Could Climate Change Be More Extreme Than We Think?
Our climate models could be missing something big. Photo Illustrations by Brendan Pattengale | Maps by La Tigre Images above: Glaciers from the Vatnajokull 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. Updated at 1:53 p.m. ET on February 11, 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 Earths 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 Earths 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, CO 2 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 CO 2 . At some points in the Earths history, lots of CO 2 has vented from the crust and leaped from the seas, and the planet has gotten warm. At others, lots of CO 2 has been hidden away in the rocks and in the oceans depths, and the planet has gotten cold. The sea level, meanwhile, has tried to keep uprising 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, CO 2 has been the primary driver of the Earths climate . And sometimes, when the planet has issued a truly titanic slug of CO 2 into the atmosphere, things have gone horribly wrong. Today, atmospheric CO 2 sits at 410 parts per million, a higher level than at any point in more than 3 million years. And humans are injecting more CO 2 into the atmosphere at one of the fastest rates ever. When hucksters tell you that the climate is always changing, theyre right, but thats 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 historyat only a few thousand years, a mere eyeblink in geologic timehas played out in perhaps the most stable climate window of the past 650,000 years. We have been shielded from the climates violence by our short civilizational memory, and our remarkably good fortune. But humanitys ongoing chemistry experiment on our planet could push the climate well beyond those slim historical parameters, into a state it hasnt seen in tens of millions of years, a world for which Homo sapiens did not evolve. When theres been as much carbon dioxide in the air as there already is todaynot to mention how much theres likely to be in 50 or 100 yearsthe 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 CO 2 stays at its current levels, much less steadily increases, it will take centurieseven millenniafor the planet to fully find its new footing. The transition will be punishing in the near term and the long term, and when its 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 its 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-CO 2 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. Read: Scientists have uncovered a disturbing climate change precedent The first couple of steps back in time wont take us to a warmer worldbut they will illuminate just what sort of ill-tempered planet were dealing with. As we pull back even slightly from the span of recorded historyour tiny sliver of geologic timewell 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 CO 2 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 CO 2 as it is now, we wont be returning to an ice age for the foreseeable future. And to reach analogues for the kind of warming well 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 todays 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 andafter 200,000 years of wanderinghad begun to stay put. Sedentary settlements started to blossom. Humans, with a surfeit of calories, began to divide their labor, and artisans plied new arts. The Earths oldest cities, such as Jericho, were bustling. Its easy to forget that the Earthcozy, pastoral, familiaris 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 civilizations first light, the Earths top half was aimed toward the sun during the closest part of its orbitan 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 Saharas 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 Doggerlanda lowland that had joined mainland Europe to the British Isleswere abandoned by nomadic humans and offered to the surging seas. Vast islands like Georges Bank, 75 miles off Massachusettswhich once held mastodons and giant ground slothssaw 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. Thats 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 civilizationsa veritable globalized economyfell 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 IIIthe last great pharaoh of Egypts New Kingdom periodspeak 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 worldone strained by souring trade relations, with fractious populaces led by unsteady, unscrupulous leaders and now stricken by plaguethe 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. This same story is told elsewhere, over and over, throughout the extremely mild stretch of time that is written history. The Roman empires imperial power was vouchsafed by centuries of warm weather, but its end saw a return to an arid coldperhaps conjured by distant pressure systems over Iceland and the Azores. In A.D. 536, known as the worst year to be alive , one of Icelands 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 wasnt 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 summerwhich brought snows to New England in Augustglobal 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. Read: The strange future Hurricane Harvey portends 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 yearsto yesterday, geologicallythe 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 makea 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 Antarcticas 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 Bonnevillethe 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 todays 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 agetents, hearths, burnt garbage pits, hoards of ivory jewelryall 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 CO 2 out of the air and deep into the ocean, freezing the planet further. Read: When a killer climate catastrophe struck the worlds oceans This parched Pleistocene world would have appeared duller from space, hosting as it did a quarter less plant life. CO 2 in the atmosphere registered only a paltry 180 ppm, less than half of what it is today. In fact, CO 2 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 CO 2 unused by plantsin 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. Its 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. 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 planets past, it has had virtually no ice whatsoever. The periods of extreme coldlike the ultra-ancient, phantasmagoric nightmares of Snowball Earth, when the oceans might have been smothered by ice sheets all the way to the tropicsare 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 Earths history, the planet was a much warmer place than it is today, with much higher CO 2 levels. This is not a climate-denying talking point; its 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-CO 2 worlda 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-CO 2 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 protoIndian Ocean and smashed into Asia in slow motion. The collision not only quieted CO 2 -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 rocksthat is, breaking them down with CO 2 -rich rainwateris one of the planets 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 CO 2 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 planets carbon sink . Over tens of millions of years, then, the stately march of plate tectonicsthe balance of volcanic CO 2 and rock weatheringseems to have driven long-term climate change, in our case toward a colder, lower-CO 2 world. As well see, humans now threaten to undo this entire epic, geologic-scale climate evolution of the Cenozoic eraand in only a few decades. When Earths blanket of CO 2 was finally thin enough , the planets 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 leisurelythe depths of planetary winter taking tens of thousands of years to arrivethe 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. Coral reefs marking the ancient sea levelbut today lying deep off the coasts of Tahiti and Indonesiareveal 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 equators 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 worlds 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 Earths 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 ices retreat. Permafrost melted, and methane bubbled up from thawing bogs. Colder, more CO 2 -soluble oceans warmed, and gave up the carbon theyd 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 CO 2 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. Weve gone tremendously far back, 129,000 years, though in some ways weve 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 Earths 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 todays Anthropocene chart-toppersor 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. Modelers have tried and mostly failed to square how a world about as warm as todays 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 aboutas a matter of simple atmospheric physicsfrom 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 its time to keep moving. We must take our first truly heroic leap into geologic time, millions of years into the past. Were 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 Polethe last members of a venerable line of once-majestic forests that had existed since long before the age of the dinosaurs. What weve 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 CO 2 levels continue their slow climb . When we arrive in the middle of the Pliocene, just over 3 million years ago, CO 2 levels are high enough that weve escaped the cycle of ice ages and warm interglacials altogether. Lucy the Australopithecus roams a heavily forested East Africa. We are now outside the evolutionary envelope of our modern world, sculpted as it was by the temperamental northern ice sheets and deep freezes of the Pleistocene. But as to atmospheric carbon dioxide, 3 million years is how far back we have to go to arrive at an analogue for 2021. Despite the similarities between our world and that of the Pliocene, the differences are notable. In the Canadian High Arcticwhere today tundra spreads to the horizonevergreen forests come right to the edge of an ice-free Arctic Ocean. Though the world as a whole is only a few degrees warmer, the Arctic, as always, gets the brunt of the extra heat. This is called polar amplification, and its why maps of modern warming are crowned by a disturbing fog of maroon. Models struggle to reproduce the extreme level of warming in the Pliocene Arctic. Its a full 10 to 15 degrees Celsius warmer in the long twilight of northern Canada, and the pine and birch woodlands of these Arctic shores are filled with gigantic forest-dwelling camels . Occasionally this boreal world erupts in wildfire, a phenomenon echoed by the blazes that today sweep ever farther north. Elsewhere, West Antarcticas ice sheet may have disappeared entirely, and Greenlands, if it exists at all, is shriveled and pathetic. A common projection for our own warming world is that, while the wet places will get wetter, the dry places will get drier. But the Pliocene seems to defy this saw for reasons not yet fully understood. Its a strangely wet world , especially the subtropics, wherein the Sahara, the Outback, the Atacama, the American Southwest, and Namibialakes, savannas, and woodlands replace deserts. This ancient wetness might come down to inadequacies in how we model clouds , which are under no obligation to behave in physical reality as they do in simplified lines of computer code. Hurricanes were almost certainly more consistently punishing 3 million years ago, just as our storms of the future will be. And a more sluggish circulation of the atmosphere might have lulled the trade winds, turning El Nino into El Padre. Perhaps this is what brought rainsand lakesto the Mojave at this time. Our modern coastlines would have been so far underwater that youd have to take great pains to avoid getting the bends if you tried scuba diving down to them. Today, traveling east through Virginia, or North or South Carolina, or Georgia, midway through your drive youll pass over a gentle 100-foot drop. This is the Orangeburg Scarp, a bluffhundreds of miles longthat divides the broad, flat coastal plain of the American Southeast. It comprises the eroded and smoothed-out rumors of once-magnificent sea cliffs. Here, waves of the Pliocene high seas chewed away at the middle of the Carolinasan East Coast Big Sur. This ancient shoreline is visible from space by the change in soil color that divides the states, and is visible on much closer inspection as well: To the east of this strange drop-off, giant megalodon-shark teeth and whale bones litter the Carolina Low Country. Though warped over the ages by the secret workings of the mantle far below, these subtle banks 90 miles inland nevertheless mark the highest shoreline of the Pliocene, when the seas were dozens of feet higher than they are today. But even within this warm Pliocene period, the sea level leaped and fell by as much as 60 feet every 20,000 years, to the rhythm of the Earths sway in space. This is because, under this higher-CO 2 regime, the unstable ice sheet in Antarctica took on the volatile temperament that, 1 million years later, would come to characterize North Americas ice sheet, toying with the ancient coastline as if it were a marionette. So this is the Pliocene, the world of the distant present. While todays projections of future warming tend to end in 2100, the Pliocene illuminates just what sort of long-term changes might inevitably be set in motion by the atmosphere weve already engineered. As the great ice sheets melt, the permafrost awakens, and darker forested land encroaches on the worlds tundra, positive feedbacks may eventually launch our planet into a different state altogether, one that might resemble this bygone world. Nevertheless, human civilization is unlikely to keep atmospheric CO 2 at a Pliocene levelso more ancient and extreme analogues must be retrieved. Were now deeper in the past, and the planet appears truly exotic. The Amazon is running backwards, and gathers in great pools at the foot of the Andes. A seaway stretches from Western Europe to Kazakhstan and spills into the Indian Ocean. Californias Central Valley is open ocean. What today is the northwestern U.S. is especially unrecognizable. Today the airy, columnated canyons of the Columbia River on the Oregon-Washington border swarm with tiny kiteboarders zipping through gorges of basalt. But 16 million years ago, this was a black, unbreathable place, flowing with rivers of incandescent rock. The Columbia River basaltsold lava flows that spread across Washington, Oregon, and Idaho, in some places more than two miles thickwere the creation of a class of extremely rare and world-changing volcanic eruptions known as large igneous provinces, or LIPs. Some LIPs in Earths history span millions of square miles, erupt for millions of years, inject tens of thousands of gigatons of CO 2 into the air, and are responsible for most of the worst mass extinctions in the history of the planet . They live up to their namethey are large. But these mid-Miocene eruptions were still rather small as far as LIPs go, and so the planet was spared mass death. Nevertheless, the billowing volcanoes raised atmospheric CO 2 up to about 500 ppm, a level that today represents something close to the most ambitious and optimistic scenario possible for limiting our future carbon emissions. In the Miocene, this volcanic CO 2 warmed up the world to at least 4 degrees Celsius and perhaps as much as 8 degrees above modern temperatures. As a result, there were turtles and parrots in Siberia. Canadas Devon Island, in the high Arctic, is today a desolate wasteland, the largest uninhabited island in the worldand one used by NASA to simulate life on Mars. In the Miocene, its flora resembled Lower Michigans. The sweeping grasslands distinctive to our cooler, drier, low-CO 2 world had yet to take over the planet, and so forests were everywherein the middle of Australia and Central Asia and Patagonia. All of this vegetation was one of the reasons it was so warm. Forests and shrubs made this planet darker than our own worldone still painted pallid hues in many places by bare land and iceand allowed it to absorb more heat. This change in the planets color is just one of the many long-term feedback loops awaiting us after the ice melts. Long after our initial pulse of CO 2 , they will make our future world warmer and more alien still. As for fauna, were now so distantly marooned in time from our own world that most of the creatures that inhabited this leafy planet range from the flatly unfamiliar to the uncannily so. There were big cats that werent cats, and rhino-size hell pigs that werent pigs. There were sloths that lived in the ocean and walruses that werent related to todays walruses. Earths largest-ever meat-eating land mammals, African juggernauts like Megistotherium and Simbakubwa, not closely related to any living mammals today, tore early elephants apart with bladed mouths. And with CO 2 at 500 ppm, the sea level was about 150 feet higher than today. Approaching Antarctica in the middle Miocene by sea, the waters would be warmer than today, and virtually unvisited by ice. To get to the ice sheet, youd have to hike far past lakes and forests of conifers that lined the coast. Trudging past the trees and finally over endless tundra, you would come at last to the edge of a much smaller ice sheet whose best days were still ahead of it. An axiom about this land-based Antarctic ice sheet in paleoclimatology is that its incredibly stubborn. That is, once you have an ice sheet atop the heart of Antarctica, feedback loops kick in to make it exceedingly hard to get rid of. Barring true climatic madness, a land-based Antarctic ice sheet is essentially there to stay. But in the middle Miocene this young Antarctic ice sheet seemed to have a temper. It might have been surprisingly dynamic, as one paper cheerfully puts it. As CO 2 increased from just below todays levels up to about 500 ppm, Miocene Antarctica shed what today would amount to 30 to 80 percent of the modern ice sheet. In the Miocene, Antarctica seemed exquisitely tuned to small changes in atmospheric CO 2 , in ways that we dont fully understand and that were not incorporating into our models of the future. There will undoubtedly be surprises awaiting us in our high-CO 2 future, just as there were for life that existed in the Miocene. In fact, the Antarctic ice sheet may be more vulnerable today to rapid retreat and disintegration than at any time in its entire 34-million-year history . In the 16 million years since this mid-Miocene heat, the volcanic hot spot responsible for the Columbia River basalts has wandered under Yellowstone. Today it powers a much tamer kind of volcano. It could cover a few states in a few inches of ash and disrupt global agriculture for years, but it couldnt launch the planet into a new climate for hundreds of thousands of years, or kill most life on the surface. Unfortunately, there is such a supervolcano active on Earth today: industrial civilization. With CO 2 likely to soar past 500 ppm from future emissions, even the sweat-soaked, Siberian-parrot-populated world of the middle Miocene might not tell us everything we need to know about our future climate. Its time to go back to a global greenhouse climate that ranks among the warmest climate regimes complex life has ever endured. In our final leap backwards, CO 2 at last reaches levels that humans might reproduce in the next 100 years or so. What follows is something like a worst-case scenario for future carbon emissions. But these worst-case projections have continued to prove stubbornly accurate in the 21st century so far , and they remain a possible path for our future. Were now about to take our largest leap, by far, into the geologic past. We hurdle over 40 million years of history, past volcanic eruptions thousands of times bigger than that of Mount St. Helens, past an asteroid impact that punched out a gigantic crater where the Chesapeake Bay sits today. The Himalayas slump; India unhitches from Asia; and the further back we go, the higher the CO 2 level rises and the warmer the Earth gets. The Antarctic ice sheet, in its death throes, vanishes altogether, and the polar continent instead gives way to monkey puzzle trees and marsupials. We have arrived, finally at the end of our journey, in the greenhouse world of the early age of mammals. Today the last dry land one steps on in Canada before setting out across the ice-choked seas for the North Pole is Ellesmere Island, at the top of the world. But once upon a time there was a rainforest here. We know this because tree stumps still erode out of the barren hillsides, and theyre more than 50 million years old . Theyre all thats left of an ancient polar jungle now whipped by indifferent Arctic winds. But once upon a time, this island was a swampy cathedral of redwoods, whose canopy naves were filled with flying lemurs, giant salamanders, and hippolike beasts that pierced the waters. At this polar latitude, on some late-fall evening of the early Eocene, the sun tried and failed to lift itself from the horizon. A pink twilight reached deep into the jungle, but soon the sun would set entirely here for more than four months. In this unending Arctic dark, the stillness would be broken by the orphaned calls of tiny early primates, who hopped fearlessly over stilled alligators that would start moving again when the sun returned from beyond the horizon. In this unending night, tapirs hunted for mushrooms and munched on leaf litter that was left over from sunny days past and that in the far future would become coal. We have no modern analogue for a swampy rainforest teeming with reptiles that nevertheless endures months of Arctic twilight and polar night. But for each degree Celsius the planet warms, the atmosphere holds about 6 percent more water vapor , and given that global temperatures at the beginning of the age of mammals were roughly 13 degrees warmer than today, its difficult to imagine how uncomfortable this planet would be for Ice Age creatures like ourselves. In fact, much of the planet would be rendered off-limits to us, far too hot and humid for human physiology. Not only was this a sweltering age, but it was also one cruelly punctuated by some of the most profound and sudden CO 2 -driven global-warming events in geologic historyon top of this already feverish baseline. Deep under the North Atlantic, the Eocene epoch kicked off in style 56 million years ago with massive sheets of magma that spread sideways through the crust, igniting vast, diffuse deposits of fossil fuels at the bottom of the ocean. This ignition of the underworld injected something like the carbon equivalent of all currently known fossil-fuel reserves into the seas and atmosphere in less than 20,000 years, warming the planet by another 5 to 9 degrees Celsius. Evidence abounds of violent storms and megafloods during this ancient spasm of climate changeepisodic waves of torrential rains unlike any on Earth today. In some places, such storms would have been routine, separated by merciless droughts and long, brutal, cloudless heat waves. Seas near the equator may have been almost as hot as a Jacuzzitoo hot for most complex life. As for the rest of the planet, all of this excess CO 2 acidified the oceans, and the worlds coral reefs collapsed. Ocean chemistry took 200,000 years to recover. The most jarring thing about the early age of mammals, though, isnt merely the extreme heat. Its the testimony of the plants. In higher-CO 2 conditions, plants reduce the number of pores on their leaves, and fossil leaves from the jungles of the early Eocene have tellingly fewer pores than todays. By some estimates, CO 2 50 million years ago was about 600 ppm . Other proxies point to higher CO 2 , just over 1,000 ppm, but even that amount has long bedeviled our computer models of climate change. For years, in fact, models have told us that to reproduce this feverish world, wed need to ramp up CO 2 to more than 4,000 ppm . This ancient planet is far more extreme than anything being predicted for the end of the century by the United Nations or anyone else. After all, the world that hosted the rainforests of Ellesmere Island was 13 degrees Celsius warmer than our own, while the current global ambition, enshrined in the Paris Agreement, is to limit warming to less than 2, or even 1.5, degrees. Part of what explains this glaring disparity is that most climate projections end at the end of the century. Feedbacks that might get you to Eocene- or Miocene-level warmth play out over much longer timescales than a century. But the other, much scarier insight that Earths history is very starkly telling us is that we have been missing something crucial in the models we use to predict the future. Some of the models are starting to catch up. In 2019, one of the most computationally demanding climate models ever run, by researchers at the California Institute of Technology, simulated global temperatures suddenly leaping 12 degrees Celsius by the next century if atmospheric CO 2 reached 1,200 ppma very bad, but not impossible, emissions pathway. And later that year, scientists from the University of Michigan and the University of Arizona were similarly able to reproduce the warmth of the Eocene by using a more sophisticated model of how water behaves at the smallest scales. The paleoclimatologist Jessica Tierney thinks the key may be the clouds. Today, the San Francisco fog reliably rolls in, stranding bridge towers high above the marine layer like birthday candles. These clouds are a mainstay of west coasts around the world, reflecting sunlight back to space from coastal California and Peru and Namibia. But under higher-CO 2 conditions and higher temperatures, water droplets in incipient clouds could get bigger and rain down faster. In the Eocene, this might have caused these clouds to fall apart and disappearinviting more solar energy to reach, and warm, the oceans. That might be why the Eocene was so outrageously hot. This sauna of our early mammalian ancestors represents something close to the worst possible scenario for future warming (although some studies claim that humans, under truly nihilistic emissions scenarios, could make the planet even warmer). The good news is the inertia of the Earths climate system is such that we still have time to rapidly reverse course, heading off an encore of this world, or that of the Miocene, or even the Pliocene, in the coming decades. All it will require is instantaneously halting the super-eruption of CO 2 disgorged into the atmosphere that began with the Industrial Revolution. We know how to do this, and we cannot underplay the urgency. The fact is that none of these ancient periods is actually an apt analogue for the future if things go wrong. It took millions of years to produce the climates of the Miocene or the Eocene, and the rate of change right now is almost unprecedented in the history of animal life. Humans are currently injecting CO 2 into the air 10 times faster than even during the most extreme periods within the age of mammals. And you dont need the planet to get as hot as it was in the early Eocene to catastrophically acidify the oceans. Acidification is all about the rate of CO 2 emissions, and we are off the charts. Ocean acidification could reach the same level it did 56 million years ago by later this century, and then keep going. When he coined the term mass extinction in a 1963 paper, Crises in the History of Life, the American paleontologist Norman Newell posited that this was what happened when the environment changed faster than evolution could accommodate. Life has speed limits. And in fact, life today is still trying to catch up with the thaw-out of the last ice age, about 12,000 years ago. Meanwhile, our familiar seasons are growing ever more strange: Flycatchers arrive weeks after their caterpillar prey hatches; orchids bloom when there are no bees willing to pollinate them. The early melting of sea ice has driven polar bears ashore, shifting their diet from seals to goose eggs. And thats after just 1 degree of warming. Subtropical life may have been happy in a warmer Eocene Arctic, but theres no reason to think such an intimately adapted ecosystem, evolved on a greenhouse planet over millions of years, could be reestablished in a few centuries or millennia. Drown the Florida Everglades, and its crocodilians wouldnt have an easy time moving north into their old Miocene stomping grounds in New Jersey, much less migrating all the way to the unspoiled Arctic bayous if humans re-create the world of the Eocene. They will run into the levees and fortifications of drowning Florida exurbs. We are imposing a rate of change on the planet that has almost never happened before in geologic history, while largely preventing life on Earth from adjusting to that change. Taking in the whole sweep of Earths history, now we see how unnatural, nightmarish, and profound our current experiment on the planet really is. A small population of our particular species of primate has, in only a few decades, unlocked a massive reservoir of old carbon slumbering in the Earth, gathering since the dawn of life, and set off on a global immolation of Earths history to power the modern world. As a result, up to half of the tropical coral reefs on Earth have died, 10 trillion tons of ice have melted, the ocean has grown 30 percent more acidic, and global temperatures have spiked. If we keep going down this path for a geologic nanosecond longer, who knows what will happen? The next few fleeting moments are ours, but they will echo for hundreds of thousands, even millions, of years. This is one of the most important times to be alive in the history of life. This article appears in the March 2021 print edition with the headline The Dark Secrets of the Earths Deep Past.