Earth’s forests lost 60% of their canopy during ancient global warming—could it happen again?

Earth’s forests endured a dramatic transformation during one of the planet’s most extreme episodes of global warming—and they took more than 100,000 years to fully recover.

The finding comes from a new study of the Paleocene-Eocene Thermal Maximum (PETM), a period roughly 56 million years ago when huge amounts of carbon entered the atmosphere and global temperatures climbed sharply. Researchers say the ancient event provides an important glimpse into how forests can respond when the climate changes rapidly.

The study, published August 13 in Science, found that forest canopy in what is now Wyoming eventually lost about 60% of its leaf coverage during the PETM. Even after conditions began to stabilize, forests remained significantly less dense for much of the warming period.


The research has drawn attention because the rate at which humans are adding carbon dioxide to the atmosphere today is considerably faster than the carbon release associated with the PETM.

Ancient Forests Changed After an Initial Growth Surge

Higher levels of carbon dioxide might appear beneficial to plants because CO₂ is essential for photosynthesis. And initially, that appears to have happened during the PETM.

But the apparent boost in plant growth did not last.

To reconstruct what happened, researchers examined fossilized leaves preserved in rocks in Wyoming’s Hanna Basin. The leaves contain microscopic clues about the amount of sunlight they received while growing, allowing scientists to distinguish between foliage that developed in bright sunlight and leaves that grew beneath a forest canopy.

As Regan Dunn, a paleobotanist at the Natural History Museum of Los Angeles County and a co-author of the study, explained to Nature, sun-exposed and shaded leaves develop differently. Shade leaves tend to have elongated cells as they adapt to limited light, while sun leaves have shorter, rounder cells.

By examining those characteristics in fossils from different stages of the PETM, the researchers were able to reconstruct changes in the ancient forest canopy.

Forest Canopy Fell by About 60%

The reconstruction reveals a striking pattern.

The forest initially benefited from the carbon-rich atmosphere. But as the PETM progressed, its canopy became dramatically thinner, eventually losing roughly 60% of its leaf area.

For the remainder of the warming event, canopy cover stayed about 35% below its original level, according to the researchers.

The impact extended beyond the trees themselves.

Broad-leaved relatives of modern elms, walnuts and avocado trees became less dominant, while ferns became more common. With less vegetation and fewer plants storing carbon, the amount of carbon held by forests and soils also declined.

The changing vegetation affected waterways as well. Reduced plant cover left more material vulnerable to erosion, contributing to greater sediment movement into rivers.

As Dunn wrote in The Conversation, the episode illustrates a two-way relationship: climate can reshape forests, while changes in forests can in turn alter the wider landscape.

Forests Took More Than 100,000 Years to Recover

One of the study’s most striking findings is how long the recovery took.

The researchers estimate that the dense forest canopy did not return to its earlier state until at least 55.7 million years ago—more than 100,000 years after the PETM began.

That long recovery period is particularly significant because the ancient forests were responding to a carbon release that unfolded over thousands of years, rather than the much faster emissions associated with modern industrial activity.

Regan Dunn told The Guardian that the rate of carbon release occurring today has no known precedent in Earth’s history.

That distinction matters because modern forests are not facing rising carbon dioxide alone.

Today’s Forests Face a Much Bigger Climate Challenge

Forests around the world are already dealing with a combination of pressures, including higher temperatures, drought, wildfires, air pollution, deforestation, habitat fragmentation and disease.

There is also evidence that the earlier global trend toward increasing vegetation has begun to change.

Satellite observations starting in the 1980s showed widespread increases in vegetation, partly linked to rising atmospheric carbon dioxide. But research has found that since around 2000, the greening trend has reversed across more than 90% of the world’s vegetated regions.

That does not mean every forest is declining in the same way. Local climate, species, water availability and land-use changes all influence how vegetation responds.

But the PETM fossil record suggests that more atmospheric CO₂ does not necessarily translate into healthier forests indefinitely.

Why the PETM Matters for Climate Change

Scientists often look to periods such as the PETM because they provide natural experiments that occurred long before humans began dramatically altering the atmosphere.

Gabriel Bowen, a palaeoclimatologist at the University of Utah who was not involved in the research, described the findings as cautionary in comments to Nature. The goal, he explained, is to use evidence from Earth’s past as a guide to what could happen under future climate conditions.

Trevor Keenan, an environmental scientist at the University of California, Berkeley, who was also not involved in the study, offered an even more sobering assessment to The Guardian.

If forests became thinner during an ancient warming event that unfolded relatively slowly, he suggested, that is not a reassuring precedent for a world where carbon emissions are rising much faster.

The PETM does not provide an exact blueprint for the future. Earth’s ecosystems, species and atmospheric conditions were very different 56 million years ago.

But the fossil leaves from Wyoming reveal one important lesson: forests can initially respond positively to higher carbon dioxide and still suffer major, long-lasting damage as warming intensifies.

For a world already experiencing rising temperatures and increasingly severe climate pressures, that ancient transformation offers a warning worth paying attention to.