The vast database underlying the model rests on decades of unsung work all over the world by legions of geologists: the fruits of lonely camping trips at outcrops in the desert; hunks of rock fed to mass spectrometers in the lab; daring helicopter trips to the Arctic to gather fossils; new measurements of old specimens gathering dust in museums. “The dataset is 150,000 data points,” Tierney said. “That took us two years all by itself just to get it together into one place.”

A data point might be a single temperature from one part of the planet, registered in the oxygen isotopes in a fossilized oyster shell dated to hundreds of millions of years ago. Alone it tells us as little about the average temperature on that ancient planet as a thermometer outside your window can say about ours. But by integrating and leveraging these dispersed data — scattered throughout the rock record and across the Earth — with a sufficiently powerful climate model mapped onto ancient geographies, Tierney’s group can simulate these worlds with an unprecedented level of detail.

“Jess’ group has just been absolutely trailblazing,” said Richard Stockey, a systems scientist at the University of Southampton, who was not involved with the work. “The assembly of this fantastic dataset has taken us beyond anything we had before.”

The model results echo what’s known from the rock record. But the sheer range of average global temperatures Tierney found over Earth’s history — from an outrageous 20 degrees Celsius (36 degrees Fahrenheit) hotter than today during ancient greenhouse climates, to more than 6 degrees Celsius (10.8 degrees Fahrenheit) colder during its icehouse stretches — is extreme. In the tropics during Earth’s hottest epochs, such as spans when alligators lived near the poles 50 million years ago, temperatures inland that approach 50 degrees Celsius (122 degrees Fahrenheit) would seem to have pushed up against the hard limits for life.

“I already had a sense of how hot the planet could get, and the numbers that we get are really high,” she said. “Some people in the community are pretty uncomfortable with [our model producing] global temperatures that high.”

But besides a sheer planetary warmth and frigidity in epochs gone by that we would find completely inhospitable, the work drives home that global temperature has always moved in lockstep with atmospheric carbon dioxide concentration.

“CO₂ is the most important thing,” said Benjamin Mills, a geochemist at the University of Leeds who independently models Earth’s past climates, about the changing climate over the age of animal life. “Aside from, you know, the brightness of the sun. But the thing about the sun is, it’s very predictable. It’s gotten brighter over billions of years, very slowly, in a very predictable way — because it’s just a big, round ball of hydrogen and stuff. But in terms of moving between greenhouse and icehouse climates, CO₂ is the primary driver.”

Despite the surprising dynamism and range of the Earth system, it has never veered so far astray as to wink out the biosphere entirely — though there have been narrow misses. Some 300 million years ago, when carbon dioxide levels dropped to staggering lows, the planet nearly froze over. And 250 million years ago, extraordinary heat nearly killed off complex life in the high-CO₂, end-Permian mass extinction known as the Great Dying, just before the rise of the dinosaurs. But otherwise the climate, through its perturbations, has maintained conditions conducive to animal life for nearly a half-billion years.

This is not simply a fortunate accident. The planet has stayed in this range because its carbon dioxide is regulated through an extraordinary interplay between plate tectonics and life. The planet has a thermostat.

The Temperature Knob

Scientists have known for some time now that CO₂ is the “principal knob governing Earth’s temperature,” as a classic paper puts it. While carbon dioxide is steadily vented into the atmosphere by Earth’s volcanoes — at one one-hundredth the rate of modern anthropogenic carbon emissions — it’s elsewhere secreted away by the biosphere. Life is carbon-based, and the ultimate carbon source for all life is carbon dioxide, which for the most part enters the churn of the biosphere through the nanomachinery of photosynthesis. Over longer timescales, the ultimate resting place for much of this carbon dioxide is the Earth’s crust, whence it came.

When silicate rocks of the sort that make up most of the planet’s crust and mantle are exposed to the atmosphere, they blessedly draw down carbon dioxide. Rainwater reacts with carbon dioxide in the air and becomes slightly acidic, wearing away at rock exposed on the Earth’s surface. This carbon from the sky is then carried by rivers to the sea where it’s laid down on the seafloor as carbonate rocks like limestone.

“The whole mantle is made of these silicate rocks,” Mills said. “When you expose them to the atmosphere or even just underwater, they just automatically form this cycle, and they’re going to start removing carbon from the system.”

Fortunately for life on Earth, this brake on warming becomes more effective as carbon dioxide levels climb. On hotter, stormier worlds, these reactions speed up, drawing more of the offending carbon dioxide into rock. While this weathering feedback loop operates on a timescale of hundreds of millennia, it’s possible to inject enough carbon dioxide in a short enough span to render this safety switch irrelevant, driving unabated warming and ocean acidification. This happened during many of the worst spasms of mass extinction in Earth’s history, and it is happening again now. But, for the most part, this cycle has been sufficient to keep the surface world within livable bounds.

Still, it’s an unnervingly tight path the planet has to navigate. So far, the greenhouse effect has kept our planet hospitably warm. But if carbon dioxide sources and sinks were mismatched by as little as 5% for 5 million years, the biosphere would be utterly destroyed in a runaway high-CO₂ greenhouse. Conversely, if carbon dioxide levels ever fell too far, the Earth would freeze nearly solid, also devastating the biosphere.

Five million years is an eyeblink in geologic time. Over the past 50 million years, as carbon dioxide levels fell from 0.1% to 0.018% of the atmosphere, palm trees that once swayed along the shores of Alaska gave way to ice sheets that smothered a third of North America, dropping sea levels to some 400 feet lower than they are today. That share of the atmosphere is currently an ominous 0.042% (roughly 420 parts per million, or ppm) and rising fast.

In Tierney’s models, the correlation between global temperature change and carbon dioxide levels is surprisingly tight. Her group found that for every doubling of carbon dioxide, the temperature consistently jumped a whopping 8 degrees Celsius. It happened on ice-free worlds far warmer than our world today and during colder episodes alike.

If this is true then the ancient greenhouse world of the dinosaurs, lasting into the early age of mammals and already known to be sweltering, was even more unthinkably hot than previously appreciated. Mills and Stockey think the extraordinarily high temperatures reached in Tierney’s reconstructions could reflect subtle biases in some of the proxies that skew hot. Tierney, though, thinks the result could reflect mechanisms unknown from recent Earth history that can drive extreme temperature jumps.

In our rather cold recent geologic past, ice sheets may have helped hold Earth in the balance. But perhaps other equivalently transformative feedback loops take the wheel once carbon dioxide has doubled, amplifying soaring temperatures. Perhaps it’s methane that awakens on a warmer world, or perhaps it’s clouds, she suggested, which represent the largest uncertainty in climate models; under some of the grimmer emissions scenarios for the coming century, in which carbon dioxide tops 1,000 ppm, clouds might return our planet to a truly ancient hothouse (the one last seen when alligators roamed the poles).

“These were not worlds inhabited by us,” Tierney said of this kind of prospective jump back to the future. “The planet is capable of much warmer climates than today. It’s just that we aren’t at all adapted to them. We’re adapted — along with all of the ecosystems we share the planet with — to an icehouse climate. So that makes us very vulnerable to rapid climate change.”

Carbon Arithmetic

Like Tierney and Judd, Mills and his colleagues have simulated the past half-billion years of climate history — but instead of recreating it from proxies for temperature, they are doing it from the ground up.

Mills’ team works from what’s known about the physical Earth over deep time, and then lets the model naïvely produce the temperature estimates on its own. In doing so, they’ve further illuminated the shifting planetary sources and sinks for carbon dioxide that have hurled Earth back and forth between ice worlds and cauterized hothouses over hundreds of millions of years. While the Earth system is unfathomably complex, Mills has been able to reproduce the broad trajectory of our climate over the ages by turning just four knobs — each of which modulates the carbon dioxide budget.

First are the carbon dioxide sources. While volcanoes steadily puff the stuff into the air and oceans, the rate at which this happens has subtly changed over millions of years in the course of a grand planetary tectonic ballet.

Second are the sinks, the most important of which are the rocks themselves. Put water on a planet, spin it up and put a blinding orb in its sky, and you’ll get weather on that world. What is critical for burying carbon dioxide is where that weather (that is, precipitation) falls. Some rocks, such as Oman’s ophiolites or Indonesia’s basalts, are far more reactive than others. Put these reactive rocks in today’s tropics, where there’s lots of weather and warmth, and they’ll draw down far more carbon dioxide than if they were stranded on some dry and desolate landmass in the Arctic.

The third knob is paleogeography. The very configuration of the continents can bring about different climates. For example, with supercontinent arrangements such as Pangaea, little weather reached the rocks of their vast arid interiors. Struggling to get rid of its carbon dioxide, the Pangaean planet suffered from high heat.

Paleogeography is especially important insofar as it also influences the fourth and final factor: biology. While silicate rocks will draw down carbon dioxide all on their own, the emergence of large land plants around 380 million years ago transformed these processes. Plants plowed through the continents with roots and organic acids, accelerating rock weathering and shuttling carbon dioxide away (some as vast stores of buried organic matter, which we are now re-releasing into the atmosphere as fossil fuels).

By putting it all together — reconstructing the lengths and types of subduction zones, the shifting locations of the continents, the types of rocks that paved them and the plants that covered them — and simply letting the model run, Mills’ simulation successfully spat out not only the extraordinary high-CO₂ hothouse of the Permian and much later Cretaceous, but also the low-CO₂ ice ages at the appropriate times. These include a surprising chill around 450 million years ago that buried the Sahara in ice sheets (instigating the end-Ordovician mass extinction), the ice caps of the coal age some 300 million years ago, along with our own recent and ongoing ice age (mile-thick ice sheets, like those in Antarctica, are rare in Earth history).

“We haven’t told the model that it’s supposed to reproduce these ice ages,” Mills said at a recent seminar at the University of Southampton. “We’ve just told it what’s happening with tectonics, and we’ve given it its own reactive biosphere.”

These complementary efforts of fieldwork geology and the more digital world of computer modeling, in which geochemical data, fossils and ancient tectonics are paired with Earth system models, have helped pull back the veil on our deep past. After many decades of often thankless effort — desert fieldwork in rusting 4×4s and sediment coring on the heaving seas; endless grant writing and rejection letters; hours spent cataloging specimens and writing monographs now yellowing in forgotten university file cabinets; ages in the lab fussing over gas chromatographs and mass spectrometers; and, more recently, Red Bull–powered late nights coding in R or coaxing deep convolutional neural networks and advanced climate models — we can now see our planet’s climate history with more clarity and insight than ever before.

What it adds up to is the biography of Earth in the age of animal life.

Animals Take Off

We pick up this thread with 90% of the planet’s history already in the rearview mirror (we’ll put aside the disconcerting eons-long delay in the biosphere’s development). Animal life finally reached takeoff some 540 million years ago. But while the Cambrian might be best known for its explosion of life, as a psychedelic menagerie of shelly, tentacled, squirming animals blossomed, it was also a period when hot, stagnant seas punished this world. Waves of trilobite extinctions swept the oceans, and an ancient planet better known for its explosion of life now saw the ambition of this frenzied new animal world periodically stifled under a high-CO₂ hothouse.

As the deep calendar turned on the Cambrian, the fever broke. Tectonic collisions formed new mountains, whose freshly exposed rock encouraged weather to wear them down and drew carbon dioxide from the air, while a tentative plant world newly fringed the edges of fresh water, further drawing down carbon dioxide. The planet gently cooled — and sea life thrived — before crossing a threshold. Some 445 million years ago, Earth plummeted into a devastating ice age and mass extinction, followed by an even more merciless rebound back into the greenhouse hundreds of thousands of years later.

High heat would continue to haunt this planet before the confident rise of the terrestrial biosphere. Around 380 million years ago, ankle-high greenery shot to the sky in strange, towering forests that coaxed fish and invertebrates ashore to try out life on land. These pioneering forests geoengineered the ancient world, sequestering carbon dioxide in their very trunks, leaves and soils as they advanced on the continents. In the process, they accelerated rock weathering with the slow, gnashing work of roots.

Then, with bursts of tropical mountain-building that drained the skies of carbon dioxide, the Earth shivered and settled into the longest ice age in the history of animal life: the 100-million-year-long Late Paleozoic Ice Age. Tropical coal swamps, streaked by titanic bugs, were first buried in this age, moving carbon dioxide into the geologic abyss (they now give up that ancient carbon dioxide through modern smokestacks and tailpipes). Far from the equatorial lowlands where these swamps formed, polar glaciers steadily marched on the midlatitudes and threatened to take over the entire planet.

The grip of this ice age was loosened only when carbon dioxide began to soar again around 300 million years ago. The planet staggered toward the most dangerous greenhouse in the fossil record.

As subduction zones burned through the carbon-rich crust at the edges of the Pangaea supercontinent, any rock weathering that might have buried carbon dioxide now sputtered in the vast interior. Rainforests dried out and carbon dioxide levels ticked steadily upward. The planet stumbled toward apocalypse. On a world fully stocked with reptiles, dazzling reefs, ammonoids, sharks, massive amphibians, trilobites, giant sea scorpions and forgotten, lopped-off branches of our family tree, Siberia turned inside out. The landmass burbled over a million square miles of lava, injecting thousands of gigatons of carbon dioxide into the air and jackknifing the temperature by 10 degrees Celsius over thousands of years, killing off most animal life on Earth.

In the dreadful wake of the greatest die-off in history, the end-Permian mass extinction, the biosphere struggled for millions of years with high carbon dioxide levels during the torrid height of Pangaea. Having lost its tropical forests — a huge carbon sink — in the mass death, it stayed hot for millions of years, then weathered occasional mass extinctions during further bouts of CO₂-spewing volcanism. But as the supercontinent finally splintered apart around 200 million years ago and the near-endless age of dinosaurs found its groove, the planet recovered to conditions more hospitable to animal life.

Newly evolved chalky plankton fed a steady conveyor belt of carbon gently snowing through oceans to the seafloor, where it was delivered to deep sea trenches, cooked and then released through the throats of volcanoes at the surface again as carbon dioxide. This helped keep this Mesozoic planet perpetually warm, culminating some 90 million years ago in the steamy, jungly dinosaur world of our public imagination. When this world was catastrophically interrupted by a hunk of space trash some 30 million years later, in the cinematic end-Cretaceous mass extinction, the mammals would inherit not only the Earth but the climate of the dinosaurs as well.

Our Mammalian Age

The climatic impact of the asteroid strike that doomed the dinosaurs, while global and catastrophic, was relatively short-lived. In these early days of our age, the age of mammals, some 50 million years ago, the planet remained hot; carbon dioxide levels topped 1,000 ppm. The alligators of Arctic Canada lay motionless in the swamps to wait out months of polar night, as dawn redwoods filtered the starlight. But then, carbon dioxide began to steadily fall from these sweltering highs. As the atmospheric concentration of carbon dioxide dropped below about 750 ppm around 33 million years ago, Antarctica swelled with a smaller version of the ice sheet we now take for granted.

Earth shuddered with yet another wave of extinctions in this initial chill. About 30 million years later, as carbon dioxide continued its slow ebb to below 300 ppm, the planet finally plummeted into another series of spectacular ice ages — when sea level fell hundreds of feet lower than it is today — punctuated by brief, warmer reprieves such as our own (some 11,000 years in progress), paced by seemingly trivial changes in sunlight reaching northern latitude summers.

Over the past 2.6 million years, this metronome between the icy, volatile, dry, dusty, low-CO₂ world of the Pleistocene and the briefer, warmer periods that have repeatedly interrupted it (like the past few thousand years) shaped our evolution. That brings us to now, this infinitesimal moment of geological history — one of the most out of control in the history of animal life.

Poised at the end of a humdrum interglacial, in an otherwise chilly corner of Earth history, industrial civilization is emitting carbon dioxide at a clip 10 times faster than the apocalyptic volcanoes of the end-Permian mass extinction. In only a few decades, we have reproduced an antique level of carbon dioxide in the atmosphere unseen on Earth for millions of years, predating the evolution of our genus, Homo — from a time when camels roamed Arctic forests and sea level was 70 feet higher. Atmospheric carbon dioxide is back up to 420 ppm and rising.

The Earth system is extraordinarily far from equilibrium, and being pushed more violently so by the year. Where does that leave us?

“The message from the geological past is that this can go very badly wrong,” said Mills, noting that we don’t have an analogue for the global chemistry experiment we’re currently running.

“The end-Permian is just an insane event, but it’s a very long period of huge amounts of CO₂ degassing,” he said. “So we have analogues in terms of the magnitude of temperature change. But we don’t have anything on this timescale. We understand how these mechanisms work when they’re slowed down, but actually understanding how it’s going to play out in timescales like decades is really, really difficult.”

“We’ve never hit the Earth system this hard, this fast before,” Stockey said.

For Tierney, the lesson of Earth’s animal history is that we live on a far more extreme planet than the relatively sheltered millennia-long human historical memory would lead us to believe. We invite a demonstration of this planetary volatility at our peril.

“Right now we’ve warmed just over 1 degree [Celsius],” Tierney said. “It’s pretty small, given that the range we’re finding in the Phanerozoic [the eon of animal life] is between, like, 11 degrees and 36 degrees [average temperature].” Today, the global average surface temperature is about 15 degrees Celsius.

“But even with that tiny bit of global temperature change,” she continued, “we already see all these major changes in climate — drought, bigger floods, bigger hurricanes, bigger fires. It just shows you how dynamic the Earth system is. It doesn’t take much of a temperature change to create a really different world.”

As we pull out to the geological scale again, and run the clock forward this time, we find that the human chemistry experiment, like all previous paroxysms of carbon dioxide, will be dealt with in good time. Over tens of millennia, no matter how much we put into the atmosphere, the rocks will weather, the carbon dioxide will be transmuted to rock, the planet will slowly cool. And perhaps hundreds of millennia from now, the violent human imposition on the climate will be relegated to the realm of Earth history, too.

Since Mills’ modeling can blindly reproduce the climates of the ancient past, he’s recently toyed with putting it into reverse to see what awaits our ever-changing planet in the far, far future. Picking up the pace a bit more, one at which we now see the continents perceptibly move across the face of the Earth, untold biomes and regimes of animals come and go, evolve and fossilize, until all the continents are reunited again some 250 million years in the future. As the sun has grown subtly brighter over this span, if this supercontinent struggled with high carbon dioxide levels like its Pangaean forebear did, then this would be an inimically hot world to animals, except for lone refuges fringing its polar far reaches.

And if another end-Permian-style bout of volcanism were to strike this “Pangaea Ultima,” then it could be the death blow for complex life. If not, then a spin or two around the galaxy later, and the sun will have grown bright enough, and the water cycle amplified enough as a result, that ramped up rock weathering will draw carbon dioxide so low that even grasses can’t hold on. Photosynthesis will stop, and that will be that.

While that might mean the end of our story, as we pan out to the galactic scale once more, perhaps other planets on other spiral arms will then find themselves similarly in full bloom for a while, miraculously managing their carbon for a rich, improbable, glorious season of life.