What the Paleocene-Eocene Thermal Maximum Can Teach Us About a Warming World

It’s a stunning summer evening in the arctic. The sun glows orange near the horizon, sparkling off the back of an alligator. Each slow sweep of the reptile’s tail sends it between warm light and shadow created by the trees; groves of redwoods are huddled together in the weedy waters. 

High arctic scenes like this one once took place on what is now Ellesmere Island in the northeastern most portion of Canada. Paleontologists had no idea what was there, even what could be there, until Mary Dawson and Robert West went looking in the 1970s. They reported alligators once occupied the 55 million-year-old rock of Ellesmere, part of a collection of animals that lived there when temperatures didn’t dip below freezing. Ellesmere’s arctic alligators and hippo-like mammals, called Coryphodon, are just extreme examples of a prehistoric trend found at other fossil sites all around the world. 

Beginning 56 million years ago, the Paleocene-Eocene Thermal Maximum was a sudden warm spike that affected everything from deep ocean currents to the size of the largest land mammals. Scientist are now examining that period for clues on what can happen to habitats and wildlife in a warming world—the very warming that is now accelerating on our planet. 

Fifty-six million years ago, when our ancestors were small and tarsier-like primates leaping through humid forests, shallow water vents in the North Atlantic shoved open and began dumping methane and carbon dioxide into the water and air above. The amount of carbon released was in the thousands of petagrams; one petagram is a billion metric tons. In a world that was already warm and lacked polar ice, the sudden emissions of these greenhouse gases kicked off a series of feedback loops and cascades that quickly warmed the average global temperature between nine and 14 degrees Fahrenheit. 

This ancient event does not offer a one-to-one comparison to how we’ve been influencing the atmosphere and climate throughout human history. Before us, volcanoes were a primary source of atmosphere-changing emissions. And scientists so far do not expect that human activity will push global temperatures to a threshold near nine to 14 degrees by 2100. But the Paleocene-Eocene Thermal Maximum warm spike can help us learn how life responded in a progressively warming world.

What we know comes from geochemical traces in ancient mammal teeth, the distribution of marine plankton, leaves chewed and gouged by ancient insects, and more. For example, vast subtropical forests grew thick around the planet, hosting early horses. Tusked herbivores in ancient Wyoming thrived while Utah dried out. Plant fossils document the shift to hot desert plants rather than tropical plants. Plants grew in new places as temperatures and rainfall patterns shifted, right on the edge of what ancient plants were able to adjust to as the lusher plant communities shifted to new places and dense forests turned to woodland and desert scrub.

Insects were happy to munch on the new accumulation of plants. Fossil plant leaves from this era show a spike in the chewing, burrowing, and nesting common of insects when temperatures are warmest. Insects are largely ectothermic, their body temperature and activity regulated by environmental temperatures, meaning they’re more active when it’s warm. But pollinators became more active, too. Fossil pollen indicates that wind-pollinated plants became rarer as animal-pollinated plants became more prevalent during the warmest stretch, perhaps indicating a response to the ecosystem disruptions caused by the heat. 

Mammals dealt with the heat in different ways. Prior to the Paleocene-Eocene Thermal Maximum, fossil mammals formed communities of herbivores and carnivores, large and small species, by habitat preferences. Hoofed herbivores the size of a medium-size dog stepped through thick undergrowth while lemur-like primates leapt through the trees, roughly wolf-like carnivores stalking along the edges of woodlands. 

But the mammals were not fussy about their favored niches. Mammals were still mostly generalists that had been evolving since the asteroid impact of 10 million years before that ended the age of dinosaurs. 

What was happening in the oceans is harder to reconstruct. We know that increased carbon dioxide in the seas turns the water acidic, which not only hampers shells and coral formation but also destroys shells that would otherwise become helpful fossils. Organisms that paleontologists often look to as environmental indicators are harder to find in the Paleocene-Eocene Thermal Maximum’s ocean rocks. Changes in ocean currents borne of warmer temperatures shifted more acidic flows to the deep sea and caused a mass extinction of a group of organisms called forams. These animals were like amoebas inside a shell, more than 35 percent of deep sea foram species vanished. These little organisms were abundant and essential parts of the deep-sea food web. Nearer the surface, bathtub temperatures caused red tides and algal blooms in addition to essential planktonic species shifting towards the cooler poles.

Life was incredibly stressed during the Paleocene-Eocene Thermal Maximum. And within that span of 200,000 years, some species vanished. But eventually, the greenhouse gases faded and global temperatures went back down. Forests began to grow thick again, toxic ocean blooms became rarer, plants and insects and mammals began to shift back to how life was before the temperature spike. The pressure was on for organisms to move or adapt. Many did.

The Paleocene-Eocene Thermal Maximum isn’t merely a preview of what we might suffer on a hotter planet. It’s a reminder of what makes this warming era different. In our case, human activity is responsible for releasing the greenhouse gas emissions heating up Earth. The animals living through climate change over 50 million years ago didn’t have a choice. We do. The fossil record is clear: life will recover if we let it.