Prepare for more of the practically unimaginable. Photo: Joe Raedle/Getty Images

COVID-19 is not a climate-change pandemic — as far as we know, nothing about the emergence or spread of the coronavirus bears the recognizable imprint of global warming. But if the disease and our utter inability to respond to it terrifies you about our future staring down climate change, it should, not just as a “fire drill” for climate change generally but as a test run for all the diseases that will be unleashed in the decades ahead by warming. The virus is a terrifying harbinger of future pandemics that will be brought about if climate change continues to so deeply destabilize the natural world: scrambling ecosystems, collapsing habitats, rewiring wildlife, and rewriting the rules that have governed all life on this planet for all of human history.

Among the many unnerving lessons the two crises share is this one: Nature is mighty, and scary, and we have not defeated it but live within it, subject to its temperamental power, no matter where it is that you live or how protected you may normally feel. As the coronavirus has paralyzed much of the northern hemisphere, for instance, 192 billion locusts, perhaps 8,000 times more than usual, are swarming East Africa in clouds as big as whole cities, thanks to weather patterns scrambled by climate change; a small swarm can destroy the food supply of 35,000 people in a single day, and they are now traveling in swathes as wide as 25 miles, imperiling the food supply of tens of millions. In the U.S., it looks likely we will now be sheltering in place into the beginning of hurricane season. “We have been living in a bubble, a bubble of false comfort and denial,” as George Monbiot wrote recently in the Guardian. “Living behind screens, passing between capsules — our houses, cars, offices and shopping malls — we persuaded ourselves that contingency had retreated, that we had reached the point all civilisations seek: insulation from natural hazards.”

COVID-19 is one such hazard we believed, until a few weeks ago, we were mostly invulnerable to. In the future, we may have to reckon also with diseases we believed we already defeated, since in addition to bringing about pandemics of the future, global warming will revive plagues of the past. There are now, trapped in Arctic ice, diseases that have not circulated in the air for millions of years — in some cases, since before humans were around to encounter them. Which means our immune systems would have no idea how to fight back when those prehistoric plagues emerge from the ice. Already, in laboratories, several microbes have been reanimated: a 32,000-year-old “extremophile” bacteria revived in 2005, an 8-million-year-old bug brought back to life in 2007, a 3.5-million-year-old one a Russian scientist self-injected, out of curiosity, just to see what would happen. (He survived.) In 2018, scientists revived something a bit bigger — a worm that had been frozen in permafrost for the last 42,000 years.

The Arctic also stores terrifying diseases from more recent times. In Alaska, researchers have discovered remnants of the 1918 flu that infected as many as 500 million, and killed as many as 50 million — about 3 percent of the world’s population, and more had died in the world war for which the pandemic served as a kind of gruesome capstone. Scientists suspect smallpox is trapped in Siberian ice, among many other diseases that have otherwise passed into human legend — an abridged history of devastating sickness, left out like egg salad in the Arctic sun. Many of these frozen organisms won’t actually survive the thaw; those that have been brought back to life have been reanimated typically under fastidious lab conditions. But in 2016, a boy was killed and 20 others infected by anthrax released when retreating permafrost exposed the frozen carcass of a reindeer killed by the bacteria at least 75 years earlier; more than 2,000 present-day reindeer died.

What concerns epidemiologists more than ancient diseases are existing scourges relocated, rewired, or even reevolved by warming. The first effect is geographical. Before the early modern period, human provinciality was a guard against pandemic — a bug could wipe out a town, or a kingdom, or even in an extreme case devastate a continent — but in most instances it couldn’t travel much farther than its victims, which is to say, not very far at all. The Black Death killed as much as 60 percent of Europe, but consider, for a gruesome counterfactual, how big its impact might have been in a truly globalized world.

Today, even with globalization and the rapid intermingling of human populations, our ecosystems are mostly stable, and this functions as another limit — we know where certain bugs can spread, and we know the environments in which they cannot. (This is why certain vectors of adventure tourism require dozens of new vaccines and prophylactic medications, and why New Yorkers traveling to London don’t need to worry.)

But global warming will scramble those ecosystems, meaning it will help disease trespass those limits as surely as Cortés did. The footprint of every mosquito-borne illness is currently circumscribed, for instance, but those borders are disappearing rapidly as the tropics expand — the current rate is 30 miles per decade. In Brazil, for generations, yellow fever sat in the Amazon Basin, where the Haemagogus and Sabethes mosquitoes thrived, making the disease a concern for those who lived, worked, or traveled deep into the jungle, but only for them; in 2016, it left the Amazon as more and more mosquitoes fanned out of the rain forest, and by 2017, it had reached areas around the country’s megalopolises, São Paulo and Rio de Janeiro — more than 30 million people, many of them living in shantytowns, facing the arrival of a disease that kills between 3 and 8 percent of those infected.

Yellow fever is just one of the plagues that will be carried by mosquitoes as they migrate, conquering more and more of a warming world — the globalization of pandemic disease. Malaria alone already kills a million people each year, infecting many more, but you don’t worry much about it if you are living in Maine or France. As the tropics creep northward and mosquitoes migrate with them, you may; over the course of the next century, more and more of the world’s population will be living under the shadow of diseases like these. You didn’t worry much about Zika before a couple of years ago, either.

As it happens, Zika may be a good model of a second worrying effect — disease mutation. One reason you hadn’t heard about Zika until recently is that it had been trapped in Uganda and Southeast Asia; another is that it did not, until recently, appear to cause birth defects. Scientists still don’t entirely understand what happened or what they missed, even now, several years after the planet was gripped by panic about microcephaly. It could be that the disease changed as it came to the Americas, the result of a genetic mutation or in adaptive response to a new environment, or that Zika produces those devastating prenatal effects only when another disease is present, possibly one less common in Africa, or that something about the environment or immunological history in Uganda protects mothers and their unborn children there.

But there are things we do know for sure about how climate affects some diseases. Malaria, for instance, thrives in hotter regions, which is one reason the World Bank estimates that by 2030, 3.6 billion people will be reckoning with it — 100 million as a direct result of climate change.

Projections like those depend not just on climate models but on an intricate understanding of the organism at play. Or, rather, organisms. Malaria transmission involves both the disease and the mosquito; Lyme disease, both the disease and the tick — which is another epidemiologically threatening creature whose universe is rapidly expanding, thanks to global warming. As Mary Beth Pfeiffer has documented, Lyme-case counts have spiked in Japan, Turkey, and South Korea, where the disease was literally nonexistent as recently as 2010 — zero cases — and now lives inside hundreds more Koreans each year. In the Netherlands, 54 percent of the country’s land is now infested; in Europe as a whole, Lyme caseloads are now three times the standard level. In the U.S., there are likely around 300,000 new infections each year — and since even many of those treated for Lyme continue to show symptoms years after treatment, the numbers can stockpile. Overall, the number of disease cases from mosquitoes, ticks, and fleas have tripled in the U.S. over just the last 13 years, with dozens of counties across the country encountering ticks for the first time. But the effects of the epidemic can be seen perhaps most clearly in animals other than humans: In Minnesota, during the aughts, winter ticks helped drop the moose population by 58 percent in a single decade, and some environmentalists believe the species could be eradicated entirely from the state within the next few years.

In New England, moose calves have been found suckling as many as 90,000 engorged ticks, which often kill the calves not through Lyme disease but simple anemia, the effect of that number of bugs each drawing a few milliliters of blood. Those that survive are far from robust, many having scratched so incessantly at their own hides to clear the ticks that they completely eliminated their own hair, leaving behind a spooky gray skin that has earned them the name “ghost moose.”

Lyme is still, in relative terms, a young disease, and one we don’t yet understand all that well. We attribute a very mysterious and incoherent set of symptoms to it, from joint pain to fatigue to memory loss to facial palsy, almost as a catchall explanation for ailments we cannot pinpoint in patients who we know have been bitten by a bug carrying the bug. We do know ticks, however, as surely as we know malaria — there are not many parasites we understand better. But there are many, many millions we understand worse, which means our sense of how climate change will redirect or remodel them is shrouded in a foreboding ignorance. And then there are the plagues that climate change will confront us with for the very first time — a whole new universe of diseases humans have never before known to even worry about.

“New universe” is not hyperbole. Scientists guess the planet could harbor more than a million yet-to-be-discovered viruses — many of them, like COVID-19, for now “quarantined” in particular susceptible species, but which could evolve or “jump” into humans, either as the result of changing climatic conditions or because the scrambling of native ecosystems and habitats brings the host species into contact with humans in a much more direct way that ever before. The more we pave over and log and deforest the natural world, disrupting stable ecosystems and turning those organisms living happily within them out into the human world, the more diseases, and pandemics, we’ll produce. That is what it means to be living entirely outside the window of climate conditions that enclose all of human history — everything we have ever taken to be stable about our relationship to the planet is thrown into chaos. That chaos will confront us, again and again, with undiscovered disease.

Bacteria are even trickier, and so we probably know about even fewer of them. Perhaps scariest are those that live within us, peacefully for now. More than 99 percent of even those bacteria inside human bodies are currently unknown to science, which means we are operating in near-total ignorance about the effects climate change might have on the bugs in, for instance, our guts — about how many of the bacteria modern humans have come to rely on, like unseen factory workers, for everything from digesting our food to modulating our anxiety, could be rewired, diminished, or entirely killed off by an additional few degrees of heat.

Overwhelmingly, of course, the viruses and bacteria making homes inside us are nonthreatening to humans — at present. Presumably, a difference of a degree or two in global temperature won’t dramatically change the behavior of the majority of them — probably the vast majority, even the overwhelming majority. But consider the case of the saiga — the adorable dwarflike antelope, native to Central Asia. In May 2015, nearly two-thirds of the global population died in the span of just days — every single saiga in an area the size of Florida, the land suddenly dotted with hundreds of thousands of saiga carcasses and not one lone survivor. An event like this is called a “megadeath,” this one so striking and cinematic that it gave rise, immediately, to a whole raft of conspiracy theories: aliens, radiation, dumped rocket fuel. But no toxins were found by researchers poking through the killing fields — in the animals themselves, in the soil, in the local plants. The culprit, it turned out, was a simple bacteria, Pasteurella multocida, which had lived inside the saiga’s tonsils, without threatening its host in any way, for many, many generations. Suddenly, it had proliferated, emigrated to the bloodstream, and from there to the animals’ liver, kidneys, and spleen. Why? “The places where the saigas died in May 2015 were extremely warm and humid,” Ed Yong wrote in The Atlantic. “In fact, humidity levels were the highest ever seen in the region since records began in 1948. The same pattern held for two earlier, and much smaller, die-offs from 1981 and 1988. When the temperature gets really hot, and the air gets really wet, saiga die. Climate is the trigger, Pasteurella is the bullet.”

This is not to say we now understand what precisely about humidity weaponized Pasteurella, or how many of the other bacteria living inside mammals like us — the one percent we have identified, or perhaps more worryingly the 99 percent we house without any knowledge or understanding — might be similarly triggered by climate, friendly symbiotic bugs with whom we’ve lived in some cases for millions of years transformed suddenly into contagions already inside us. That remains a mystery. But ignorance is no comfort. Presumably climate change will introduce us to some of them.

This essay was adapted from The Uninhabitable Earth: Life After Warming, published by Tim Duggan Books in 2019; the paperback edition came out earlier this month.

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