How hot can Earth get? Our planet’s climate history holds clues

Our species likes it cold.

Homo sapiens evolved in — and still inhabits — one of Earth’s rare and fragile ice ages, periods distinguished not by an abundance of saber-toothed cats and woolly mammoths but by ice caps at the poles. For most of its 4.5-billion-year history, our planet was too warm for polar ice. Tyrannosaurus rex’s steamy Cretaceous kingdom 66 million years ago was in many ways a more representative slice of history than our own. Back then, reefs blanketed the beds of shallow seas as warm as bathwater, and jungle creatures watched the southern lights dance behind gaps in the thick canopies of Antarctic rainforests.

Not every warm period was so pleasant. In the Permian Period, some 270 million years ago, all animal life on Earth very nearly ended in a catastrophic mass extinction accompanied by intense, rapid global warming. But while the Permian world ended in fire, life on Earth has nearly perished more than once — and sometimes, it ended in ice. The polar ice caps crept down to the equator and the planet froze over in states known as “Snowball Earth,” which each lasted for millions of years.

Earth’s history confronts us with the fleeting fragility of our moment: Earth doesn’t have to look like it does now. In fact, it usually doesn’t. Between our world’s fiery infancy and its (for now) chilly present, it has been many planets, thanks to a multitude of geologic forces.

Understanding why Earth’s climate changed in the past — and what happened to life when it did — can help us understand our unusual moment today. Because while our species likes it cold, we’ve taken the reins of climate in hand and veered our planet onto a hot new trajectory. What does the past have to teach about where we might end up?

The Hadean Eon was hell on Earth

An ocean of magma stretches to the horizon in every direction, an expanse of liquid rock beneath a cracked crust of black-gray basalt. The sunlight beating down on this dead expanse is weak, dim — but heat rises from the depths below.

This is the Hadean Eon, Earth’s turbulent infancy, which began some 4.6 billion years ago when clumps of material coalesced out of the disk of hot dust and gas that swirled around the young sun. This disk was toasty, at least a few hundred degrees Celsius where the rocky planets formed. But it got a lot hotter when, about 100 million years later, a Mars-sized rock called Theia smacked into the young Earth. That run-in released the energy equivalent of trillions of H-bombs — enough “to pretty much vaporize most of Theia and melt what becomes the Earth,” says planetary scientist Norman Sleep of Stanford University.

That collision left the planet a hellish ocean of magma beneath a sky of rock vapor. And in the sky hung yet another ball of magma, an incandescent orb: the moon, which had coalesced out of impact debris potentially within a few short hours of the collision. Depending on exactly how the moon kaboom played out, the hottest vaporized bits of whatever was left in the impact’s aftermath could have reached temperatures of about 10,000° C, says geologist Mark Harrison of UCLA. “No part of the Earth would ever have subsequently reached more than about 7,000 kelvins,” or about 6700° C.

Surface temperatures on the solid rock that survived the impact were far lower, probably around 2000° C. Anything above that, and there wouldn’t have been a surface at all.

Over the next 1,000 years, Earth cooled enough for the rock vapor in the atmosphere to condense out; perhaps in showers of lava, perhaps in flakes of rocky snow. It took longer for the magma ocean to solidify. The freshly formed moon heated Earth via gravitational forces, which kneaded Earth’s interior and kept the planet molten for millions, perhaps tens of millions, of years. But when the magma ocean finally crystallized into rock, the planet crossed a threshold, Sleep says.

The sun overtook Earth’s smoldering heart as the most important source of energy. From there on out, Earth’s climate would be dictated by how much solar energy the planet received, reflected and retained.

Earth’s thermostat turned on in the Archean Eon

As the Hadean Earth cooled, it eventually started to rain. And rain. And rain. Water vapor poured out of the atmosphere and over the barren plains until Earth’s surface drowned beneath a global ocean once more — this time, of water.

The Archean Eon, 4 billion to 2.5 billion years ago, begins with the rock record itself, when the surface finally cooled enough for rock to stay solid. And the rocks from this time, when land first peeked above the seas in arcs of volcanic islands, paint a picture of a world that’s a bit chilly, especially the poles. Simulations suggest that surface temperatures ranged between a frosty zero degrees and a toasty 40° C — perfectly habitable. In fact, the earliest signs of life date to this period.

But the Archean presents a climate conundrum. At its onset, the sun was only about 70 to 80 percent as bright as it is today. The sun’s energy comes from the fusion of hydrogen to helium. As hydrogen gets used up, the core gets denser, which speeds up fusion and makes the sun brighter and hotter over time. The energy coming from the faint young sun on its own would not have been enough to keep the planet as warm as it was. So in theory, Earth should have transformed into the ninth circle of Dante’s hell — iced over.

The answer to the paradox? Greenhouse gases like carbon dioxide and methane. These gases allow sunlight to beat down on Earth’s surface, which heats it up, but don’t allow heat to radiate back out as infrared light. This traps heat around Earth like a blanket. “There was a bigger greenhouse effect” than today, says planetary scientist David Catling of the University of Washington in Seattle. “That sort of is the basic story of the Archean: fainter sun, more greenhouse gases.”

As the Hadean magma ocean cooled, it outgassed a thick, steamy atmosphere rich in water vapor and carbon dioxide. Despite the faint sun, temperatures could have been around 200° C right after the magma ocean solidified. However, sometime between the hellish Hadean and clement Archean, the planet’s natural thermostat came online: the carbon cycle.

Atmospheric carbon dioxide gets transformed into chalky white carbonate minerals through chemical weathering. This process traps carbon dioxide in rock, but it doesn’t stay trapped forever. Over hundreds of thousands of years, Earth constantly recycles its surface into the interior through plate tectonics. When carbonates end up in the mantle, they eventually break down and get belched back up by volcanoes as carbon dioxide. This cycle is sensitive to temperature: Chemical weathering speeds up in warm climates and slows down in cold ones.

At least by the beginning of the Archean, the carbon cycle had locked away enough carbon dioxide to bring the planet’s surface temperature into a habitable range. With carbon dioxide levels between 10 and 1,000 times as high as today and methane levels 100 to 10,000 times as high, the Achaean Earth was alien but livable, its seas strewn by lumpy mounds of microbes huddled together below the hazy, orange sky.

The geologic thermostat has regulated Earth’s temperature ever since and never once has it gotten hot or cold enough to end all life.

But it has come close.

A deep freeze during Snowball Earth

Between 2.4 billion and 2.1 billion years ago, near the beginning of the Proterozoic Eon, Earth froze over. Thick sheets of ice encased the planet from pole to equator. Temperatures may have plummeted to as low as −50° C — low enough to cause frostbite within minutes — and stayed low for tens of millions of years. It was, perhaps, the scene of one of Earth’s first mass extinctions. But since the only casualties were microbes, almost no fossils remain to record the death toll.

This climate cataclysm was one of several icy episodes called Snowball Earths. Those episodes bookend the otherwise toasty Proterozoic Eon, which stretched from 2.5 billion to 541 million years ago. They were the result of a runaway feedback loop: Sparkling white ice is more reflective than land or seawater. So, the more ice grows, the more sunlight Earth reflects. This increase in reflectivity, or albedo, lowers temperatures, encouraging more ice to form in a positive feedback loop. Once polar ice creeps past a latitude of about 30° North or South, the planet will become a snowball.

“Once you reach that tipping point in the area of sea ice, then it takes on the order of 200 or 300 years to reach the fully glaciated state,” says field geologist Paul Hoffman of the University of Victoria in Canada. “That’s pretty quick on a geological time scale.”

Earth’s thermostat won’t let a Snowball go on forever. With the land frozen over, chemical weathering shuts down. But volcanoes don’t. They keep pumping carbon dioxide into the atmosphere. Eventually, the greenhouse effect will thaw out the planet. Ice melts, the planet becomes less reflective, the planet warms even more and then more ice melts.

We know that the Snowballs happened thanks to glacial rock deposits left behind in areas that were near the equator back then. How they started is more mysterious, but one theory blames biology for the very first deep freeze.

The transition from the Archean to Proterozoic is, in some places, an almost literal red line in the rock record. Thick bands of red, iron-rich stone appear about 2.5 billion years ago. These banded iron formations probably formed with the emergence of photosynthetic microbes that started to fill the oceans with oxygen. Iron dissolved in the seas rusted out as solid particles, which accumulated on the seafloor in sediments that would become the banded iron formations.

As the oceans bloomed with photosynthetic organisms, more and more oxygen rose into the air. The oxygen oxidized the methane, which had served as an atmospheric blanket keeping Earth warm for 1.5 billion years.

“On a timescale of 10,000 years, you destroy your methane as oxygen rises,” Catling says. “That can’t be compensated for by the geologic carbon cycle, because that’s slow. So then you can get into the runaway albedo, and you could grow ice sheets and make a Snowball Earth.”

Global warming and the Permian extinction

Near the end of the Permian Period some 252 million years ago, the super­continent Pangaea would have been a good setting for a Western: It was a sunbaked, dusty wasteland from horizon to horizon. Daytime air temperatures in the tropics hovered around 50° C. On the hottest days, they climbed to 73° C — hot enough to denature protein. Any animal that hadn’t yet fled to the poles, where forests sprung up despite the long polar nights, would have been cooked alive.

The climate had been becoming less hospitable to life for some 20 million years, in part thanks to the assembly of Pangaea, says geologist Neil Tabor of Southern Methodist University in Dallas. With more land crammed together, coastlines shrunk, sea levels dropped, everything dried out and temperatures in the desiccated continental interior swung wildly.

“In marine environments, you still have functional ecosystems in the tropics and at high latitudes,” Tabor says. “But on land, it just goes to hell.” At least, that is, before the death blow.

The mass extinction at the end of the Permian was the worst our planet has ever seen. And while marine ecosystems initially remained mostly unscathed, they were ultimately hit hardest. In a climate meltdown that lasted a few hundred thousand years, 95 percent of marine and 70 percent of terrestrial species disappeared.

About 300,000 years before the peak of the extinction, volcanoes in what’s now Siberia erupted and didn’t stop for 1 million years. This volcanic region, called the Siberian Traps, belched up enough lava to bury an area as large as the continental United States in 50 meters of molten rock. With all that lava came lots and lots of carbon dioxide.

In a geologic blink, perhaps as quickly as 60,000 years, Earth’s average surface temperature soared by up to 10 degrees C to around 30° C. Oceans sweltered and grew too sluggish to circulate oxygen. Much marine life suffocated, and bacteria that thrived in the anoxic depths poisoned the water with hydrogen sulfide. That deadly gas might have bubbled up to poison the land, too. Volcanic gas mixed with water to rain acid on the barren, dusty wastes.

“It’s just these toxic, salty, shallow acid lakes and lots of windblown, red dust,” says geologist Kathleen Benison of the University of West Virginia in Morgantown, who uses bubbles of liquid trapped in salt left behind by ancient lakes to study the Permian climate. It took life 5 million years or more to recover.

But perhaps the most chilling aspect of the Permian is what it might suggest about our current moment. “Icehouse” periods like the one we’re in now, when Earth has polar ice, are few and far between.

To reach the previous icehouse, you have to go back to the early Permian, when the average temperature was probably 15 degrees C cooler than today. Ice sheets reached the midlatitudes. Earth might have looked a bit like it did at the height of the last glacial period 20,000 years ago when woolly mammoths roamed the frosty steppes of Paris. Just swap our continents for Pangaea and the saber-toothed cats for lizard-like protomammals.

This cold spell lasted for 105 million years before climate change transformed Pangaea into a scorched, parched and quite possibly toxic wasteland. Scientists still aren’t exactly sure why Earth stayed so cool for so long. Perhaps Pangaea itself was the culprit. Stitching together a supercontinent involves building mountain ranges, which exposes fresh rock to chemical weathering and ultimately contributes to cooling.

Plants might have played a role, too. After true trees evolved, it took about 60 million years before biology caught up and evolved ways to break them down. Since they didn’t decompose well, dead trees ended up getting buried over geologic time. That stored an enormous amount of organic carbon as coal; 90 percent of all coal deposits date back to this time.

We don’t know why this ice age began, but we do know how it ended: in the greatest mass extinction of all time.

“We’re still technically in an icehouse, but we’re rapidly going towards a greenhouse,” Benison says. “Looking back at the [end of the Permian] is a good way to try to say what happens with these big changes — and not just what happens with climate, but what happens to life.”

A hot Cretaceous but no mass extinction

Given the deadly consequences of the Permian, it might be surprising that Earth’s hottest period since the evolution of complex life was more Garden of Eden than Paradise Lost.

Ninety million years ago in the Cretaceous Period, the planet was a verdant jungle world. Vast swaths of the continents, including huge strips of the American West, were flooded by shallow seas. In some areas, carnivorous dinosaurs like Spinosaurus prowled the shores. At 36° C, the average surface temperature was a degree shy of human body temperature. You could barely cool off by taking a dip in polar seawater; it was a soupy 27° C.

But given all that, “there’s no mass extinction” during this hot part of the Cretaceous Period, says geologist Brian Huber of the Smithsonian National Museum of Natural History in Washington, D.C.

Last year, Huber and colleagues published the results of a project that pooled paleoclimate data to reconstruct the last 485 million years of surface temperature. According to this new temperature timeline, the Cretaceous super-greenhouse was the hottest Earth has ever been since the evolution of life more complex than a microbe. Scientists aren’t sure what drove temperatures so high.

But it’s clear, at least, that the walk-up to the peak temperatures was much more gradual than the 10-degree jump that rocked the Permian. Earth had been hot for a long time. In fact, it never really cooled down after the Permian extinction. The poles were effectively ice-free for the entirety of the dinosaurs’ nearly 180-million-year reign, and global mean surface temperatures mostly remained above 20° C (5 degrees C hotter than in 2024). Perhaps the transition from icehouse to greenhouse during the Permian put ecosystems under additional stress. That would be bad news, considering what’s happening today.

What’s next for Earth’s climate?

The last several million years of Earth’s climate — and the entire history of our genus, Homo — is written in ice. That’s why climate scientists are so eager to hunt for old ice, including a 6-million-year-old sample retrieved from Antarctica last year. The ice tells a story echoed in seafloor sediments and countless other clues from the rock record and computer modeling. For the last 2.3 million years, the climate has swung to the rhythm of several long-term variations in Earth’s orbit. These Milankovitch cycles subtly change the amount of sunlight Earth receives and where it is distributed. So, at first every 40,000 years and later every 100,000 years, Earth has cycled between clement interglacial periods and frosty glacial periods some 5 degrees C cooler.

The cyclical freeze and thaw of our current icehouse period began at the tail end of a long-term cooling trend that started 50 million years earlier. Perhaps due to the rise of the Himalayas, which exposed an enormous amount of fresh rock to chemical weathering, atmospheric carbon dioxide levels steadily declined. By 34 million years ago, Antarctica was cold enough for permanent ice to collect at the south pole. By 800,000 years ago, carbon dioxide levels dropped to below about 300 parts per million. As the planet cooled, it crossed a threshold: It became sensitive enough to subtle variations in sunlight to respond dramatically to Milankovitch cycles.

Our species has never seen an iceless planet. But within two short centuries, industrial carbon emissions from coal-fired power plants and gas-fueled cars have nearly doubled the carbon dioxide level from 280 ppm to 426 ppm. Average temperature has ticked up by 1.47 degrees C. We’re on track to blow past the 1.5-degree warming target set by the Intergovernmental Panel on Climate Change. Meeting that ambitious target might not even be enough to prevent total ice sheet collapse.

If nothing significant changes in our approach to climate change, that will be just the beginning: Carbon dioxide levels will reach 600 ppm by 2100, or soar above 1,000 ppm, under less optimistic scenarios. That could result in 4 degrees C of warming relative to the preindustrial average temperature.

A period 55 million years ago called the Paleocene-Eocene Thermal Maximum, or PETM, offers a view on a world with carbon dioxide levels that high. It was the hottest period in the history of our Earth; the planet we know, with its familiar continents and ecosystems dominated not by dinosaurs but by mammals. Leading up to the PETM, temperatures rose between 5 and 8 degrees C to an average of up to 34° C. Unlike us, the creatures that endured this hot period were already accustomed to an iceless planet. The PETM didn’t see a mass extinction, but it did reshuffle ecosystems. Local extinctions were common, even if a species could hold on elsewhere. And some species did disappear entirely.

If we’d been around in the PETM, we’d have had to migrate to the poles to survive. But cities can’t exactly get up and move. That’s a problem, because the world in 2100 will not be the world we know today. By the end of the century, billions of people will routinely endure heat and humidity extremes beyond the limits of human survival, even if we limit warming to 2 degrees C. We’ve already delayed the next glacial period, if not canceled it. And by 2500, 40 percent of all land area will have become unsuitable for its current biome, scientists predict.

This will be the end of the world as we know it, but not the end of the world. Even if we do create a climate catastrophe on the scale of the Permian mass extinction, Earth’s history shows that the planet will recover. The carbon thermostat will correct our error — just not nearly fast enough for it to matter for our species. Perhaps we’ll push Earth into a new greenhouse regime, like the jungle world of the dinosaurs. That would be anathema to our species, but it’s nothing Earth hasn’t seen before. Life will go on, with or without us. At least, for a while.

Temperatures today aren’t too different from what they were all the way back in the Archean. Because chemical weathering speeds up when it’s hot, Earth’s natural thermostat has trapped more and more carbon dioxide in rock as the sun warms — and it’ll keep doing that as the sun continues to heat up. Eventually, that’ll be a problem for plants; if carbon dioxide gets too low, they can’t photosynthesize.

About 500 million years from now, atmospheric carbon dioxide will dip below 100 ppm, scientists predict — low enough to kill 95 percent of plants alive today. About 1 billion years from now, carbon dioxide will sink even lower, and the sun will be about 10 percent brighter than today. At that point, any remaining plants will disappear. With photosynthesis shut down, oxygen will rapidly disappear from the atmosphere.

Eventually, the thermostat will break altogether, Hoffman says. “There will come to be a time when we will lose that CO₂ lever.” Like a beachgoer on a hot day who’s run out of clothes to take off, Earth will run out of carbon dioxide to strip away. But the sun will keep getting hotter.

Temperatures will soar higher than ever since the Hadean, and Earth will spend about 3 billion years as a hellscape before the sun starts dying and takes our planet with it.

That’s just about as long as Earth has been habitable up until now — and far longer than it will support life complex enough to consciously engineer a climate crisis.