The PETM began about 56 million years ago, when a massive release of carbon into the atmosphere and oceans drove a sharp global temperature rise. Although the exact sources and sequence of carbon emissions remain the subject of continuing research, the event is widely regarded as one of the closest geological analogues to modern human-driven climate change. The ancient warming unfolded over thousands of years rather than decades, but it produced profound ecological consequences. Temperatures increased, rainfall patterns shifted, soils were disturbed and ecosystems reorganized across continents. Because the PETM lasted long enough for forests and other plant communities to adjust, its fossil record offers scientists an opportunity to examine responses that cannot be captured by short-term experiments.

Regan Dunn and colleagues investigated these responses in the Hanna Basin, a sedimentary region in southern Wyoming that preserves a detailed record of ancient landscapes. The researchers combined several independent types of evidence, including fossil pollen, plant remains, sediment characteristics and geochemical measurements. Together, these records allowed them to track not only which plants lived in the region, but also how the structure of the forest changed as the climate warmed. The study focuses on a crucial distinction in climate biology: a forest can contain more productive individual leaves while simultaneously becoming less dense, storing less carbon overall and providing less continuous habitat.

A central element of the work is a new method for estimating Leaf Area Index, or LAI, from fossil leaf cuticles. LAI describes the total one-sided leaf surface area relative to the area of ground beneath it. In modern ecology, it is a key measurement of canopy density, light interception, water use and plant productivity. High LAI generally indicates a thick, multilayered canopy, while lower values point to a more open forest. Leaf cuticles are the waxy, chemically resistant outer layers that protect leaves from water loss and environmental damage. Even when the soft tissues and much of the plant have decayed, cuticles can remain preserved in sediments. By analyzing their abundance and characteristics, the researchers reconstructed changes in ancient canopy cover that would otherwise be nearly impossible to observe directly.

The fossil evidence indicates that the PETM was accompanied by rapid forest canopy opening. The decline in canopy density was not an isolated botanical change: it coincided with increased erosion across the landscape, suggesting that vegetation became less effective at shielding soil from rainfall and runoff. Dense forests slow precipitation, bind soil with roots and organic matter and reduce the energy of water moving across the ground. When canopies thin and plant cover becomes more discontinuous, soil can be exposed, making erosion more likely. The sedimentary record therefore provides an environmental link between forest structure and broader landscape instability during prolonged warming.

The plant community itself also underwent a dramatic transformation. Temperate broad-leaved angiosperms, which had formed an important component of the pre-PETM vegetation, declined as heat-tolerant plants became more prominent. Palms and ferns expanded in the altered environment, reflecting a shift toward species capable of tolerating warmer conditions and, in some cases, different combinations of moisture and seasonality. Such changes are more than a simple replacement of one set of species by another. Plant identity determines how much carbon is stored in trunks, branches, roots and soils, how efficiently water is used, how quickly organic matter decomposes and how habitats are structured for animals and microorganisms.

The findings challenge a common assumption about the relationship between carbon dioxide and forests. Elevated CO2 can stimulate photosynthesis by increasing the raw material available for plants to manufacture sugars. It can also improve water-use efficiency in some species because plants may partially close the microscopic pores, known as stomata, through which they exchange gases. But these benefits are not unlimited. High temperatures can damage photosynthetic machinery, increase respiratory losses and raise atmospheric demand for water. Drought can restrict the supply of water needed to transport nutrients and maintain leaf function. Over time, heat and water stress can overwhelm the initial CO2 fertilization effect, causing forests to thin even while atmospheric carbon dioxide remains high.

That possibility is especially important today because modern vegetation has already shown signs that its capacity to absorb carbon is under pressure. For decades, rising CO2 and longer growing seasons helped increase plant growth in many regions, allowing forests and other ecosystems to remove a portion of human emissions from the atmosphere. In recent years, however, extreme heat, drought, wildfire, insect outbreaks and land-use change have weakened or reversed some of those gains. The Wyoming fossils cannot provide a direct forecast for any particular modern forest, and the PETM climate was not identical to today’s climate. Nevertheless, the record demonstrates that the apparent resilience of vegetation under elevated CO2 can erode when warming persists long enough to alter water availability, species composition and ecosystem structure.

The study’s broader message is that forests should not be treated as permanent carbon-storage machines. Their ability to remove carbon depends on the interaction of atmospheric chemistry, temperature, precipitation, soils and biodiversity. A forest may initially grow more rapidly under higher CO2, yet later lose canopy cover and carbon-storage capacity as heat and drought intensify. Once plant communities shift toward more open and heat-tolerant vegetation, the consequences can extend beyond the carbon cycle, affecting erosion, habitat quality, local climate regulation and the movement of water through the landscape. By combining fossil cuticles with pollen, sediments and geochemical signals, Dunn and colleagues have reconstructed a detailed example of this process from deep time. The ancient forest’s decline suggests that the most important question is not whether plants can benefit from extra carbon dioxide, but whether forests can remain structurally intact as the planet continues to warm.

Subject of Research: Ancient forest structure, plant community change and carbon storage during the Paleocene–Eocene Thermal Maximum

Article Title: Forest canopy decline under elevated CO2 during the Paleocene-Eocene Thermal Maximum

Web References: https://doi.org/10.1126/science.aec4776

References: Dunn et al., “Forest canopy decline under elevated CO2 during the Paleocene-Eocene Thermal Maximum,” Science. DOI: 10.1126/science.aec4776

Keywords: Paleocene–Eocene Thermal Maximum, PETM, forest canopy, Leaf Area Index, fossil leaves, fossil pollen, Wyoming, Hanna Basin, elevated carbon dioxide, climate change, forest carbon storage, global warming, palms, ferns, vegetation response, erosion

The study, led by Le Yao of the Nanjing Institute of Geology and Paleontology with contributions from University of Cincinnati geosciences professor Thomas Algeo and international colleagues, examined a short-lived warming episode during Earth’s penultimate ice age. Their findings suggest that global sea surface temperatures eventually increased by more than 7 degrees Celsius, or approximately 12 degrees Fahrenheit. The rise was not necessarily caused by one enormous initial pulse of carbon. Instead, it may have developed through a chain reaction in which warming activated additional natural sources of greenhouse gases.

The Late Paleozoic Ice Age extended across a vast span of geological time, when large ice sheets covered parts of the supercontinent Gondwana. Although the planet was in an “icehouse” state, atmospheric carbon dioxide and methane still had the power to destabilize the climate. The newly analyzed warming event is particularly significant because it occurred under ice-age conditions rather than during an already warm greenhouse world. That makes it a valuable analog for the modern Earth, which still contains continent-scale ice masses in Greenland and Antarctica.

According to the researchers, the initial climate disturbance may have been linked to volcanic activity. Volcanic emissions can inject carbon dioxide and other gases into the atmosphere, where they absorb outgoing infrared radiation and prevent some heat from escaping into space. The warming may also have been assisted by changes in Earth’s orbit. Over long cycles, variations in orbital shape alter the distribution of sunlight across the planet, influencing the growth and retreat of ice sheets and changing the seasonal balance of solar energy.

At first, the additional carbon may have produced only a limited temperature increase. But once warming crossed a critical threshold, frozen ground in high-latitude regions began to thaw. Permafrost contains large quantities of organic carbon preserved in frozen soils. When that material becomes available to microbes, decomposition can release carbon dioxide and methane. Methane is especially powerful over shorter atmospheric timescales because, molecule for molecule, it absorbs considerably more infrared radiation than carbon dioxide, even though it remains in the atmosphere for less time.

This process can create a positive climate feedback. An initial warming causes permafrost to thaw, thawing releases greenhouse gases, and the additional gases produce further warming. That extra heat can accelerate more thawing, allowing the cycle to reinforce itself. The researchers describe the ancient event as a transient global warming episode in which a comparatively modest carbon release appears to have crossed a climatic tipping point and initiated much more extensive carbon emissions from natural reservoirs.

The geological timescale of the ancient warming is also central to the study’s relevance. Algeo said the temperature change unfolded over tens of thousands of years, with warming occurring at an average rate of roughly one-tenth of a degree Celsius per 1,000 years. Modern warming is progressing far more rapidly. Researchers have observed an increase of nearly 1 degree Celsius in global sea surface temperature over approximately the past century, a rate many times faster than the average change reconstructed for the ancient event.

The comparison does not mean that Earth is destined to repeat the precise conditions of 304 million years ago. Continents, ecosystems, atmospheric composition and ice-sheet geometry were all different in the Late Paleozoic. However, the physics of greenhouse warming and the existence of carbon feedbacks remain relevant. Ancient climate records reveal how a system that appears stable can respond nonlinearly when warming activates processes capable of releasing additional greenhouse gases.

The findings also place human history in a broader planetary context. All recorded human civilization has developed during the most recent ice age, despite the fact that the term can sound incompatible with today’s climate. In scientific usage, Earth remains in an ice age because substantial permanent ice sheets still exist in Greenland and Antarctica. The ancient warming episode matters precisely because it demonstrates that an ice age does not guarantee a permanently cold or stable climate. Even within an icehouse world, a small disturbance can trigger rapid and substantial changes when reinforcing feedbacks are engaged.

The research, published in the Proceedings of the National Academy of Sciences, underscores why scientists closely monitor permafrost, polar ice and other climate-sensitive carbon stores. The ancient record cannot provide an exact forecast of future warming, but it shows that Earth’s climate has repeatedly responded to carbon disturbances with amplifying feedbacks. As modern emissions continue to raise temperatures at an exceptional rate, the geological evidence offers a clear message: the danger lies not only in the carbon released directly by human activity, but also in the natural carbon sources that warming can unlock.

Subject of Research: Ancient global warming, permafrost methane feedbacks, and climate tipping points during the Late Paleozoic Ice Age

Article Title: A transient global warming event during Earth’s penultimate icehouse

Web References: https://doi.org/10.1073/pnas.2601643123

References: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.2601643123

Image Credits: Andrew Higley, University of Cincinnati

Keywords: climate change, global warming, ice age, Late Paleozoic Ice Age, permafrost, methane, climate feedback, tipping point, paleoclimate, geology, volcanic activity, Earth’s climate history