Forests Didn’t Fare Well During an Ancient Global Warming Event, Offering a ‘Cautionary’ Tale About Today’s Climate Change

An intense global warming event 56 million years ago caused a forest in modern-day Wyoming to lose around 60 percent of its canopy cover, and it took more than 100,000 years to bounce back, a study suggests

Sara Hashemi | Daily Correspondent

August 28, 2026 2:43 p.m.

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About 56 million years ago, Earth experienced one of its most intense periods of global warming, known as the Paleocene-Eocene Thermal Maximum, or PETM. As atmospheric carbon dioxide levels surged, forests suffered a massive decline in canopy cover, and they took more than 100,000 years to recover, researchers report in a study published on August 13 in the journal Science.

The findings offer a sobering look into what our planet’s future might look like because of ongoing human-caused climate change.

“The Earth has never experienced a carbon release at the pace we’re creating today,” says Regan Dunn, a study co-author and paleobotanist at the Natural History Museum of Los Angeles County, in a statement. “The PETM gives us our best window into how Earth’s climate and ecosystems respond to a massive carbon injection before humans began reshaping the planet.”

Humans are currently putting carbon dioxide in the atmosphere at an unprecedented rate. Predicting the effects of the planet-warming greenhouse gas on vegetation is complicated. Since plants need the gas for photosynthesis, elevated carbon dioxide levels should theoretically lead to enhanced plant growth. But too much heat and its consequences, like extreme weather events and an increase in pathogens, can also stress and kill plants.

That’s why Dunn and her colleagues turned to the past to see what might happen to today’s forests. They collected fossilized leaf fragments hidden in the rocks of Hanna Basin in south-central Wyoming. The specimens preserve the shapes of the leaves’ outermost cells, which can reveal how much sunlight the leaf received as it grew.

“There’s this difference between sun leaves and shade leaves, even on the same plant,” Dunn tells James Dinneen at Nature. Leaves that grow in the shade have longer cells that stretch out looking for light, while those exposed to the sun are shorter and rounder.

The information helped the team calculate the density of the forest canopy, which they tracked across the entire PETM. At first, the forest flourished thanks to the elevated carbon dioxide. But that growth spurt didn’t last. The canopy then quickly lost around 60 percent of its leaf coverage, and it stayed around 35 percent thinner than it started through the rest of the PETM.

“This should be a concern because the rate of carbon release has never been higher than now,” Dunn tells the Guardian’s Oliver Milman.

This change affected the surrounding environment too. Ferns replaced the broad-leafed relatives of elms, walnuts and avocado trees. Plants and soils stored less carbon, and rivers carried more sediments.

“The changing climate changed the forest, and the forest changed the landscape,” Dunn writes for the Conversation. Leaf coverage didn’t return to its extremely dense state until at least 55.7 million years ago, the team estimated.

These changes mirror what we’re seeing in our forests today. Starting in the 1980s, satellite images showed a greening trend because of anthropogenic carbon dioxide emissions. But around the year 2000, researchers found that the pattern had reversed in more than 90 percent of the world’s vegetated area.

What happened during the PETM is certainly “cautionary,” says Gabriel Bowen, a palaeoclimatologist at the University of Utah who wasn’t involved in the study, to Nature. “What we’re looking for is guides from the past about what can happen.”

And worryingly, the stress we’re putting on our forests today is nothing like the slow release of carbon dioxide, likely from volcanic eruptions, that happened over thousands of years during the PETM. Modern-day plants face rising temperatures along with pollution, deforestation, fragmentation, wildfires and other stressors.

“If forests thinned under the slower and cleaner version of this experiment, that is not a reassuring precedent,” Trevor Keenan, an environmental scientist at the University of California, Berkeley who wasn’t involved in the work, tells the Guardian.

Sara Hashemi is a science writer and fact-checker currently based in New York City. Her work has appeared in Sierra, The Body, Maisonneuve magazine and more.