Name: The Last Time the Globe Warmed
Uploaded: 2021-04-30T14:26:34.000Z
Duration: 10 min 54 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

Thanks to Curiosity Stream for supporting PBS Digital Studios.

Imagine an enormous rainforest teeming with life: trees, insects, pretty little birds.

Primates are climbing in the canopy, while crocodiles and turtles swim in the rivers

Now imagine this lush rainforest ... in the Arctic.

There was a time -- and not too long ago -- when the world warmed more than any human has ever

This ancient warming took place over the course of just 200,000 years -- the blink of an eye

in geologic time -- and it ended much like it began: suddenly and mysteriously.

It all started 56 million years ago, at the very end of the Paleocene

Back then, life was still recovering from the unpleasantness of the Cretaceous-Paleogene

extinction event, which wiped out the non-avian dinosaurs.

And things were already warm by today's standards.

There were no polar ice caps, which meant sea levels were much higher.

And the continents -- which were just beginning to take a familiar shape -- were covered in

habitats like temperate forests, and deserts, and a belt of rainforests around the equator.

But this environment was about to change.

In fewer than 20 thousand years, the global average temperature increased by 5 to 8 degrees

And the warming was greatest at higher latitudes.

So, at the poles, temps on land reached an average of 23 degrees, while the ocean waters

got up to a balmy 20 degrees, This means you could've gone for a comfy swim in the seas

This remarkable and sudden warming event is known as the Paleocene-Eocene Thermal Maximum,

or PETM, and it had a massive effect on life on Earth.

For one thing, when the PETM reached its peak, rainforests had expanded much farther than

Fossils from North America, Europe, and Asia reveal habitats rich in plant life that today

are associated with tropical rainforests -- even though these forests were nowhere

The fossilized fronds of palm trees have been found as far north as Wyoming, for example.

And some places within the Arctic Circle, like Ellesmere Island in Canada, show evidence

How was all of this -- any of this -- possible?

Our best clues can be found in ancient sediments.

Marine sediment samples from Maryland to Antarctica show that, about 56 million years ago, there

was a sudden spike in the amount of carbon dioxide and other greenhouse gases in the

And judging by the types of carbon found in these sediments, the gases likely came from

See, plants, like most living things, prefer to use the lighter and more common isotope

of carbon, carbon-12, as opposed to heavier isotopes, like carbon-13.

So, this biogenic carbon -- which we've talked about before -- has a different chemical

signature than carbon that's never been part of a living organism.

And, sediments that date to the start of the PETM, show a large and sudden drop in the

ratio of carbon-13, compared to carbon-12.

This means that a bunch of biogenic carbon must have suddenly been released into the

atmosphere, in the form of carbon dioxide, methane, and other gases.

Well, one hypothesis is that there was a rash of massive wildfires that unleashed tons of

Another model proposes that giant seams of coal were exposed to the heat of volcanic

activity, which would have released the carbon from fossilized plants.

Or it could be that an otherwise mild warming event triggered the release of greenhouse

gases, by melting deposits of a compound known as methane hydrate.

Methane hydrate is similar to ice, but it contains molecules of methane trapped by molecules

And hydrates are usually stable, as long as they're under a lot of pressure, like deep

in the oceans, or if they're kept cold, like in permafrost -- the thick layer of frozen

But if these places warm up, the hydrates melt, releasing bursts of methane, which is

an even more potent greenhouse gas than CO2.

And of course, the more warming that happens, the more melting there is, which releases

even more greenhouses gases, creating a classic positive feedback loop.

Now, no matter how it started, it's worth noting that, during the PETM, carbon was released

into the atmosphere at only a fraction of the rate at which it's being emitted today.

One study of marine sediments from the Arctic showed that, at the peak of the PETM, as much

as 1.7 billion metric tons of carbon were released into the atmosphere every year, for

A similar study of sediments from New Jersey put the figure at about 1.1 billion tons of

Now, compare that to the amount of carbon being released today.

In 2014 alone, it was 9.8 billion metric tons of carbon.

So, 56 million years ago, carbon was being released less quickly than it is now, but

those emissions continued for thousands of years.

And it was more than enough to create a potent greenhouse effect.

With more carbon in the atmosphere than plants could absorb, the planet started to change

In many places, the climate delivered a combination of humidity and heat that allowed vast rainforests

And among the animals that thrived in these warm forests were reptiles.

Fossils of alligators, crocodiles, and turtles can be found in nearly every fossil bed from

the PETM -- even in the polar forests of Canada and Greenland.

And these lush forests were also where many early mammal groups diversified -- including

In fact, the earliest true primates appear in the fossil record just as the PETM was

starting to take off, 56 million years ago.

They adapted quickly to a world covered in trees, developing things like forward-facing

eyes, fingernails instead of claws, and opposable thumbs.

These features gave primates such an edge, that by 53 million years ago, they could be

found across the northern hemisphere -- from tiny Eosimias in China

But in the oceans, life in hothouse Earth became much harder.

In fact, in some places it was almost impossible.

At the equator, ocean temperatures were unbearably hot, sometimes reaching as high as 36 degrees,

Subtitles ListPlay Video

The Last Time the Globe Warmed

massive

period

thrive

matter