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There was a time when alligators slid through weed-choked swamps near the North Pole. Some 55 million years ago—just around 10 million years after the mass extinction that killed T. rex and most of its kin—the average global temperature sat more than 20°F higher than it does today. Subtropical forests spread to northern latitudes, and mammals thrived in lush new habitats.

The toasty weather had nothing to do with the event that killed the dinos. The driver for the climatic shift came not from above, but from below—in Earth’s oceans. Paleontologists and geologists suspect that some amount of natural warming that took place during the Paleocene, or the period following the die-off, caused great deposits of crystallized methane to transform into gas. Seabeds belched the excess out into the water and the air, which was bad news for the planet: Methane is a greenhouse gas far more potent than carbon dioxide. The globe rapidly warmed in response—jumping about 10°F in less than 20,000 years—and held steady for some 70,000 more before starting a long and slow recovery.

Paleontologists call this hot spot the Paleocene-​Eocene Thermal Maximum (PETM). It’s a time when subtropical forests spread over the continents and new animals got to stake their claims on the planet, all thanks to an atmosphere and oceans in turmoil. This part of the fossil record is a remnant of the past, but it may also be a preview of our future.

Hungry, hungry insects

An ancient era of global warming could hint at our scorching future

The PETM was a good time to be a bug. Warmth spurred the spread of dry tropical forests north, making ancient Wyoming look more like modern Texas. Because many insects are ectothermic (their body temperatures and physiological needs are tied to the climes of their habitats), the spike opened the gates for a flood of tiny critters to move in.

[Related: Not convinced that humans are causing climate change? Here are the facts.]

The evidence is in fossil leaves found in rocks dated to the period. A sample of more than 5,000 petrified bits of flora from before, during, and after the PETM shows an uptick in the amount and diversity of insect damage. In one study of Wyoming’s Bighorn Basin, more than half the fossil leaves from the PETM had been damaged by bugs—20 percent more than before or after. Bugs nibbled the edges of plants, bored holes into them, and created little trails on their surfaces as they chewed away at changing forests. It’s likely even more critters would have thrived if the grub had been better: Plants grown in elevated carbon dioxide tend to be less nutritious, and this greenhouse gas was abundant during the PETM.

While some modern pests suffer in heat—​in Puerto Rico, struggling insects used to stable temps are imperiling the food chain—others, like some mosquitoes and ticks, are moving into new territories. One 2019 study estimated that by 2080, the number of people exposed to mosquito-borne illness around the world could increase by nearly a billion.

Oceanic extinctions

An ancient era of global warming could hint at our scorching future

In a sense, the ocean is like a big conveyor belt. Typically, cold air and salty water mix in the Southern Hemisphere to create dense, cool “deep water,” which keeps things moving. The toastier PETM climate, though, caused more rain to fall at the North Pole, which weakened currents and shifted things around. In less than 5,000 years, cold air and salty oceans were instead mixing in the North Atlantic. The change in flow warmed the ocean even more. Higher temperatures increased the metabolisms of local critters and, as a result, their demand for food. But hotter water also holds less oxygen, so it’s not hard to see how the conditions of the PETM put marine life in an impossible situation: Animals needed more food to get by, while the lack of oxygen made the environment harsher and kept nutrients scarce.

The climate effects lasted 100,000 years, and some organisms couldn’t keep up with the change. Deep-sea varieties of so-called “armored amoebas” (aka benthic foraminifera or forams), a favorite of paleontologists studying evolution and extinction because of their abundance in the fossil record, suffered a major die-off. More than 35 percent of their species went extinct, marking their only significant crisis in the last 90 million years. Forams have long been a staple food of many small ocean creatures, so paleontologists suspect their absence made a big difference.

Rapid evolution

An ancient era of global warming could hint at our scorching future

New species are always evolving just as others are going extinct. Paleontologists refer to the rate at which new species replace older ones as turnover. Fossils from the remnants of Paleocene oceans suggest that up near the surface, the process happened at breakneck speed during the PETM—in evolutionary terms, at least.

[Related: What’s in a packrat’s petrified pee? Just a few thousand years of secrets.]

In shallow waters near the coasts, existing types of snails and clams died off, but were quickly replaced by similar mollusks that took up the same ecological roles—sifting through sand and grazing on algae. Other changes were more dramatic. Triggerfish and puffer fish suffered a mass extinction, and it took nearly 20 million years for these swimmers to evolve enough new species to regain their lost diversity. Near the equator, corals similar to those alive today retreated, and disk-shaped organisms called larger foraminifera filled their niche as reef builders until the seas finally cooled back down several million years later.

Shrinking mammals

An ancient era of global warming could hint at our scorching future

Before an asteroid devastated all nonavian dinos 66 million years ago, the largest furry critter on Earth was about 11 pounds, or the size of an American badger. Just shy of a million years later, with new ecological niches cleared out by the mass extinction, the biggest topped the size of a German shepherd. Beasts were proliferating through the world’s warm forests, diversifying into new forms of herbivores, carnivores, and omnivores. Then, under the heat of the PETM, some began to shrink.

Biologists see this same phenomenon among living mammals today. In colder climates, for example, moose are often about 80 pounds heavier than their southern compatriots. Bigger builds are better at retaining internal heat: Having a smaller proportion of body surface exposed to the elements relative to overall mass means creatures lose warmth more slowly. But in hot times, it becomes less important to stay toasty and more crucial to be able to dump excess heat—something that’s easier with the greater surface-area-to-body-mass ratio of smaller creatures. An early horse known as Sifrhippus sandrae, for example, shrank by nearly one-third during the PETM, and an early primate called Cantius abditus went down in size by about 10 percent between the middle of the period and its end.

Strange new rains

an illustration of santa claus standing in a tropical forest
Renaud Vigourt

Swampy polar forests didn’t just spring up on their own when temperatures jumped during the PETM. Plants need hydration, after all, and changes to the world’s rain cycle provided an essential assist in the proliferation of subtropical canopies.

Much of our planet’s weather patterns—the ways air and water circulate through sea and sky—are influenced by the differences between temperatures at the hot equator and those at the chilly poles. Prior to the PETM, H2O that evaporated near Earth’s midsection formed rain clouds that dropped water both in the tropics and at polar latitudes. But the warming climate’s alterations to air currents caused more of the equator’s moisture to travel as far as the poles before coming back down. This is part of what allowed huge pecan and cypress trees to grow in the ancient Arctic, giving lemur-​like primates a place to clamber.

[Related: The 4 biggest lessons from the latest IPCC climate report]

But rain has to come from somewhere; this surge in wet weather in some parts of the world meant arid regions saw even more of their moisture lost to evaporation. Modern climate change may have similar effects. Wetter winters are already increasing the frequency of damaging floods in northwestern Europe. Meanwhile, the southwestern United States is on track to become even more parched than it already is. Research suggests that by the end of the century, soils in the region will be 10 to 20 percent drier than they are now, increasing the risk of drought by at least 20 percent.

Danger zones

An ancient era of global warming could hint at our scorching future

By the time the PETM started, the oceans were already hot. The ancient Atlantic, for instance, was 92°F at the equator before the warming pulse, or nearly 10°F higher than it is today. The PETM lifted that to above 98°F, more than 16°F above the modern average equatorial Atlantic temperature. The seas quickly became heat stressed. With such toasty surface waters, the deeper seas lost their cooling source. Cold water is better at retaining oxygen, so O2 levels plummeted down below. The influx of carbon dioxide from the global warming also caused a sharp increase in the ocean’s acidity.

Few forms of life can survive in such suffocating waters. In fossil deposits from the PETM, paleontologists have found that dinoflagellates—tiny organisms that ooze toxins and can create deadly algal blooms called “red tides”—flourished in the nearly 100°F surface water of the equator. These blooms choke oxygen out of the ocean, which is why they’re so deadly. Our modern waters are coping with a similar scenario: With hundreds of so-called dead zones documented across the world, including in the Gulf of Mexico and multiple spots in the Baltic Sea, we risk turning back the clock to the PETM.

This story originally appeared in the Heat issue of Popular Science. Current subscribers can access the whole digital edition here, or click here to subscribe.

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