The Last Time the Globe Warmed

Summary

The script explores the Paleocene-Eocene Thermal Maximum (PETM), a period 56 million years ago when Earth experienced rapid warming, leading to Arctic rainforests. It discusses the potential causes, such as wildfires or methane hydrate melting, and the significant impact on life, including the diversification of mammals like primates and the decline of marine life due to ocean acidification. The PETM serves as a stark comparison to current climate change, highlighting its potential extremes.

Takeaways

• 🌿 The Paleocene-Eocene Thermal Maximum (PETM) was a period of rapid global warming that occurred 56 million years ago, lasting around 200,000 years.
• 🌡️ Global average temperatures increased by 5 to 8 degrees Celsius in less than 20,000 years, with the most significant warming at higher latitudes.
• 🌳 The warming allowed rainforests to expand to regions like the Arctic, where tropical plant life, such as palm trees, thrived.
• 🐊 Reptiles, including alligators, crocodiles, and turtles, diversified and thrived in the warm, humid forests of the PETM.
• 🌊 Ocean temperatures rose, causing issues for marine life, particularly plankton which forms the base of many ocean food webs.
• 🌐 The event was triggered by a sudden release of greenhouse gases, likely from organic matter, into the atmosphere.
• 🔥 Hypotheses for the release include massive wildfires, exposure of coal seams to volcanic heat, or the melting of methane hydrates due to mild warming.
• 🌿 The PETM was a significant period for early mammal groups, including primates, which diversified and spread across the northern hemisphere.
• 🌊 Ocean acidification due to increased CO2 levels led to the extinction of 30-50% of foram species and a decline in coral reefs.
• ❄️ The PETM ended with a slow cooling period, possibly influenced by the dying off of carbon-absorbing plants like Azolla.
• ⏱️ Although the PETM's rate of carbon release was less than today's, it continued for thousands of years, illustrating the long-term effects of greenhouse gas emissions.

Q & A

What was the Paleocene-Eocene Thermal Maximum (PETM)?

The Paleocene-Eocene Thermal Maximum (PETM) was a remarkable and sudden warming event that occurred approximately 56 million years ago, causing the global average temperature to increase by 5 to 8 degrees Celsius, with the most significant warming at higher latitudes.

How did the PETM affect the distribution of rainforests?

During the PETM, rainforests expanded much farther than they ever had before, with evidence of tropical rainforest-like habitats appearing as far north as Wyoming and within the Arctic Circle, such as Ellesmere Island in Canada.

What was the average temperature at the poles during the PETM?

At the poles, temperatures on land reached an average of 23 degrees Celsius, while the ocean waters got up to a balmy 20 degrees Celsius during the PETM.

What evidence do marine sediment samples provide about the PETM?

Marine sediment samples from Maryland to Antarctica show a sudden spike in the amount of carbon dioxide and other greenhouse gases in the oceans about 56 million years ago, likely coming from organic matter like plants.

How did the carbon isotope ratio change during the PETM?

Sediments dating to the start of the PETM show a large and sudden drop in the ratio of carbon-13 compared to carbon-12, indicating a sudden release of biogenic carbon into the atmosphere.

What are some hypotheses for the source of the increased greenhouse gases during the PETM?

Hypotheses include massive wildfires releasing CO2 from plants, exposure of coal seams to volcanic heat, or the melting of methane hydrate deposits due to mild warming events, all of which could have released significant amounts of greenhouse gases.

How does the rate of carbon release during the PETM compare to today's rate?

During the PETM, carbon was released into the atmosphere at a rate of up to 1.7 billion metric tons per year, whereas in 2014 alone, the rate was 9.8 billion metric tons of carbon per year, indicating a much faster release today.

What impact did the PETM have on marine life?

The PETM led to ocean acidification due to increased CO2 levels, which depleted the water's concentration of carbonates, affecting organisms that build shells and structures. This resulted in the extinction of 30 to 50 percent of all foram species and the near disappearance of complex coral reefs.

How did the PETM influence the evolution of mammals?

The PETM was an important period for the diversification of early mammal groups, including primates. The earliest true primates appeared in the fossil record around the time the PETM began, and they adapted quickly to the forested world, spreading across the northern hemisphere.

What is the significance of the Azolla event in the cooling period following the PETM?

Arctic sediments from the early Eocene contain fossilized aquatic ferns known as Azolla, which thrived in the warm Arctic. Their subsequent death and deposition on the seafloor, taking carbon with them, is hypothesized to have contributed to the cooling and the eventual formation of polar ice caps.

How does the PETM help us understand the effects of current global warming?

The PETM provides a historical analogue for understanding the potential effects of global warming, showing how polarizing climate change can be for life on Earth, with some organisms thriving while others faced disaster.

Outlines

00:00

🌿 Paleocene-Eocene Thermal Maximum (PETM)

This paragraph discusses the Paleocene-Eocene Thermal Maximum (PETM), a period 56 million years ago when the Earth experienced a sudden and dramatic warming event. The global average temperature rose by 5 to 8 degrees Celsius in less than 20,000 years, with the most significant warming occurring at higher latitudes. The PETM led to the expansion of rainforests, with tropical plant life appearing as far north as Wyoming and within the Arctic Circle. The cause of this warming is believed to be a sudden release of greenhouse gases, possibly from massive wildfires, volcanic activity exposing coal seams, or the melting of methane hydrate deposits. The PETM had profound effects on life, with reptiles and early primates thriving in the new environments, while many marine species, particularly plankton and foraminifera, suffered from the heat and ocean acidification.

05:03

🐊 Life During the PETM

The second paragraph delves into how life on Earth was affected during the PETM. Reptiles such as alligators, crocodiles, and turtles thrived in the warm forests, even at the poles. Early mammals, including the first true primates, diversified and spread across the northern hemisphere. However, the oceans experienced significant challenges due to the high temperatures and ocean acidification. Plankton, the base of the marine food web, struggled to survive in the hot equatorial waters, and many foraminifera species went extinct. Coral reefs also declined as the acidification hindered the growth of hard corals. The PETM ended with a cooling period, the cause of which remains a mystery, but it's speculated that the die-off of the Azolla ferns in the Arctic played a role by sequestering carbon as they fell to the seafloor.

10:06

📺 Engaging with PBS Digital Studios

The final paragraph serves as a call to action for viewers to engage with PBS Digital Studios. It invites viewers to share their interests and questions about the history of life on Earth in the comments. It also encourages viewers to subscribe to the Eons channel on YouTube and to share the episode with others who might find it fascinating. The paragraph promotes other PBS Digital Studios channels and ends with a sponsored message for Curiosity Stream, offering a promotional code for Eons viewers.

Keywords

💡Rainforest

A rainforest is a dense, warm, and wet forest filled with a high diversity of plant and animal species. In the video, the concept of rainforests is used to illustrate the lush and diverse ecosystems that existed during the Paleocene-Eocene Thermal Maximum (PETM). The script mentions rainforests expanding to areas like the Arctic, which is unusual by today's standards, highlighting the extreme warmth of the PETM.

💡Paleocene-Eocene Thermal Maximum (PETM)

The PETM refers to a geological time period approximately 56 million years ago when there was a rapid increase in global temperatures. The video uses the PETM to discuss the significant climate change event that led to the expansion of rainforests and had profound effects on life on Earth. It's a key concept for understanding the video's exploration of past climate change.

💡Greenhouse gases

Greenhouse gases are gases in the atmosphere that trap heat, leading to the greenhouse effect and global warming. In the script, a spike in carbon dioxide and other greenhouse gases is identified as the cause of the PETM. The video connects this to the current climate crisis, drawing parallels between the PETM and human-induced global warming.

💡Carbon isotopes

Carbon isotopes are variants of the carbon element with different numbers of neutrons. The script discusses how a sudden drop in the ratio of carbon-13 to carbon-12 in ancient sediments indicates a release of biogenic carbon into the atmosphere during the PETM. This is a scientific detail that helps explain the mechanisms behind the PETM.

💡Methane hydrate

Methane hydrate is a type of ice that contains a large amount of methane within its crystal structure. The video suggests that the release of methane from melting methane hydrates could have contributed to the PETM. It's an important concept for understanding the potential feedback loops that can exacerbate climate change.

💡Ocean acidification

Ocean acidification is the process by which the pH of the ocean decreases, becoming more acidic, often due to the absorption of carbon dioxide. The video explains how increased CO2 levels during the PETM led to ocean acidification, which had devastating effects on marine life such as plankton and corals.

💡Foraminifera

Foraminifera, or forams, are a type of protozoan that build calcium carbonate shells. The video discusses how the increase in ocean acidity during the PETM led to the extinction of many foram species, as their shells could not form in the more acidic conditions. This is an example of how climate change can impact marine ecosystems.

💡Primates

Primates are a group of mammals that include humans, apes, and monkeys. The script mentions that the PETM was a time of diversification and adaptation for early primates, allowing them to spread across the northern hemisphere. This illustrates how some species can thrive during periods of climate change.

💡Arctic Circle

The Arctic Circle is the northernmost circle of latitude on Earth, demarcating the northern boundary of the Arctic. The video uses the Arctic Circle to highlight the extreme warmth of the PETM, as it suggests that even within the Arctic Circle, there were tropical-like rainforests.

💡Fossil record

The fossil record is the preserved history of life on Earth as discovered in the fossilized remains of plants, animals, and other organisms. The video refers to the fossil record to provide evidence of the types of life that existed during the PETM, such as palm trees in Wyoming and various plant life within the Arctic Circle.

💡Climate feedback loop

A climate feedback loop is a process that can either amplify or dampen the initial climate change. The video describes a positive feedback loop where warming leads to the release of more greenhouse gases, which in turn causes more warming. This concept is crucial for understanding how climate change can accelerate.

Highlights

Ancient warming event occurred over 200,000 years, a short time in geologic terms.

The Paleocene-Eocene Thermal Maximum (PETM) began 56 million years ago, causing a sudden rise in global temperatures.

Global average temperature increased by 5 to 8 degrees Celsius in less than 20,000 years.

The poles experienced the most significant warming, with land temperatures reaching an average of 23 degrees Celsius.

Rainforests expanded much farther than ever before during the PETM.

Fossils from North America, Europe, and Asia reveal the presence of tropical rainforest habitats outside the tropics.

Palm trees and other tropical plants were found as far north as Wyoming.

Evidence of ferns, redwoods, and gingkos within the Arctic Circle suggests a much warmer climate.

Ancient sediment samples indicate a sudden spike in carbon dioxide and other greenhouse gases in the oceans.

The carbon isotope ratio in sediments suggests the release of biogenic carbon from organic matter.

Hypotheses for the PETM include massive wildfires, volcanic activity releasing coal seams, or the melting of methane hydrate deposits.

Carbon emissions during the PETM were a fraction of the rate at which they're emitted today.

In 2014, carbon emissions were 9.8 billion metric tons, much higher than the peak of the PETM.

The PETM's warming allowed for the flourishing of vast rainforests and the thriving of reptiles.

The earliest true primates appeared during the PETM, adapting quickly to a world covered in trees.

Ocean life suffered due to high temperatures and ocean acidification during the PETM.

Ocean acidification led to the extinction of 30 to 50 percent of all foram species.

The PETM ended with a slow cooling of the climate, eventually leading to the formation of polar ice caps.

The cause of the initial cooling that stopped the PETM remains a mystery.

The PETM serves as a comparison for understanding the effects of current global warming.

The rate of annual carbon emissions has been more than five times greater than at the peak of the PETM.

The PETM's lessons on climate change's polarizing effects on life are relevant to our current warming period.

Transcripts

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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

below.

Beautiful, isn’t it?

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

seen.

So far.

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

Epoch.

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

Celsius.

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

around Antarctica!

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

they ever had before.

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

near the tropics.

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

of ferns, redwoods, and gingkos.

So.

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

oceans.

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

organic matter, like plants.

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.

But, where did these gases come from?

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

CO2 that had been locked up in plants.

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

of water.

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

soil that forms in cold climates.

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

at least 4,000 years.

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

carbon every year.

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

rapidly.

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

to flourish.

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

our every own lineage, the primates.

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

to Notharctus in Wyoming.

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,

almost as hot as your average hot tub.

This was probably too hot for many kinds of plankton, which were -- and are -- the basis

for most ocean food webs.

But an even more devastating side effect of high CO2 levels was ocean acidification.

When ocean water absorbs CO2, it becomes more acidic.

And this in turn depletes the water’s concentration of carbonates -- the compounds that many organisms

use to build shells and other structures.

And this is why one of the clearest effects of the thermal maximum can still be found

in core samples from the deep sea.

Sediments that date back to before the warming are typically pale in color, because they’re

rich with the skeletons of deep sea foraminifera.

Also known as forams, these are tiny protozoans that build shells of calcium carbonate.

And where forams were abundant, the chalky fossils of their shells turned the ocean bed

white.

But when the oceans became more acidic, the sediments turned dark.

Because … most of the forams just disappeared.

During the PETM, between 30 and 50 percent of all foram species went extinct.

The same phenomenon also stunted the growth of hard corals, which need carbonates to build

their skeletons, too.

So, during the PETM and for millions of years afterward, big, complex coral reefs all but

disappeared from the fossil record.

All told, the thermal maximum was a mixed bag for life on Earth – proving to be an important

period for us mammals, but a major loss for some marine life.

And, like all dramatic events, the PETM did come to an end.

Although, we’re not sure how, or why.

Over the course of the Eocene epoch, the climate slowly began to cool.

And although the temperature occasionally spiked again, it never reached the extremes

of the maximum.

Temperatures kept dropping during the Eocene -- so much so, in fact, that by the end of

the epoch, 34 million years ago, polar ice caps had begun to form.

But, how did we get from rainforests near the poles to ice caps?

Well, the cause of the initial cooling that actually stopped the PETM 53 million years

ago remains a mystery.

But something allowed that cooling to take hold, and make the world even colder.

And the answer here might have to do, again, with plants.

Arctic sediments that date back to the early Eocene -- 49 million years ago -- have been found

to contain huge swaths of fossilized aquatic ferns known as Azolla.

These plants thrived in the lush, warm Arctic.

But as the environment changed, they died off.

And as they dropped to the seafloor, the thinking goes, they took tons of carbon with them,

which caused temperatures to drop even further.

Despite how little we know about its end, or its beginning, the Paleocene-Eocene Thermal

Maximum shows us just how polarizing climate change can be for life on Earth.

For some organisms, like early primates, the warming was a chance to develop new forms

and spread to new locations.

But for corals, forams, and other marine life, such extreme heat spelled disaster.

It gives us, in the midst of our own period of warming, a view of how extreme the effects

of climate change can be.

And it allows us to make some pretty striking comparisons.

Remember when I said that, during the PETM, the globe warmed more than humans had seen

so far?

Well, keep in mind that, in recent years, the rate of annual carbon emissions have been

more than five times greater than they were at the peak of the PETM.

As a result, our world is warming faster than it did back in the Eocene.

Just over the past hundred years, the average global temperature has increased by about

0.7 degrees Celsius.

But that’s just been over the past century.

During the PETM, it took perhaps thousands of years for temperatures to rise that much.

So the PETM is the closest we can get to understanding the effects of global warming today.

And it has a lot to teach us about the extremes that life experienced, on land and in the

seas.

Yes, rainforests full of primates and insects and reptiles is beautiful.

But I think you’ll agree with me that most of us like them right where they are today.

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