From The Atlantic -
The Terrifying Warning Lurking in the Earth’s Ancient Rock Record
Our climate models could be missing something big.
Part 1 [article is too long for Base character limit]
Photo Illustrations by Brendan Pattengale | Maps by La Tigre
Images above: Glaciers from the Vatnajökull ice cap, in Iceland
Brendan Pattengale is a photographer who explores how color can convey emotions in an image. In his photo illustrations throughout this article, the colors of the original photos have been adjusted, but the images are otherwise unaltered.
This article was published online on February 3, 2021.
We live on a wild planet, a wobbly, erupting, ocean-sloshed orb that careens around a giant thermonuclear explosion in the void. Big rocks whiz by overhead, and here on the Earth’s surface, whole continents crash together, rip apart, and occasionally turn inside out, killing nearly everything. Our planet is fickle. When the unseen tug of celestial bodies points Earth toward a new North Star, for instance, the shift in sunlight can dry up the Sahara, or fill it with hippopotamuses. Of more immediate interest today, a variation in the composition of the Earth’s atmosphere of as little as 0.1 percent has meant the difference between sweltering Arctic rainforests and a half mile of ice atop Boston. That negligible wisp of the air is carbon dioxide.
Since about the time of the American Civil War, CO2’s crucial role in warming the planet has been well understood. And not just based on mathematical models: The planet has run many experiments with different levels of atmospheric CO2. At some points in the Earth’s history, lots of CO2 has vented from the crust and leaped from the seas, and the planet has gotten warm. At others, lots of CO2 has been hidden away in the rocks and in the ocean’s depths, and the planet has gotten cold. The sea level, meanwhile, has tried to keep up—rising and falling over the ages, with coastlines racing out across the continental shelf, only to be drawn back in again. During the entire half-billion-year Phanerozoic eon of animal life, CO2 has been the primary driver of the Earth’s climate. And sometimes, when the planet has issued a truly titanic slug of CO2 into the atmosphere, things have gone horribly wrong.
Today, humans are injecting CO2 into the atmosphere at one of the fastest rates ever over this entire, near-eternal span. When hucksters tell you that the climate is always changing, they’re right, but that’s not the good news they think it is. “The climate system is an angry beast,” the late Columbia climate scientist Wally Broecker was fond of saying, “and we are poking it with sticks.”
The beast has only just begun to snarl. All of recorded human history—at only a few thousand years, a mere eyeblink in geologic time—has played out in perhaps the most stable climate window of the past 650,000 years. We have been shielded from the climate’s violence by our short civilizational memory, and our remarkably good fortune. But humanity’s ongoing chemistry experiment on our planet could push the climate well beyond those slim historical parameters, into a state it hasn’t seen in tens of millions of years, a world for which Homo sapiens did not evolve.
When there’s been as much carbon dioxide in the air as there already is today—not to mention how much there’s likely to be in 50 or 100 years—the world has been much, much warmer, with seas 70 feet higher than they are today. Why? The planet today is not yet in equilibrium with the warped atmosphere that industrial civilization has so recently created. If CO2 stays at its current levels, much less steadily increases, it will take centuries—even millennia—for the planet to fully find its new footing. The transition will be punishing in the near term and the long term, and when it’s over, Earth will look far different from the one that nursed humanity. This is the grim lesson of paleoclimatology: The planet seems to respond far more aggressively to small provocations than it’s been projected to by many of our models.
To truly appreciate the coming changes to our planet, we need to plumb the history of climate change. So let us take a trip back into deep time, a journey that will begin with the familiar climate of recorded history and end in the feverish, high-CO2 greenhouse of the early age of mammals, 50 million years ago. It is a sobering journey, one that warns of catastrophic surprises that may be in store.
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The first couple of steps back in time won’t take us to a warmer world—but they will illuminate just what sort of ill-tempered planet we’re dealing with. As we pull back even slightly from the span of recorded history—our tiny sliver of geologic time—we’ll notice almost at once that the entire record of human civilization is perched at the edge of a climate cliff. Below is a punishing ice age. As it turns out, we live on an ice-age planet, one marked by the swelling and disintegration of massive polar ice sheets in response to tiny changes in sunlight and CO2 levels. Our current warmer period is merely one peak in a mountain range, with each summit an interglacial springtime like today, and each valley floor a deep freeze. It takes some doing to escape this cycle, but with CO2 as it is now, we won’t be returning to an ice age for the foreseeable future. And to reach analogues for the kind of warming we’ll likely see in the coming decades and centuries, we will need to move beyond the past 3 million years of ice ages entirely, and make drastic jumps back into the alien Earths of tens of millions of years ago. Our future may come to resemble these strange lost worlds.
Before we move more dramatically backwards in time, let us briefly pause over the history of civilization, and then some. Ten thousand years ago, the big mammals had just vanished, at human hands, in Eurasia and the Americas. Steppes once filled with mammoths and camels and wetlands stocked with giant beavers were suddenly, stunningly vacant.
The coastlines that civilization presumes to be eternal were still far beyond today’s horizon. But the seas were rising. The doomed vestiges of mile-thick ice sheets that had cloaked a third of North American land were retreating to the far corners of Canada, chased there by tundra and taiga. The roughly 13 quintillion gallons of meltwater these ice sheets would hemorrhage, in a matter of millennia, raised the sea level hundreds of feet, leaving coral reefs that had been bathed in sunlight under shallow waves now drowned in the deep.
By 9,000 years ago, humans in the Fertile Crescent, China, Mexico, and the Andes had independently developed agriculture and—after 200,000 years of wandering—had begun to stay put. Sedentary settlements blossomed. Humans, with a surfeit of calories, began to divide their labor, and artisans plied new arts. The Earth’s oldest cities, such as Jericho, were bustling.
By 5,000 years ago, sunlight had waned in the Northern summer, and rains drifted south toward the equator again. The green Sahara began to die, as it had many times before.
It’s easy to forget that the Earth—cozy, pastoral, familiar—is nevertheless a celestial body, and astronomy still has a vote in earthly affairs. Every 20,000 years or so the planet swivels about its axis, and 10,000 years ago, at civilization’s first light, the Earth’s top half was aimed toward the sun during the closest part of its orbit—an arrangement today enjoyed by the Southern Hemisphere. The resulting Northern-summer warmth turned the Sahara green. Lakes, hosting hippos, crocodiles, turtles, and buffalo, speckled North Africa, Arabia, and everywhere in between. Lake Chad, which today finds itself overtaxed and shrinking toward oblivion, was “Mega-Chad,” a 115,000-square-mile freshwater sea that sprawled across the continent. Beneath the Mediterranean today, hundreds of dark mud layers alternate with whiter muck, a barcode that marks the Sahara’s rhythmic switching from lush green to continent-spanning desert.
Imprinted on top of this cycle were the last gasps of an ice age that had gripped the planet for the previous 100,000 years. The Earth was still thawing, and amid the final approach of the rising tides, enormous plains and forests like Doggerland—a lowland that had joined mainland Europe to the British Isles—were abandoned by nomadic humans and offered to the surging seas. Vast islands like Georges Bank, 75 miles off Massachusetts—which once held mastodons and giant ground sloths—saw their menagerie overtaken. Scallop draggers still pull up their tusks and teeth today, far offshore.
By 5,000 years ago, as humanity was emerging from its unlettered millennia, the ice had stopped melting and oceans that had been surging for 15,000 years finally settled on modern shorelines. Sunlight had waned in the Northern summer, and rains drifted south toward the equator again. The green Sahara began to die, as it had many times before. Hunter-fisher-gatherers who for thousands of years had littered the verdant interior of North Africa with fishhooks and harpoon points abandoned the now-arid wastelands, and gathered along the Nile. The age of pharaohs began.
By geologic standards, the climate has been remarkably stable ever since, until the sudden warming of the past few decades. That’s unsettling, because history tells us that even local, trivial climate misadventures during this otherwise peaceful span can help bring societies to ruin. In fact, 3,200 years ago, an entire network of civilizations—a veritable globalized economy—fell apart when minor climate chaos struck.
“There is famine in [our] house; we will all die of hunger. If you do not quickly arrive here, we ourselves will die of hunger. You will not see a living soul from your land.” This letter was sent between associates at a commercial firm in Syria with outposts spread across the region, as cities from the Levant to the Euphrates fell. Across the Mediterranean and Mesopotamia, dynasties that had ruled for centuries were all collapsing. The mortuary-temple walls of Ramses III—the last great pharaoh of Egypt’s New Kingdom period—speak of waves of mass migration, over land and sea, and warfare with mysterious invaders from afar. Within decades the entire Bronze Age world had collapsed.
Historians have advanced many culprits for the breakdown, including earthquakes and rebellions. But like our own teetering world—one strained by souring trade relations, with fractious populaces led by unsteady, unscrupulous leaders and now stricken by plague—the eastern Mediterranean and the Aegean were ill-prepared to accommodate the deteriorating climate. While one must resist environmental determinism, it is nevertheless telling that when the region mildly cooled and a centuries-long drought struck around 1200 B.C., this network of ancient civilizations fell to pieces. Even Megiddo, the biblical site of Armageddon, was destroyed.
The Jökulsárlón glacier lagoon in Iceland
This same story is told elsewhere, over and over, throughout the extremely mild stretch of time that is written history. The Roman empire’s imperial power was vouchsafed by centuries of warm weather, but its end saw a return to an arid cold—perhaps conjured by distant pressure systems over Iceland and the Azores. In A.D. 536, known as the worst year to be alive, one of Iceland’s volcanoes exploded, and darkness descended over the Northern Hemisphere, bringing summer snow to China and starvation to Ireland. In Central America several centuries later, when the reliable band of tropical rainfall that rings the Earth left the Mayan lowlands and headed south, the megalithic civilization above it withered. In North America, a megadrought about 800 years ago made ancestral Puebloans abandon cliffside villages like Mesa Verde, as Nebraska was swept by giant sand dunes and California burned. In the 15th century, a 30-year drought bookended by equally unhelpful deluges brought the Khmer at Angkor low. The “hydraulic empire” had been fed and maintained by an elaborate irrigation system of canals and reservoirs. But when these canals ran dry for decades, then clogged with rains, invaders easily toppled the empire in 1431, and the Khmer forfeited their temples to the jungle.
Hopscotching through these human disasters to the present day, we pass perhaps the most familiar historical climate event of all: the Little Ice Age. Lasting roughly from 1500 to 1850, the chill made ice rinks of Dutch canals, and swelled up Swiss mountain glaciers. Tent cities sprung up on a frozen Thames, and George Washington endured his winter of cold and privation at Valley Forge in 1777 (which wasn’t even particularly harsh for the times). The Little Ice Age might have been a regional event, perhaps the product of an exceptional run of sunlight-dimming volcanism. In 1816, its annus horribilis, the so-called year without a summer—which brought snows to New England in August—global temperatures dropped perhaps a mere half a degree Celsius. While it is perennially plumbed by historians for insights into future climate change, it is not even remotely on the same scale of disruption as that which might lie in our future.
As Europe emerged from its chill, coal from 300-million-year-old jungles was being fed into English furnaces. Although the Earth was now in the same configuration that, in the previous few million years, had invited a return to deep, unthinkable ice ages, for some reason the next ice age never took. Instead the planet embarked on an almost unprecedented global chemistry experiment. Halfway through the 20th century, the climate began behaving very strangely.
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So this is the climate of written history, a seemingly eventful stretch that has really been the random noise and variability of a climate essentially at peace. Indeed, if you were to find yourself in an industrial civilization somewhere else in the universe, you would almost certainly notice such similarly strange and improbably pleasant millennia behind you. This kind of climate stability seems to be a prerequisite for organized society. It is, in other words, as good as it gets.
As we jump back 20,000 years—to yesterday, geologically—the world ceases being recognizable. Whereas all of recorded history played out in a climate hovering well within a band of 1 degree Celsius, we now see what a difference 5 to 6 degrees can make—a scale of change similar to the one that humans may engineer in only the next century or so, though in this case, the world is 5 to 6 degrees colder, not warmer.
An Antarctica’s worth of ice now rests atop North America. Similar sheets smother northern Europe, and as a result, the sea level is now 400 feet lower. The midwestern United States is carpeted in stands of stunted spruce of the sort that would today look at home in northern Quebec. The Rockies are carved up, not by wildflower-dappled mountain valleys, but by overflowing rivers of ice and rock. California is a land of dire wolves. Where the Pacific Northwest edges up against the American Antarctica, it is a harsh and treeless place. Nevada and Utah fill up with cold rains.
During World War II, at Topaz, the desolate Japanese American internment camp in Utah, prisoners combed the flats of the Sevier Desert for unlikely seashells, fashioning miraculous little brooches from tiny mussel and snail shells to while away their exile. The desert seashells were roughly 20,000 years old, from the vanished depths of the giant Pleistocene-era Lake Bonneville—the product of a jet stream diverted south by the ice sheet. This was once a Utahan Lake Superior, more than 1,000 feet deep in places. It was joined by endless other verdant lakes scattered across today’s bleak Basin and Range region.
Elsewhere, the retreat of the seas made most of Indonesia a peninsula of mainland Asia. Vast savannas and swamps linked Australia and New Guinea, and of course Russia shared a tundra handshake with Alaska. There were reindeer in Spain, and glaciers in Morocco. And everywhere loess, loess, and more loess. This was the age of dust.
Ice is a rock that flows. Send it in massive sterilizing slabs across the continents, and it will quarry mountainsides, pulverize bedrock, and obliterate everything in its path. At the height of the last ice age, along the crumbling margins of the continental ice sheets, the rocky, dusty spoils of all this destruction spilled out onto the tundra. Dry winds carried this silt around the world in enormous dust storms, piling it up in seas of loess that buried the central U.S., China, and Eastern Europe under featureless drifts. In Austria, not far from the site of the voluptuous Venus of Willendorf figurine, carved some 30,000 years ago, are the remains of a campground of the same age—tents, hearths, burnt garbage pits, hoards of ivory jewelry—all abandoned in the face of these violent, smothering haboobs. Ice cores from both Antarctica and Greenland record a local environment that was 10 times dustier than today. All of this dust seeded the seas with iron, a vital nutrient for carbon-hogging plankton, which bloomed around Antarctica and pulled gigatons of CO2 out of the air and deep into the ocean, freezing the planet further.
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This parched Pleistocene world would have appeared duller from space, hosting as it did a quarter less plant life. CO2 in the atmosphere registered only a paltry 180 ppm, less than half of what it is today. In fact, CO2 was so low, it might have been unable to drop any further. Photosynthesis starts to shut down at such trifling levels, a negative-feedback effect that might have left more CO2—unused by plants—in the air above, acting as a brake on the deep freeze.
This was the strange world of the Ice Age, one that, geologically speaking, is still remarkably recent. It’s so recent, in fact, that today, most of Canada and Scandinavia is still bouncing back up from the now-vanished ice sheets that had weighed those lands down.
The floods carried 30-foot boulders on biblical waves, through what were suddenly the world’s wildest rapids.
In 2021, we find ourselves in an unusual situation: We live on a world with massive ice sheets, one of which covers one of the seven continents and is more than a mile deep. For most of the planet’s past, it has had virtually no ice whatsoever. The periods of extreme cold—like the ultra-ancient, phantasmagoric nightmares of Snowball Earth, when the oceans might have been smothered by ice sheets all the way to the tropics—are outliers. There were a few other surprising pulses of frost here and there, but they merely punctuate the balmy stretches of the fossil record. For almost all of the Earth’s history, the planet was a much warmer place than it is today, with much higher CO2 levels. This is not a climate-denying talking point; it’s a physical fact, and acknowledging it does nothing to take away from the potential catastrophe of future warming. After all, we humans, along with everything else alive today, evolved to live in our familiar low-CO2 world—a process that took a long time.
How long, exactly? Fifty million years ago, as our tiny mammalian ancestors were still sweating through the jungly, high-CO2 greenhouse climate they had inherited from the dinosaurs, India was nearing the end of an extended journey. Long estranged from Africa and the august, bygone supercontinent of Gondwana, the subcontinent raced northeast across the proto–Indian Ocean and smashed into Asia in slow motion. The collision not only quieted CO2-spewing volcanoes along Asian subduction zones; it also thrust the Himalayas and the Tibetan Plateau toward the stars, to be continually weathered and eroded away.
As it turns out, weathering rocks—that is, breaking them down with CO2-rich rainwater—is one of the planet’s most effective long-term mechanisms for removing carbon dioxide from the atmosphere, one that modern geoengineers are frantically trying to reproduce in a lab, for obvious reasons.
Adding to this colossal Himalayan CO2 sink, the more recent buckling, tectonic mess that lifted Indonesia and its neighbors from the sea over the past 20 million years or so also exhumed vast tracts of highly weatherable ocean crust, exposing it all to the withering assault of tropical rainstorms. Today this corroding rock accounts for roughly 10 percent of the planet’s carbon sink. Over tens of millions of years, then, the stately march of plate tectonics—the balance of volcanic CO2 and rock weathering—seems to have driven long-term climate change, in our case toward a colder, lower-CO2 world. As we’ll see, humans now threaten to undo this entire epic, geologic-scale climate evolution of the Cenozoic era—and in only a few decades.
When Earth’s blanket of CO2 was finally thin enough, the planet’s regular wobbles were at long last sufficient to trigger deep glaciations. The ice ages began. But the climate was not stable during this period. The ice advanced and retreated, and while the descent into the wild episodes of the Pleistocene epoch could be leisurely—the depths of planetary winter taking tens of thousands of years to arrive—the leap out of the cold tended to be sudden and violent. This is where positive feedback loops come in: When the last ice age ended, it ended fast.
Glacial ice near the Torfajökull volcano, in Iceland
Coral reefs marking the ancient sea level—but today lying deep off the coasts of Tahiti and Indonesia—reveal that about 14,500 years ago, the seas suddenly jumped 50 feet or so in only a few centuries, as meltwater from the late, great North American ice sheet raged down the Mississippi. When a 300-foot-deep lake of glacial meltwater spanning at least 80,000 square miles of central Canada catastrophically drained into the ocean, it shut down the churn of the North Atlantic and arrested the seaborne flow of heat northward. As a result, tundra advanced to retake much of Europe for 1,000 years. But when ocean circulation kicked back into gear, and the dense, salty seawater began to sink again, the system rebooted, and currents carried the equator’s heat toward the Arctic once more. Temperatures in Greenland suddenly leaped 10 degrees Celsius in perhaps a decade, fires spread, and revanchist forests reclaimed Europe for good.
In Idaho, ice dams that had held back giant lakes of glacial meltwater about six times the volume of Lake Erie collapsed as the world warmed, and each released 10 times the flow of all the rivers on Earth into eastern Washington. The floods carried 30-foot boulders on biblical waves, through what were suddenly the world’s wildest rapids. They left behind a labyrinth of bedrock-scoured canyons that still covers the entire southeastern corner of the state like a scar. When the Earth’s climate changes, this is what it can look like on the ground.
As the ice sheets of the Northern Hemisphere finally lost their grip, darker land around the melting margins became exposed to the sun for the first time in 100,000 years, accelerating the ice’s retreat. Permafrost melted, and methane bubbled up from thawing bogs. Colder, more CO2-soluble oceans warmed, and gave up the carbon they’d stolen in the Ice Age, warming the Earth even more. Relieved of their glacial burden, volcanoes in Iceland, Europe, and California awoke, adding even more CO2 to the atmosphere.
Soon the Sahara would green again, Jericho would be born, and humans would start writing things down. They would do so with the assumption that the world they saw was the way it had always been. “We were born only yesterday and know nothing,” one of them would write. “And our days on earth are but a shadow.”
As we leap back in time again, we emerge before the final Pleistocene glaciation. We’ve gone tremendously far back, 129,000 years, though in some ways we’ve only returned to our own world. This was the most recent interglacial period, the last of many breaks between the ice ages, and the last time the planet was roughly as warm as it is today. Once more, the seas have risen hundreds of feet, but something is awry.
As the Earth’s wobble and orbit conspired to melt more ice than the poles have shed so far today, the planet absorbed more sunlight. As a result, global temperatures were little more than 1 degree warmer than today’s Anthropocene chart-toppers—or maybe even the same. But sea level was 20 to 30 feet higher than it is now. (A full third of Florida was sunk beneath the waves.) This is “sobering,” as one paper put it.
The Dallol sulfur springs in the Danakil Depression, Ethiopia, one of the hottest places on Earth
Modelers have tried and mostly failed to square how a world about as warm as today’s could produce seas so strangely high. Provisional, if nightmarish, explanations like the runaway, catastrophic collapse of monstrous ice cliffs more than 300 feet tall in Antarctica, which may or may not be set into motion in our own time, are fiercely debated in conference halls and geoscience departments.
Very soon, we may well have warmed the planet enough to trigger similarly dramatic sea-level rise, even if it takes centuries to play out. This is what the Exxon scientist James Black meant in 1977 when he warned higher-ups of the coming “super-interglacial” that would be brought about—as a matter of simple atmospheric physics—from burning fossil fuels. But our trajectory as a civilization is headed well beyond the warmth of the last interglacial, or any other interglacial period of the Pleistocene, for that matter. So it’s time to keep moving. We must take our first truly heroic leap into geologic time, millions of years into the past.
We’re more than 3 million years in the past now, and carbon dioxide in the atmosphere is at 400 parts per million, a level the planet will not again see until September 2016. This world is 3 to 4 degrees Celsius warmer than ours, and the sea level is up to 80 feet higher. Stunted beech trees and bogs line the foothills of the Transantarctic Mountains not far from the South Pole—the last members of a venerable line of once-majestic forests that had existed since long before the age of the dinosaurs.
What we’ve glossed over in our journey back to this ancient present: the entire evolutionary history of Homo sapiens , three Yellowstone super-eruptions, thousands of megafloods, the last of the giant terror birds, a mass extinction of whales, and the glacial creation and destruction of innumerable islands and moraines. As we make our way backwards in time to the Pliocene, the glaciations get briefer, and the ice sheets themselves become thinner and more temperamental. About 2.6 million years ago they all but disappear in North America, as CO2 levels continue their slow climb.
This article appears in the March 2021 print edition with the headline “The Dark Secrets of the Earth’s Deep Past.”