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

Yedoma deposits represent some of the most carbon-dense permafrost globally, containing an estimated 210 to 500 gigatons of organic carbon bound within frozen soils formed during the Late Pleistocene. The process of thawing converts this once sequestered organic matter into dissolved forms that can leach into lakes, rivers, and wetlands. Previously, the focus has primarily been on particulate organic carbon and methane emissions from these landscapes, but this study shifts attention to the dissolved fraction, which can be more bioavailable and rapidly cycled in aquatic ecosystems. By measuring DOC concentrations in lakes within the Yedoma region of Siberia, the researchers have revealed that these waters hold extraordinary quantities of aged carbon, some of it thousands of years old.

Using advanced radiocarbon dating techniques coupled with detailed geochemical analyses, the team systematically sampled lakes across Siberia’s Yedoma region during recent summer field campaigns. These lakes, formed through thermokarst processes—where ice-rich permafrost collapses upon thawing—serve as natural traps and conduits for organic carbon exported from thawing soils. The analytical methods employed included accelerator mass spectrometry for radiocarbon age determinations and high-resolution mass spectrometry to characterize the molecular structure of the dissolved carbon compounds. Such rigorous technical approaches enabled the scientists to differentiate recently produced organic materials from those that have remained preserved since the Pleistocene, a critical distinction for modeling carbon cycle feedbacks.

The findings revealed DOC concentration gradients with values exceeding prior expectations by nearly an order of magnitude in the studied lakes. These concentrations were not only high but also demonstrated considerable age heterogeneity, indicating a complex mixture of carbon sources. The aged fraction of DOC often consisted of highly degraded compounds with chemical signatures distinct from modern plant-derived organic matter, suggesting that permafrost thaw mobilizes deep, previously inaccessible carbon pools. The significance of these observations is underscored by the potential for microbial communities within lake waters to decompose this ancient carbon, leading to enhanced carbon dioxide and methane production and release to the atmosphere.

This discovery complicates our understanding of Arctic carbon cycling feedbacks because the fate of the ancient DOC once it enters aquatic environments remains poorly constrained. While bacterial metabolism can convert some fraction of DOC to greenhouse gases, part of it may also be transported downstream and eventually buried in sediments or even exported to the Arctic Ocean. The research emphasizes the importance of including dissolved organic matter from thawing permafrost in climate models, which have historically underestimated these fluxes. The complex biogeochemical interactions that govern the transformation and persistence of DOC in lakes following permafrost thaw represent a critical knowledge gap for projecting future carbon dynamics in a warming Arctic.

Moreover, this study highlights that the Yedoma regions, which span millions of square kilometers across Siberia and Alaska, could serve as hotspots for massive DOC release in coming decades. The accelerating rate of permafrost degradation, driven by rising Arctic temperatures, is expected to enhance thermokarst lake formation and expansion, creating widespread pathways for organic matter mobilization. As these processes intensify, the quantity of aged DOC entering lake systems could reach levels that significantly amplify radiative forcing linked to anthropogenic climate change, underscoring an urgent need to monitor and quantify these changes more precisely.

The implications extend beyond the Arctic as well, particularly in understanding how permafrost carbon feedbacks may induce global nonlinear responses in Earth’s climate system. The release of ancient carbon that had been locked away for millennia represents a critical tipping element, with the potential to trigger cascades of biogeochemical and ecological shifts. Such feedbacks could accelerate warming trajectories and pose challenges for global emission mitigation strategies. This research thus urges policymakers and climate scientists to reassess risk assessments related to permafrost carbon emissions in their future projections and adaptation plans.

One of the more surprising revelations from this study is the variability in DOC composition and aging across different lakes that formed through thermokarst processes. Factors such as lake size, depth, hydrology, and sediment composition were found to influence the quality and quantity of DOC present. This heterogeneity suggests that localized conditions exert a strong control over carbon release patterns and biogeochemical processing, demanding more nuanced models that incorporate spatial variability in permafrost landscapes rather than relying solely on large-scale averages.

In addition to biological and chemical factors, physical processes such as mixing and thermal stratification within these lakes were found to modulate the residence time and distribution of dissolved organic carbon. The researchers observed that during summer months, stratification limits oxygenation of deeper waters, creating conditions favorable for anaerobic microbial metabolism, which can produce methane from ancient DOC. Conversely, turnover events in autumn and winter seasonally redistribute DOC, potentially influencing microbial degradation rates and greenhouse gas fluxes. Such seasonal dynamics add an additional layer of complexity to the carbon cycling in thaw lakes.

From a methodological perspective, the integration of multi-disciplinary techniques combining organic geochemistry, radiocarbon dating, and hydrological modeling represents a pivotal advancement in tracing the pathways of dissolved organic carbon in permafrost-affected regions. This comprehensive approach has proven essential for disentangling sources of carbon and appreciating the temporal scales over which these organic compounds are mobilized and transformed. Future research will benefit from coupling these techniques with in situ monitoring and remote sensing technologies to capture long-term trends in DOC fluxes and permafrost degradation.

Importantly, the work also reinforces emerging evidence from other Arctic regions that lakes constitute a major interface for carbon transfer from land to atmosphere or ocean. Given the vast surface area covered by thermokarst lakes in permafrost terrains, understanding their role in global carbon budgets is more urgent than ever. This study’s emphasis on Yedoma permafrost enriches the global narrative by spotlighting some of the oldest and most carbon-rich deposits reacting to climate warming, provoking broader scientific inquiries into permafrost resilience and carbon sequestration potentials.

The study’s findings prompt a re-evaluation of carbon management strategies in Arctic environments. Conservation efforts, infrastructure planning, and indigenous knowledge systems all must incorporate the risks posed by permafrost thaw-induced DOC release. These results highlight opportunities for targeted monitoring networks in Yedoma regions and underscore the necessity for international cooperation to mitigate climate impacts associated with permafrost carbon mobilization. The scientific community’s responsiveness to these challenges will be pivotal in modeling future scenarios with greater accuracy.

Beyond the immediate technical and environmental implications, the research also touches on the socio-ecological dimensions of permafrost thaw. Indigenous populations that depend on stable land and water resources may face disruptions as dissolved organic carbon alters lake chemistry, affecting fisheries and water quality. Thus, the study connects global climate processes with local livelihoods, emphasizing the interconnectedness of environmental changes in the Arctic. This integrative perspective enriches the dialogue around sustainable development in cold regions facing rapid transformation.

In conclusion, the discovery of massive concentrations of ancient dissolved organic carbon in Siberian Yedoma thaw lakes represents a major leap forward in understanding Arctic carbon cycle complexities. It challenges prior assumptions about permafrost carbon stability, highlighting the urgent need for advanced observational networks and predictive models to capture this newly recognized carbon flux. As the Arctic continues to warm at unprecedented rates, insights such as these are crucial for framing climate resilience and global mitigation objectives. The full ramifications of this research will undoubtedly influence the trajectory of climate science and policy in the years ahead.

Subject of Research:
Ancient dissolved organic carbon release from Yedoma permafrost thaw in Siberian lakes and its implications for Arctic carbon cycling and climate feedbacks.

Article Title:
Massive concentrations of old dissolved organic carbon from Yedoma thaw in lakes in Siberia.

Article References:
Ollivier, S., Séjourné, A., Hatté, C. et al. Massive concentrations of old dissolved organic carbon from Yedoma thaw in lakes in Siberia. Commun Earth Environ 7, 200 (2026). https://doi.org/10.1038/s43247-026-03229-0

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s43247-026-03229-0