Los Angeles, CA (Aug 13, 2026)—If you want to know how rapidly pumping carbon dioxide into the atmosphere will impact future forest ecosystems, you have to look into the distant past. The researchers behind a new study published in Science have done just that, reconstructing the forest canopy of 56 million years ago, during a massive and abrupt emission of carbon into the atmosphere during the Paleocene-Eocene Thermal Maximum (PETM). For the first time, the authors of the study used fossilized leaf cells to reconstruct the tree canopy of the PETM, Earth's most recent period of comparable global warming, with chilling implications for our near future.
"We are putting CO₂ into the atmosphere faster than any known natural process," says lead author and paleobotanist Dr. Regan Dunn, Assistant Deputy Director and Associate Curator of the Samuel Oschin Global Center for Ice Age Research at La Brea Tar Pits and the Natural History Museum of Los Angeles County. "The Earth has never experienced a carbon release at the pace we're creating today. 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."
The authors found that the PETM was marked by a widespread browning of Earth's landscapes. Against a backdrop of elevated atmospheric CO₂, global warming, and reduced rainfall triggered by volcanically driven carbon release, plant species migrated northwards, and canopy cover declined, accelerating erosion and disrupting the terrestrial water cycle. The human-caused climate change happening now is pumping CO2 at an order of magnitude faster than the ancient parallel. Trees are at the root of these drastic changes.
"There are significant tree mortality events everywhere on Earth right now," says Dunn. "Forests are in decline because of warming temperatures, drought stress, pathogen and insect infestations, and wildfires—when you start losing the trees, our canaries in the coal mine, you're in trouble. It's almost universally true in all of Earth's five major extinction events."
Regrowing the Leaf Area Index
How do you rebuild a forest from more than 50 million years in the past? With the fossilized cells of leaves called leaf cuticle fragments.
Introducing an entirely new method, the authors used leaf cuticle fragments to calculate leaf area index (LAI) for the first time. LAI is a measure of how much of the sky is filled by leaves, giving researchers a way to quantify the density of a forest canopy. Dunn and her co-authors calculated the LAI of a variety of modern forests in South and Central America, setting up a camera with a fish-eye lens on a tripod pointed upward while collecting leaf cuticles in soil samples directly beneath it.
"What excites me about LAI is that it lets us move beyond somewhat subjective descriptions of vegetation, like 'open forest' or 'woodland.' Instead, the leaves themselves provide a quantitative estimate of canopy structure, allowing us to compare ecosystems using an objective, reproducible metric," says Dunn. Assigning a numerical value lets researchers more accurately measure changes over time.
Dunn's earlier work used phytoliths, microscopic silica-filled plant cells, to measure LAI, but a new calculation was needed for leaf cuticle fragments to account for preservational biases of the different fossil types. To get the fuller picture of how trees responded to the explosion of CO2 during the PETM, Dunn turned to those cuticle fragments preserved in organic-rich sedimentary rocks from Wyoming.
The shape of a leaf's epidermal cells reflects its exposure to sunlight, whether it's a shade leaf further down the tree with less access to sunlight, or a canopy leaf near the top of a tree, fully exposed to the sun's rays. Dunn discovered that the shape of the leaf cells is highly correlated to the density of the vegetation (LAI). "Leaves that grow in denser forests—in shadier habitats–have cells with higher aspect ratios, or more elongate and skinny cells than leaves receiving more sunlight," says Dunn.
"This is the shade response of plants; they lengthen the leaf by lengthening each cell," Dunn says. Picture each cell stretching itself to get sunlight, compared to a fat and happy cell bathed in full sunlight. "Once the leaves fall from the tree, and begin to break down in the soil, the cell shapes preserved in the fragments can tell us how dense the canopy was where the leaf originally grew."
LAI was calculated from photographs, while Dunn and her colleagues measured thousands of epidermal cells from leaf fragments recovered from the soils, and compared their measurements with the LAI values. "This is the process of setting up a proxy to reconstruct an environmental variable from fossil deposits. You go into modern environments, establish the relationship, and create a model from which you can reconstruct that variable, in this case LAI, from rocks 56 million years old," says Dunn.
Unearthing Wyoming's Fossil Forests
"I love telling my undergraduate classes that Wyoming has the best fossil record in the world," says co-author, Dr. Ellen D. Currano, paleoecologist, paleobotanist, and professor at the University of Wyoming. "This work is another demonstration of that."
"What's unique about the Hanna Basin is that it's a coal-forming basin. So you have all these organic-rich rocks, like lignites and coals, that are lacking in other places where the PETM interval is well-known, such as the Bighorn Basin in Northwestern Wyoming," adds Dunn. "Contained within these organic-rich rocks are the leaf fragments we need to reconstruct the ancient forest canopy."
The forests of ancient Wyoming teemed with giant dawn redwood (Metasequoia) trees overlooking sycamores, alders, palms, and other subtropical and tropical plant species, in stark contrast to the sagebrush habitat seen today. Over more than a decade of work, Currano, Dunn, and their team collected hundreds of samples from Wyoming's Hanna Basin, methodically looking for fossil evidence of those bygone forests from before and after the PETM. Using the fossil record from Hanna Basin, Dunn, Currano, and their colleagues have recreated the saga of those forests.
"Understanding how forest structure changed during the PETM is really important because it tells us about how plant growth, biomass, and productivity are affected by adding a lot of carbon dioxide to the atmosphere. It turns out that too much carbon dioxide is a bad thing for forests because the accompanying warming and drying stresses the trees and kills many of them," says Currano. "Our work shows that during the PETM, forest canopies became more open, with fewer big trees, and this change affected climate, nutrient cycling, weathering, and, of course, the animals that inhabited the forests. We are starting to see similar changes to forests occurring today, particularly in the Amazon, and the Wyoming plant fossil record gives us insight into where Earth might be headed."
A Browning Earth
Measured since the 1980s via satellite, our modern global LAI tells a parallel story to that of the authors' reconstruction of the PETM.
"Earth has been on a greening trajectory because of anthropogenic carbon dioxide emissions that have fertilized plants," says Dunn. "But as Earth has heated up because of those emissions, this greening trend is reversing, and many parts of Earth are now browning. This suggests that we've passed a critical threshold where heightened temperatures are now adversely affecting forests."
Dunn and her colleagues' findings suggest that continued climate warming could push Earth's forests toward the kind of widespread browning observed during the PETM. Unlike the ancient world, however, today's forests are confronting multiple human-driven pressures simultaneously. In addition to rapidly rising atmospheric CO₂ and warming temperatures, deforestation, increasingly frequent and severe wildfires, habitat fragmentation, invasive species, and land-use change are reducing forest resilience and limiting their ability to recover. These combined stresses threaten to amplify carbon release and ecosystem degradation beyond what occurred during the PETM. Protecting and restoring forests is therefore not only essential for conserving biodiversity but also for preserving one of Earth's most important natural defenses against accelerating climate change.
"Trees are awesome. All those forests are taking up hundreds of millions of tons of carbon for us every day," Dunn adds. "But when you start losing the forests, then you're losing those critical carbon sinks. The PETM reminds us that when forests decline, the consequences ripple through the climate system. We would be wise to protect and restore them while they can still help buffer the unprecedented pace of human-caused climate change."
