The Last Time the Globe Warmed

Thanks to Curiosity Stream for supporting PBS Digital Studios. Imagine a huge rainforest full of life: trees, insects, cute little birds. Primates climb the treetops, 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 long ago, when the world got hotter than any human being has ever seen. Until now. This ancient warming took place over the course of just 200,000 years (a blink of an eye in geological

time

) and ended much like it began: suddenly and mysteriously. It all started 56 million years ago, at the end of the Paleocene.

Back then, life was still recovering from the unpleasantness of the Cretaceous-Paleogene extinction, which wiped out non-avian dinosaurs. And things were already warm by today's standards. There were no ice caps, which meant sea levels were much higher. And the continents, which were just beginning to take on a familiar shape, were covered in habitats such as temperate forests, deserts, and a belt of rainforests around the equator. But this environment was about to change. In less than 20 thousand years, the average global temperature increased between 5 and 8 degrees Celsius. And the warming was greater at higher latitudes. So, at the poles, temperatures on land reached an average of 23 degrees, while ocean waters reached a balmy 20 degrees.

This means you could have comfortably gone swimming 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 peaked, tropical forests had expanded much more than ever before. Fossils from North America, Europe, and Asia reveal habitats rich in plant life that are today associated with rainforests, although these forests were nowhere near the tropics. Fossilized palm leaves have been found as far north as Wyoming, for example. And some places within the Arctic Circle, such as Ellesmere Island in Canada, show evidence of ferns, sequoias and gingkos.

So. How was all this, all this possible? Our best clues can be found in ancient sediments. Samples of marine sediments from Maryland to Antarctica show that, about 56 million years ago, there was a sudden increase 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 probably came from organic matter, such as plants. You see, plants, like most living things, prefer to use the lightest and most common carbon isotope, carbon-12, rather than heavier isotopes, such as carbon-13. So this biogenic carbon, which we've talked about before, has a different chemical signature than carbon that has never been part of a living organism.

And sediments dating to the start of the PETM show a large and sudden drop in the proportion of carbon-13, compared to carbon-12. This means that a large amount of biogenic carbon must have been suddenly released into the atmosphere, in the form of carbon dioxide, methane and other gases. But where do these gases come from? Well, one hypothesis is that there was a series of massive forest fires that unleashed tons of CO2 that had been locked up in plants. Another model proposes that giant coal seams were exposed to the heat of volcanic activity, which would have released 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 contains methane molecules trapped by water molecules. And hydrates are usually stable, as long as they are under a lot of pressure, such as in the deep oceans, or if they remain cold, such as in permafrost, the thick layer of frozen soil that forms in cold climates. But if these places warm, the hydrates melt, releasing bursts of methane, which is an even more potent greenhouse gas than CO2.

And of course, the more warming that occurs, the more it melts, releasing even more greenhouse 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 is emitted today. A study of Arctic marine sediments showed that at the height of the PETM, up to 1.7 billion metric tons of carbon were released into the atmosphere each year, for at least 4,000 years. A similar study of New Jersey sediments put the figure at about 1.1 billion tons of carbon each year.

Now let's compare that to the amount of carbon being released today. In 2014 alone, there were 9.8 billion metric tons of carbon. So, 56 million years ago, carbon was released less rapidly than it is now, but those emissions continued for thousands of years. And it was more than enough to create a powerful greenhouse effect. With more carbon in the atmosphere than plants could absorb, the planet began to change rapidly. In many places, the climate generated a combination of humidity and heat that allowed vast tropical forests to flourish. And among the animals that thrived in these warm forests were reptiles.

Fossils of alligators, crocodiles and turtles can be found in almost all PETM fossil sites, including in the polar forests of Canada and Greenland. And these lush forests were also where many of the earliest groups of mammals diversified, including our own lineage, the primates. In fact, the first true primates appear in the fossil record just as the PETM was beginning to take off, 56 million years ago. They quickly adapted to a tree-covered world, developing things like forward-facing eyes, nails instead of claws, and opposable thumbs. These characteristics gave primates such an advantage that 53 million years ago, they could be found throughout the Northern Hemisphere, from tiny Eosimias in China to Notharctus in Wyoming.

But in the oceans, life on greenhouse Earth became much more difficult. In fact, in some places it was almost impossible. At the equator, ocean temperatures were unbearably high, sometimes reaching up to 36 degrees, almost as hot as the average hot tub. It was probably too hot for many types of plankton, which were (and are) the basis of most oceanic 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 that's why one of the clearest effects of the thermal maximum can still be found in core samples from the deep sea. Sediments dating from before warming are usually pale in color, because they are rich in skeletons of deep-sea foraminifera. Also known as foramia, they are small protozoa that form calcium carbonate shells. And where foramia were abundant, the calcareous fossils of their shells stained the ocean floor white. But when the oceans became more acidic, the sediments darkened. Because... most of the forums just disappeared. During the PETM, 30 to 50 percent of all forum species became extinct. The same phenomenon also slowed the growth of hard corals, which also need carbonates to build their skeletons.

Thus, during the PETM and for millions of years afterward, large, complex coral reefs virtually disappeared from the fossil record. All told, the thermal maximum was a mixed period for life on Earth: it turned out to be an important period for us mammals, but a major loss for some marine life. And, like every dramatic event, the PETM came to an end. Although we're not sure how or why. Throughout the Eocene, the climate began to slowly cool. And although the temperature occasionally rose again, it never reached the extremes of the maximum. Temperatures continued to drop during the Eocene; in fact, so much so that at the end of the era, 34 million years ago, the polar caps had begun to form.

But how do we get from the tropical rainforests near the poles to the polar 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 cool the world even further. And the answer here might have to do, again, with plants. Arctic sediments dating 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 warm, lush Arctic. But as the environment changed, they disappeared. And when they fell to the bottom of the sea, it is believed, they took with them tons of carbon, causing temperatures to drop even further.

Despite how little we know about its end or beginning, the Paleocene-Eocene Thermal Maximum shows us how polarizing climate change can be for life on Earth. For some organisms, such as early primates, warming was an opportunity to develop new forms and spread to new places. But for corals, foramas and other marine species, such extreme heat spelled disaster. It gives us, in the midst of our own warming period, a glimpse of how extreme the effects of climate change can be. And it allows us to make some pretty surprising comparisons. Remember when I said that during PETM the planet got hotter than humans had ever seen before?

Well, let's keep in mind that, in recent years, the rate of annual carbon emissions has been more than five times higher than at the peak of the PETM. As a result, our world is warming faster than in the Eocene. In the

last

hundred years, the average global temperature has increased by about 0.7 degrees Celsius. But that has only happened during the

last

century. During the PETM, it may have taken thousands of years for temperatures to rise that high. The PETM is therefore the closest we can come to understanding the effects of current global warming. And it has much to teach us about the extremes that life experienced, on land and in the seas.

Yes, rainforests full of primates, insects and reptiles are beautiful. But I think you'll agree with me that most of us like them exactly where they are today. PBS Digital Studios is sponsored by Curiosity Stream, where you can stream documentary films and shows about science, nature, and history, including exclusive originals! For example, check out Rise of the Continents, a BBC series that uses geological clues to recreate the supercontinent of Pangea. Curiosity Stream offers unlimited streaming, and for Eons viewers, the first two months are free when you sign up at Curiositystream.com/eons and use the promo code EONS.

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