by Bryan Walsh
In a 2009 computer simulation of Toba’s climate effects, Alan Robock of Rutgers University estimated that global temperatures would have fallen by 18 degrees on average for several years after the eruption,13 and potentially by as much as 30 degrees. Precipitation would have been fallen by 45 percent, and vegetation cover would have shrunk dramatically, with broadleaf evergreen trees and tropical deciduous trees dying out.14 Imagine a winter that lasted for years, like something out of Game of Thrones, shriveling life on land. According to Michael Rampino, a geologist at New York University, up to three-quarters of plant species in the Northern Hemisphere might have perished after Toba, along with countless animals.15
For our hunter-gatherer ancestors, who at the time were still mostly concentrated in Africa and parts of southern Asia, what would it have felt like to live through Toba’s volcanic winter? Imagine waking up day after day, year after year, the dim sun obscured behind a film of sulfate aerosols that never lifted. That 18-degree drop in temperatures would have felt like moving from New York City to Anchorage, Alaska. Warmth would become nothing more than a memory, as the plants and animals you depended on for survival died out in the lingering chill. The weakest members of your community—the sick, the old, and the very young—would follow. Though climate models indicate that temperatures returned to close to normal within a decade, it must have felt in that prolonged darkness and cold like the end times had come.
They nearly did. In the 1990s, Rampino and Stanley Ambrose, an anthropologist at the University of Illinois, as well as the science journalist Ann Gibbons, put forward a theory that the effects of the supereruption might have indirectly reduced the total human population at the time to as few as four thousand people. This was an existential risk that actually played out. “Toba might have caused such dire environmental conditions that it would have led to a die-off,” said Rampino. “It’s a reminder that we are at the mercy of geology.”
As the existential risk scholars Nick Bostrom and Milan Ćirković write in the introduction to the book Global Catastrophic Risks, Toba “is perhaps the worst disaster that has ever befallen the human species.”16 It is likely the closest we have ever come to extinction before or since.
We know that the population of Homo sapiens went through a bottleneck around the same general time as the Toba eruption because we can read the evidence in our genes. Unlike most of our fellow primates, and even other mammals, human beings across races and nations show an unusual degree of genetic uniformity. The science writer Charles Mann has noted that two human beings from two different parts of the world might share all but 0.1 percent of their DNA. Compare that to two different strains of the simple bacteria E. coli, which might share only 95 percent of their genes,17 even though their genome is perhaps a quarter the size of the human genome. That genetic similarity—along with evidence that human populations suddenly exploded about fifty thousand years ago18—indicates that contemporary human beings descend from a relatively small group of ancestors. Rampino and Ambrose believe that the extreme if short-term climate change caused by the Toba supereruption left just a small group of survivors—our forebears.
We don’t know how they made it, though Ambrose believes communities that were able to work together cooperatively had a better chance of survival. “You might think that in an apocalyptic event that people would be stealing from each other,” Ambrose told me. “It’s not true. In a stable environment, when population density is high, you don’t need to rely on your neighbors to get the next meal, and you may even need to defend yourself from them. But in a situation where it’s the equivalent of a small lifeboat and everyone needs to cooperate, selfish people will be weeded out. You end up with a population that is more sharing and caring.”
We’re lucky that the survivors pulled together. You and I and everyone we know—those who came before us and everyone who might come after us—are here because the human beings who lived through Toba found a way to survive the eruption and its long, cold aftermath. Without their resourcefulness, the human story would have ended in its earliest chapters. Extinction was a possibility.
In fact extinction has been the rule over the 3.8 billion years that life has existed on Earth, not the exception. Some species go extinct due to disease, some due to predators, some through the process of evolution, as one species gradually gives way to a successor better suited to the environment. This is what scientists call the background rate of extinction, which amounts to an estimated one to five species per year dying out19—biological business as usual. But five times over the course of the planet’s history, life has been washed away by mass extinction events, waves of death that resulted in the loss of half or more of the species existing on Earth at the time. These were the actual end times, played on repeat.
The demise of the dinosaurs from the Chicxulub asteroid is now so well known that when we imagine a mass killer, we picture a flaming rock screaming across the sky. Over the course of the Earth’s history, however, volcanoes have been the cause of far more extinctions than anything from space. That may even include the dinosaurs as well—some scientists believe that the Chicxulub impact might have accelerated the ongoing eruptions of sprawling volcanoes called the Deccan Traps, in what is now India, and that the two catastrophes would have combined to cause the icy global cooling that ultimately killed off life.20 But we know for certain that the most severe mass extinction event the Earth has ever experienced—one where an estimated 75 percent of life on land and 90 percent of life in the ocean went extinct, an event so terrible that it has been called the “Great Dying”—was due to volcanic activity.21 And not just any volcanoes—these were eruptions that make Toba look like a popgun.
The Siberian Traps are now large plateaus and rolling grassland that cover a broad expanse of northern Russia. But beneath the grass is roughly a million square miles of hardened basalt rock, the cooled remains of an eruption that turned the Earth inside out. About 300,000 years before the start of the Great Dying, colossal amounts of lava began pouring through the surface, enough to cover the continental United States in magma half a mile deep.22 Unlike Toba, though, this wasn’t a single powerful explosion, but rather a flow—the technical term is “Large Igneous Province,” or LIP. Seven hundred thousand cubic miles of lava flowed for hundreds of thousands of years. When the magma running underground reached a massive coal basin in modern Siberia, it sparked a raging inferno that released vast amounts of carbon into the atmosphere. That tremendous pulse of greenhouse gases pushed climate change into overdrive, particularly in the oceans, where temperatures rose by as much as 18 degrees.23
Life simply couldn’t adjust. In a blink of geologic time, Earth went from hospitable to hostile. As the science writer Peter Brannen tells us in his excellent book The Ends of the World, the fossil record went silent. And it took 10 million years for life on Earth to begin to recover.24
If Toba marked the moment when humanity was nearly driven to extinction, the Great Dying nearly ended the story of life altogether. And both began with a volcano—a reminder that, as the historian Will Durant wrote, “Civilization exists by geological consent, subject to change without notice.” No natural force on Earth puts humans at greater existential peril than a supervolcano.
There’s almost no chance that the Earth will experience another LIP like what caused the Great Dying—at least not over the next 10–100 million years. Supervolcanoes like Toba, however, are known to erupt more frequently. Toba isn’t even the planet’s most recent supereruption. About 26,500 years ago, the Taupo volcano in New Zealand hit 8 on the VEI scale, spewing 260 cubic miles of volcanic material into the air,25 making it a little more than a third the size of Toba.
About 50,000 years separate the Toba and Taupo eruptions. That’s almost eternity on a human scale. It’s long enough to rerun the entire history of civilization multiple times over. But geologic time runs at a very different speed than human time. To geologists, 50,000 years ago is like yesterday. Or to be more precise, if you think
of the entire history of the planet as a single day, it’s just a second ago.
Geologists can comprehend the endless sweep of Earth’s multibillion-year history—but most of us are not geologists. We are confined to the brief human time horizons of our own experience. What happened in the deep past or what could happen in the far future has little bearing on our lives as we live them, the seven or eight or nine decades we count ourselves lucky to get. It’s as if we are born, grow up, and die all on the same spot, ignorant of the vastness that surrounds us. “Think about it in terms of a mayfly that lives and dies in a single afternoon in a creek on the mountains,” said Jake Lowenstern, a geologist with the U.S. Geological Survey. “A mayfly doesn’t really believe in nighttime or hailstorms or winter or ice. They never see anything but light and water, so why would they believe in anything else?
“We’re the same way.”
Just because the mayfly doesn’t live to see hailstorms or winter doesn’t mean they don’t exist, and just because we’ve never experienced a supereruption doesn’t mean one couldn’t strike in our future. Humans aren’t mayflies—through the practice of science, we have the ability to extend ourselves into the deepest corners of the past and the farthest reaches of the galaxy. But human psychology is a very different matter. Our species has been fortunate enough over the past ten thousand or so years to develop in a stable environment that—despite the usual earthquakes and hurricanes and droughts and floods—hasn’t been marred by a megacatastrophe that might have set back civilization to zero, or even threatened human extinction. Our inability to see beyond the narrow boundaries of human history—the availability heuristic I mentioned earlier—means that we suffer from a blind spot that can leave us vulnerable to an array of extremely rare but extremely lethal existential risks, including supervolcanoes.
We assume that our experience is normal, that yesterday was like today and that tomorrow will be the same. But there’s nothing special about our experience so far, either as individuals or as a species. Over the 4.5-billion-year life span of our planet, our experience is just an infinitesimal and random sampling, no more representative of the Earth’s full potential than a few snatched seconds would be of your entire life. So to protect ourselves from supervolcanoes and other existential threats, we need to understand that our experience isn’t normal—and we need to see beyond it. We need to see like a geologist.
So how vulnerable are we today to a supereruption? It helps first to understand the checkered history of the word “supervolcano.” It is not exactly a scientific term. “Supervolcano” wasn’t used in a geology publication until 2002, in an article by R. B. Trombley, who duped people into believing he was a trained geologist and could predict when volcanoes would erupt. (Trombley was uncovered as a fraud after the eruption of the Icelandic volcano of Eyjafjallajökull in 2010.) The term took off in 2005 after a BBC/Discovery docudrama about a future eruption at Yellowstone National Park called, of course, Supervolcano. The film and the title proved popular enough that geologists were basically forced to begin using it whether they wanted to or not. Professional volcanologists still prefer “supereruption” to “supervolcano,” though in practice they mean the same thing.
There are about twenty known supervolcanoes on Earth,26 and if any of them were to erupt at full power, the effects would be spectacular, and spectacularly destructive. The authors of a 2005 report for the Geological Society of London put it in perspective this way: if a supervolcano were to erupt in London’s central Trafalgar Square, it would produce at least enough magma to bury all of Greater London and everything in it to a depth of 700 feet.27 But what really sets a supervolcano apart from anything else the Earth can throw at us—tsunamis, earthquakes, hurricanes—is that they are the only natural disaster that can truly go global. What happened after Toba could happen again—ash and sulfur from a supereruption would reach the stratosphere, and depending on the location of the volcano and its eruptive strength, it could spread around the globe, blocking incoming sunlight and casting the world into winter. A supervolcanic eruption in Trafalgar Square would be very bad news for London, but the entire world would feel it.
Because the last supervolcano blew tens of thousands of years ago, scientists employ the tools of paleoclimatology and computer modelling to reconstruct what might have happened in the aftermath of a supereruption. But those models are imprecise, which leaves room for debate—including about Toba. While Alan Robock’s models projected double-digit temperature declines after Toba, Claudia Timmreck of the Max-Planck Institute for Meteorology in Germany argued in a 2010 study that cooling from the supereruption might have been much less, perhaps just five or six degrees on average.28 That would have still made for abnormally cold weather—roughly the difference between a winter in Boston and one in Washington, D.C.—but Timmreck concluded that the cooling would not have been “large enough to severely affect the species survival of modern humans.”
Scientists have also tried to piece together archaeological evidence from around the time of the Toba eruption, in hopes of better understanding how the aftermath affected humans. In a 2018 study published in Nature, researchers excavated a cliff near the town of Mossel Bay on South Africa’s southern coast and discovered a layer of microscopic glass shards called cryptotephra that could be traced back to Toba.29 That was clear evidence that ash from the supervolcano had reached as far as southern Africa, more than 5,400 miles from Toba in Indonesia. Just as important were the bones, tools, and other signs of human activity found above and below the cryptotephra layer. In this settlement at least, prehistoric humans seemed largely unaffected by the distant volcano. In fact, signs of human occupation in the settlement seemed to grow in the years following the supereruption.
In villages in southern India, the University of Cambridge archaeologist Sacha Jones found stone tools above and below a layer of Toba cryptotephra—more evidence that human settlement in the region hadn’t been overly disrupted by the blast. (Jones told me she discovered the Toba deposits when she found out that Indian villagers were mining and selling the ash—which she described as “bright white, gritty, and abrasive”—as a skin exfoliant.)
The new research doesn’t necessarily mean that our ancient ancestors simply shrugged off Toba. Settlements like the kind Jones found in India may have served as refugia, safe zones where animals—including human beings—ride out environmental catastrophes. But conditions beyond those refugia might have been much harsher. “We will find that Toba had very varied effects on different regions of the world, and that they could be especially negative if the population was quite vulnerable to begin with,” Jones told me. “If you’re doing well, then maybe you’re fine. But if you’re vulnerable, if your resilience is not so good, you’re in trouble.” Any attempt to reconstruct a world 74,000 years gone will inevitably be imprecise, whether the tools employed are climate computer models or the shovels and spades of an archaeologist. And any attempt to predict exactly how a supervolcano would disrupt our world today would be almost as imprecise. That’s an occupational hazard of dealing in existential risk. We look to past analogues to try to forecast how a future event will unfold, but existential risks by definition are on a level that we have never known.
While we haven’t experienced a supereruption in recorded history, we have come close, and more recently than you might know. What happened afterward could help us understand why a supervolcano would represent an existential threat like no other.
Tambora might have once been among the tallest peaks in Indonesia, though we’ll never know for sure, because on the evening of April 5, 1815, this mountain on the Indonesian island of Sumbawa detonated. In his book Tambora: The Eruption That Changed the World, the historian Gillen D’Arcy Wood writes that “huge plumes of flame issued from the mountain for three hours, until the dark mist of ash became confused with the natural darkness, seeming to announce the end of the world.”30 As the eruption unfolded over days, whole villages were vaporized by flame, ash, and hurri
cane-strength winds, instantly killing some 10,000 people living in the immediate vicinity of the volcano.31
The force of the eruption sent tsunamis racing across the sea. Even those living far from Tambora could hear the rumble of the eruption as it continued, on and off, for days. Sir Stamford Raffles, the British imperialist and the founder of modern Singapore, reported that a detachment of troops had been mobilized because officials assumed the booming sound in the distance could only be cannon fire. Raffles wrote that local Indonesians assured him that the sound came not from a military clash, but from a battle between the devil and the souls of their departed ancestors.32
What they had all witnessed was a VEI 7 volcano eruption. That is one ranking below a supervolcano, but it still sufficed to make Tambora the most powerful eruption in known history. Nearly ten cubic miles of ash, gases, and magma were blasted into the atmosphere, and the mountain itself lost more than two-thirds of its estimated height.33 In the days that followed the eruption a blanket of toxic volcanic ash draped over the island of Sumbawa, suffocating rice paddies and poisoning wells. Many of the islanders who survived the initial eruption succumbed to sickness and starvation in the following weeks, raising the estimated death toll to 100,000 and making Tambora not only the strongest but the deadliest eruption known to man.34
But the deaths didn’t stop there. As Tambora’s ash cloud spread across Southeast Asia, darkness fell over the region. For weeks after the eruption, British colonial officials were forced to conduct business by candlelight, even during the day.35 The cloud—higher than 27 miles36—spread by stratospheric winds until the entire globe was affected. As happened after Toba, the ash and sulfuric gases in the atmosphere reduced the amount of sunlight reaching the Earth’s surface, leading to global cooling. Average temperatures fell by 2.7 degrees between 1810 and 1819, making this decade the coldest in the historical record.37 A drop of a little less than three degrees Fahrenheit may not seem like much of a change—you’ll experience a much greater range of temperatures in the course of a typical day—but that average masks extremes that yielded famine and death.
