Fifty-six million years ago, Earth's forests crossed a tipping point. During one of the planet's most intense episodes of greenhouse warming, the dense, lush canopies that had flourished began to thin. As global temperatures rose by as much as 11 degrees Fahrenheit (6 degrees Celsius), heat and drought stressed the forests and killed large numbers of trees. The openings let more sunlight reach the ground and altered the movement of water through the landscape.

In southern Wyoming, ferns briefly took over ground where relatives of elms, walnuts, dawn redwoods and avocado trees had thrived. Palms and other warmth-loving plants then spread northward.

In a new study published in the journal Science, researchers show that those Wyoming forests lost 60% of their canopy during this period — known as the Paleocene-Eocene Thermal Maximum, or PETM — and needed well over 100,000 years to recover. The PETM was marked by a massive release of carbon into the ocean-atmosphere system, and its warming and recovery together spanned roughly 200,000 years. It was Earth's closest natural analog to the warming the world is experiencing today, although humans are now releasing carbon dioxide roughly 10 times faster than the planet's natural processes did then. Understanding what happened to those forests may help humanity recognize similar thresholds before the planet crosses them again.

Reading the forest from fossil leaves

Paleobotanists conventionally use plant fossils to identify which species once lived in a place. The study's authors set out to answer a harder question: What did the forest itself look like, and how did it change? A forest's structure — above all its canopy — controls the amount of light that reaches the forest floor, as well as the temperature, the water habitat and the amount of carbon the forest can store, making it one of the clearest indicators of ecosystem function.

But how do you measure the density of a forest that disappeared 56 million years ago? Ecologists measure canopy density using what is known as the leaf area index. Dense forests with multiple layers of leaves intercepting sunlight score high, while open forests that allow more light to reach the forest floor score lower. Because the canopy influences shade, temperature, water loss and photosynthesis, the index provides a powerful measure of forest function. Satellites routinely measure it today to track forest condition across the globe, but no such record exists for forests that grew tens of millions of years ago.

The clues to canopy density millions of years ago came from microscopic plant cuticles — the thin, waxy outer skin of leaves, which can survive for millions of years in organic-rich sediments. The shapes of epidermal cells are still visible in these fossil leaf fragments, and that matters: cell shape reflects the amount of sunlight a leaf received while it grew. Leaves that develop in shade produce longer, more elongated cells as they stretch toward sunlight; those exposed to more sun develop shorter, rounder ones.

The researchers turned that relationship into a tool for reconstructing ancient forests. To calibrate it, they collected soils from forests across Central and South America spanning a wide range of canopy densities. Each handful of soil contains cuticles shed by many different plants across the canopy, reflecting the structure of the forest as a whole — and the fossil record preserves this same fragmented leaf litter.

Comparing the shapes of thousands of epidermal cells with the leaf area index they measured revealed a remarkably strong relationship: the more elongated the cells, the denser the forest canopy above them. That correlation allowed the team to reconstruct the structure of Wyoming's forests millions of years ago and trace how it changed over time.

When forests reach their limits

One of the most surprising discoveries was that the forests did not enter the Paleocene-Eocene Thermal Maximum in decline. Just before rapid warming began, the canopies reached their greatest density in hundreds of thousands of years, likely reflecting favorable growing conditions as atmospheric carbon dioxide began to increase. A leading theory for the source of that carbon dioxide involves volcanic eruptions.

That flourishing did not last. As temperatures climbed, heat and drought overwhelmed the benefits of higher carbon dioxide levels. The canopy thinned rapidly as trees died, and it remained much thinner for over 100,000 years.

The forests functioned very differently in this diminished state, and the surrounding environment felt the effects. Ancient soils gave way to coarser river deposits, suggesting that the loss of canopy altered how water and sediment moved through the basin. The changing climate changed the forest, and the forest changed the landscape.

Lessons for today

This sequence carries an important lesson for the present. Higher carbon dioxide levels like those the world is experiencing now — concentrations have climbed from roughly 280 parts per million before the industrial era to more than 420 parts per million today — can stimulate plant growth, but only while temperatures and water remain within the limits that trees can tolerate. Beyond those limits, heat, drought, insects, pathogens and wildfire can overwhelm any fertilization effect that would boost growth.

Around the world, many forests are already showing signs of diminishing as temperatures rise — researchers have documented episodes of heat- and drought-driven tree mortality on every forested continent in recent decades — in addition to deforestation for timber, crops and rangeland that further reduces their resilience.

Forests recovered, but it took over 100,000 years

The story of the ancient forests of 56 million years ago does not end with collapse. Over time, the increased breakdown of rocks in the warmer climate — a process known as weathering — gradually pulled carbon from the air, storing it in marine sediments. That allowed the climate to cool and water to become more available. Forest canopies recovered, eventually becoming even denser than before the warming began.

As the forests expanded, they likely restored their ability to stabilize soils, regulate the water cycle and draw carbon from the atmosphere, helping reduce the greenhouse effect and support the planet's long-term recovery.

The study shows that carbon dioxide emissions have pushed forests beyond their physiological limits before, triggering changes that ripple from vegetation to rivers and across entire landscapes. It also shows that forests are remarkably resilient when given time to recover — but that the time required is far longer than a human lifespan, spanning thousands of generations.

Today, human-caused carbon emissions and warming are unfolding vastly faster than during the PETM. The fossil record reminds us that forests can recover, but only if humanity avoids pushing them beyond thresholds from which recovery takes tens of thousands of years.

Regan E. Dunn is Associate Curator at the La Brea Tar Pits and Museum and Adjunct Professor of Earth Sciences at USC Dornsife College of Letters, Arts and Sciences.

This article is republished from The Conversation under a Creative Commons license and was originally featured on Fortune.com.