Once upon a time, Earth started to heat up.

Greenhouse gases like carbon dioxide poured into the atmosphere. Temperatures across the planet soared.

Ice caps melted. Oceans acidified. And when all was said and done, the world was changed forever.

Sound familiar? It should, since something pretty similar is happening to us right now. But the thing is, that story didn’t happen today— it happened around 56 million years ago.

This long-ago geologic event has a lot to teach us about how Earth heats up— and what might happen to us as it does. Hi! I’m Sage, and this is Crash Course Geology. [THEME MUSIC] That super hot time period millions of years ago is called the Paleocene-Eocene Thermal Maximum. “Paleocene-Eocene” because it happened on the cusp of those two epochs, or chunks of time in Earth’s geologic history.

And “thermal maximum” because, well, it was really freakin’ hot. For now, we’ll call it the PETM. Or, pet ‘em.

Meh, just kidding, nobody calls it that. This heatwave was intense. Over the course of 15 or 20 thousand years, surface temperatures rose more than 5 degrees Celsius, topping off at a global average of over 34 degrees.

That’s about what you’d find in the line at Disney World mid-summer. Compare that to 2024, when the global average  surface temperature was  roughly 15.2 degrees Celsius. But that 34-degree temperature during the PETM was an average, pulled down by the cool-ish temps at the Earth’s poles.

And I do mean -ish. At the time, the poles were hot enough to support palm trees and crocodile ancestors. The ocean off the coast of Antarctica, which sits close to freezing today, hit around 17 degrees Celsius.

That’s about the temperature of Lake Michigan in early summer. Meanwhile, off the coast of West Africa near the equator, the ocean was about 32, like a nice, warm bath. As for how we know all this?

Throughout this series, we’ve learned that Earth is a great record-keeper. In fossils, rocks, and ice cores, we’ve found evidence of what the planet used to be like— including how hot it was, and what kinds of life it could support. Today, geologists use that evidence to make pretty good guesses about what Earth’s climate was like throughout its history.

And it shows us that the climate has changed a lot— and so has the kind of life that can live in it. In the past 4 billion years, we’ve gone from ice ages to thermal maximums, from woolly mammoths in Granada to palm trees in Greenland. As for what the evidence shows us about the PETM?

Let’s turn to some of my geology rock stars… In 1987, Dutch scientist Ellen Thomas was on a boat in the Antarctic. She was drilling into the ocean floor and pulling up sediment samples to learn about Earth’s ancient past. The ocean floor holds a pretty good history of life on Earth.

As ocean critters die, they settle on the bottom. Many of those creatures leave fossils in layers that correspond to the different epochs of geologic history. Thomas is an expert in the tiniest of fossils: benthic foraminifera, or foram for short, single-celled organisms that live on the ocean floor.

In those samples, Thomas found drastic shifts in species. A variety of foram fossils in the samples from the end of the Paleocene were nowhere in the more recent sediments— a telltale sign of mass extinction. Other scientists had spotted this shift before, but Thomas and her colleagues took an even closer look.

The team analyzed the chemistry of the fossils to figure out what might have killed the forams. See, as water temperature shifts, it leaves a chemical fingerprint. And Thomas’s samples indicated that the temperature of deep-ocean water had been really, really warm.

The last puzzle piece slid into place thanks to the work of paleoclimatologist James Zachos. Around the same time Thomas was drilling into the ocean floor, Zachos was digging up secrets thousands of miles away on land. In the rock layers that marked the Paleocene-Eocene boundary in Wyoming, he also found something weird: an abundance of the isotope carbon-13, which is slightly different from natural carbon.

This suggested that, right around the time that things were getting deathly hot in the deep ocean, tons of carbon had been released into the atmosphere. Zachos confirmed Thomas’s theory: around the Paleocene-Eocene boundary, the planet got hot enough  to kill up to half of all  these deep-sea forams, the biggest mass extinction in the deep ocean for the last 90 million years. That tracks with what we already know about the relationship between carbon and climate.

The geologic record shows us that Earth’s  temperature depends a lot  on the greenhouse effect. The greenhouse gases in our atmosphere, like carbon dioxide and methane, trap the sun’s heat and warm the surface of the earth. Geologists have looked at records of greenhouse gases in ice cores, and compared them to temperature records in the composition of fossils.

It’s clear that the greenhouse effect has been driving temperature changes for basically all of Earth’s history— including during the PETM. See, about 56 million years ago, more than 2000 gigatons of carbon were released into the atmosphere— a truly incomprehensible amount. The exact culprit of this release is uncertain, but there are a number of possibilities, like wildfires blazing across continents, or huge volcanic eruptions.

However it happened, that release of carbon ‘ trapped a lot of heat in the atmosphere, just like it had multiple  times in Earth’s history. And just like it’s doing right now. Even though we don’t know exactly where all that carbon came from back then, we do know one thing for sure: it didn’t come from humans.

Primates had just barely evolved, and I don’t think these guys were driving around in unnecessarily big pick-up trucks. But these days, humans are really good at emitting carbon. Between deforestation, industrial farming, and burning fossil fuels, we’re on course to warm things at least as much as those volcanoes or wildfires did 56 million years ago.

Which means we are staring down the barrel at another huge change in Earth’s climate. But while geologists are pretty good at seeing the past, they can’t see the future. Not even those crystal-gazing gemologists.

It’s pretty impossible to know exactly what the next era of Earth has in store, but we can get an idea. While it sounds like a long time ago, 56 million years ago is recent history in geologic time. The PETM is the closest comparison we’ve got to climate change happening today.

By looking back, we can at least make some guesses about what we’ll be seeing going forward. For starters, all that heat and carbon during the PETM impacted basically every ecosystem across the world. The oceans warmed up, and all the carbon in the air made them acidify.

That drove the mass extinction of those benthic forams, and made things pretty weird for life bobbing near the ocean’s surface. Coastal areas saw huge blooms of harmful algae called dinoflagellates, and the warming water threatened all kinds of plankton. Some of them went extinct, while others found their way to cooler spots closer to the poles.

Some might have even adapted to living deeper in the water to escape the heat. The PETM also mixed things up on land. Plants and animals closer to the equator were driven out and replaced by more heat- and drought-tolerant species.

And the warming planet opened up new habitats closer to the poles— hence the palm trees and crocodiles in the Arctic Circle. But not everything packed up and moved. Some plants and animals adapted to their new environments.

All that heat allowed cold-blooded reptiles— whose body temperatures depend on their environment— to get a lot bigger. In the tropics of South America, the giant Titanoboa grew to over twice the length of an anaconda. That’s too much snake.

Meanwhile, we warm-blooded mammals, who need to shed heat, adapted by becoming smaller. The PETM even gave us early horses, which weighed only about 4 kilograms, or eight and a half pounds. That’s not enough horse!

All those changes set the stage for tons of evolution and diversity, and basically allowed mammals to take over the world. And it goes to show that life is resilient, even in really harsh circumstances. So, even though the lines at Disney World seem like they’ll be the death of me, maybe I could just move into the shade.

Or… become smaller. Any tips, Dwayne? Another thing the PETM has taught us is that adding more carbon to the atmosphere makes it more sensitive to carbon.

Which means that further carbon emissions have even more of an impact on temperature and climate. We can also expect the poles to warm faster than the rest of the earth, melting ice caps and contributing to sea-level rise— which is already happening. A warming climate also means more extreme weather like monsoons and winter storms, but less rainfall in areas like the edge of the tropics, where it's really needed.

But when it comes to specific places, weather patterns, or species that might be impacted, geology can only predict so much. And even though current climate change is a lot like the PETM, it’s not exactly the same. Compared to the PETM, the heat today doesn’t seem so bad.

Like, in April 2023, the average global temperature of ocean surface water was around 21 degrees Celsius. But at the same time, carbon is pouring into the atmosphere way faster than it did during the PETM. Scientists say that, back then, the Earth was spitting out carbon at a rate of no more than 4 billion metric tons a year.

In 2024, energy-related carbon dioxide emissions measured about 37.8 billion metric tons. Our global warming is outstripping the PETM, too. That 5-degree temperature rise over 20,000 years was incredibly fast.

That’s, like, geologic breakneck speed! But in the past 150-ish years, our global average temperature has already risen at least 1 degree Celsius. And since 1975, it’s been shooting up as much as 0.2 degrees every ten years.

Which is, like, geologic ludicrous speed. If the PETM is any indication, we’re looking at a long, hot future. Those heavy carbon emissions probably only lasted a few thousand years, but the climate likely took up to 200,000 years to cool all the way back down.

That's about two-thirds as long as our species has been around. All of that can all feel pretty, well, depressing. But there is good news.

Like, in 2025, a report by the climate think tank Ember found that the production of renewable energy from the world’s wind and solar farms outperformed coal plants for the first time in history, leading to a small — but noticeable — decrease in coal and gas use. Another report from the International Energy Agency found that globally, renewable energy could more than double by the end of the decade, potentially preventing tons of future greenhouse gas emissions. So, we may be in for a lot of changes, but they’re not all set in stone… yet.

We can still apply lessons from Earth’s past to protect its future— and ours, too. It’s never too late to do the right thing for the climate. Next time, we’ll talk about another chapter of geologic history: the Anthropocene.

See you then. Thanks for watching this episode of Crash Course Geology, which was filmed at our studio in Indianapolis, Indiana, and was made with the help of all these nice people. If you’d like to help keep Crash Course free for everyone, forever, you can join our community on Patreon.