickly than other mammals, and to carry their viruses farther. And bats in Southeast Asia are highly diverse, and tend to have small ranges that don’t overlap. “You shake that like a snowglobe and you get a lot of first encounters,” Carlson said.

Such events will also be problematic elsewhere in the world. In Africa, bats are probably the natural reservoirs for Ebola. Thirteen species could potentially carry the virus, and as global warming forces them to disperse, they’ll encounter almost 3,700 new mammal species, leading to almost 100 spillovers. So far, the biggest Ebola outbreaks have occurred in West Africa, but Carlson said that within decades, the disease could easily become a bigger problem for the continent’s eastern side too. “And that’s emblematic of everything,” he told me: Every animal-borne disease will likely change in similarly dramatic ways.

These reshufflings are bad news for bats and other animals, which will have to cope with unfamiliar infections on top of the hardships of climate change. Even one newly introduced disease can reshape an ecosystem, and many such wildlife epidemics have occurred in recent decades. “For species in poor health, showing up in new habitats and being bombarded by disease is probably not good for their conservation,” Carlson said. And spillovers that initially occur between other mammals could someday affect us: The original SARS virus hopped from bats to humans via civets, and HIV reached us from monkeys via chimpanzees and gorillas. For an animal virus to jump into humans, geography, biological compatibility, and other factors must line up in just the right way. Each event is unlikely: Imagine playing Russian roulette using a gun with a million chambers. But as the climate changes, we’re loading more of those chambers with bullets, and pulling the trigger more frequently. Carlson can’t say whether climate-induced viral reshuffling directly led to the current pandemic, but they certainly make such events more likely.

The Iceberg simulation also showed that such events will be disproportionately common in areas that are likely to be settled by humans or used as cropland. “Species are going to move to spaces that are a little uphill and environmentally stable—and that’s where we have built cities,” Carlson told me. This unhappy coincidence means that the places where their viruses will explore new hosts “just happen to be our backyards.”

Several case studies suggest that the Iceberg’s predictions are just the tip of the … well, you know. For example, melting sea ice recently allowed a virus that normally infects North Atlantic seals to jump into the Pacific Northwest’s sea otters. But Iceberg, ironically, didn’t consider either melting ice or marine mammals. It didn’t consider birds, which harbor their own coterie of viruses, including several dangerous influenza strains. It didn’t consider potential pathogens other than viruses, such as fungi or bacteria. “I don’t think they’re overstating the problem,” Raina Plowright, a spillover expert at Montana State University, told me. The effects of climate change are compounded by habitat loss and other destructive forces in unpredictable ways, she said, which could force species to move and mingle even more radically than Iceberg simulated.

This is already happening. At first, Carlson and Albery assumed that the changes they simulated would occur in the later half of this century. But instead, their simulation suggested that we could be living through the peak era of spillovers right now. The problem will worsen as the world warms, but it’s plenty warm already—so most of the predicted viral reshufflings are either happening or about to. And to Carlson’s surprise and dismay, that remains true even if we successfully curb greenhouse-gas emissions henceforth. There are many good reasons to slow the pace of climate change, but the Pandemicene, having been unleashed, cannot be rebottled.

The revelations are “so large and heavy to behold that even as we were writing them, we didn’t want to,” Carlson said. But despite every attempt that he and Albery made to naysay their own work, the simulation kept spitting out the same results. They confirm that three of our greatest existential threats—climate change, pandemics, and the sixth mass extinction of wildlife—are really intertwined parts of the same mega-problem. To tackle it, “we need atmospheric scientists talking to ecologists talking to microbiologists talking to demographers,” Rachel Baker, whose research at Princeton focuses on climate and disease, told me.

The study “is not uplifting,” but being informed is useful, Sadie Ryan, a medical geographer at the University of Florida, told me. Efforts to surveil wild animals for worrying viruses can now be targeted to hot spots where those creatures will be, instead of simply their current whereabouts, Ryan said. Artificial intelligence makes identifying the most concerning pathogens easier. Vaccines can be prepared ahead of time, and more quickly than before.

But pandemics are inherently unpredictable, and no amount of prevention will fully negate their risk. The world must be ready to meet the viruses that slip through the net. That means fortifying public health and health-care systems, strengthening social safety nets, and addressing all the weaknesses of the pre-COVID normal that made the world so vulnerable to the current pandemic and will leave it susceptible to the next. The world, in its desire to move past COVID, is already forgetting the lessons of the recent past, and perhaps assuming that a generation-defining crisis will occur only once a generation. “But no, all of this could happen again tomorrow,” Carlson said. And “if this many viruses are undergoing host jumps this much,” multiple pandemics could strike together.

Carlson sounded rattled when I spoke with him, his anxiety about his findings cloaked by gallows humor. He told me that the Iceberg study was both the hardest and the most meaningful thing he’s ever done. He and Albery worked on the project from the small apartment they shared during the early pandemic. They kept the bar napkin on which they first drafted the title of their eventual paper on their fridge. The study’s grim results have been a huge burden to bear, but they and their team bore it together. Today, their paper has been published, the world will know, and Carlson will testify before Congress about the need to prepare for spillovers. “And then,” he told me, “we’ll start the project of fixing it.”

Ed Yong is a staff writer at The Atlantic. He won the Pulitzer Prize for Explanatory Reporting for his coverage of the COVID-19 pandemic.

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