The discovery is especially significant because most ancient hyperthermal events used to understand rapid warming occurred under greenhouse conditions, when Earth already lacked the extensive permanent ice sheets that characterize an icehouse climate. Those events therefore provide imperfect comparisons with the modern world. Today, the planet is warming while still operating within an icehouse state, and the consequences include ocean acidification, declining oxygen levels in seawater, and mounting stress on marine ecosystems. The Late Paleozoic Ice Age is the only deep-time icehouse interval known to have maintained atmospheric carbon dioxide concentrations broadly comparable to those of the Quaternary, the period that includes the present. The KGTM thus provides an unusual natural experiment: a rapid warming episode that took place against a low-carbon, glaciated background similar in some important respects to the modern climate system.

The event occurred near the boundary between the Kasimovian and Gzhelian stages of the Carboniferous–Permian interval. Earlier evidence from fossil conodonts and brachiopods had hinted at warming around this boundary, but the available oxygen-isotope records were too widely spaced to establish how large or rapid the temperature changes had been. To resolve the event at higher precision, the researchers analyzed the oxygen-isotope composition of conodont apatite using secondary ion mass spectrometry, or SIMS. Conodonts were small, eel-like marine vertebrates whose tooth-like feeding elements are exceptionally useful for reconstructing ancient seawater temperatures. Oxygen isotopes preserved in their apatite reflect the combined influence of seawater chemistry and temperature, allowing scientists to identify shifts in ancient marine conditions when the records are carefully calibrated and compared across regions.

Samples came from the Naqing and Narao sections in South China and the Usolka section in the southern Urals of Russia. All three locations recorded large negative excursions in apatite oxygen-isotope values at the Kasimovian–Gzhelian boundary. In paleoclimate studies, a negative oxygen-isotope shift in a marine fossil commonly indicates warming, although changes in ice volume and seawater composition must also be considered. The geographic agreement among these sites suggests that the signal was not a local anomaly but part of a broad climatic disturbance. The researchers divided the KGTM into two distinct phases. The initial phase produced an estimated sea-surface temperature increase of about 2 degrees Celsius and lasted approximately 60,000 years. A second, main warming phase raised sea-surface temperatures by roughly 3.5 degrees Celsius over about 35,000 years. When global effects and polar amplification are taken into account, the total increase in global mean surface temperature is estimated to have approached 7.5 degrees Celsius.

The temperature reconstruction was supported by climate-model experiments using the Community Earth System Model. By comparing the reconstructed temperatures from Usolka with simulations under different carbon dioxide concentrations, the team inferred that atmospheric carbon dioxide climbed from approximately 300 to around 700 parts per million during the KGTM. This range overlaps the modern atmospheric range, which has risen from about 300 parts per million before industrialization to more than 400 parts per million today. The comparison, however, does not mean that the ancient event unfolded at the same speed as modern warming. The estimated rate during the KGTM was approximately 0.04 to 0.10 degrees Celsius per thousand years, whereas current warming rates are roughly 10 to 15 degrees Celsius per thousand years when expressed on the same timescale. Modern emissions are therefore driving temperature change vastly faster, even though the ancient event demonstrates that a relatively modest initial disturbance can eventually produce much larger climate consequences.

Geochemical evidence points to volcanic activity as the trigger for the first phase. The researchers examined mercury isotopes, particularly Δ199Hg and δ202Hg, because volcanic emissions can transport mercury into the atmosphere, where it is deposited in marine and terrestrial sediments. Similar mercury-isotope values, ranging from approximately 0 to +0.04 per mille, appeared during the initial warming phase in sections separated by thousands of kilometers in South China and the southern Urals. That consistency indicates a broadly contemporaneous increase in volcanic influence across local and regional environments. Volcanic carbon dioxide and other greenhouse gases could have initiated warming, while volcanic emissions may also have altered nutrient supply and marine chemistry. The mercury record changes during the main warming phase, however, showing a distinct negative shift. This suggests that volcanism did not remain the dominant source of forcing throughout the entire event.

Instead, orbital dynamics appear to have amplified the initial disturbance. Sedimentary cycles at Naqing show coordinated and rhythmic changes in apatite oxygen isotopes and a carbon-isotope parameter known as Δ13C, calculated from the difference between carbonate and organic carbon isotope values. These cycles correspond to the approximately 405,000-year long-eccentricity cycle, a highly stable pattern produced by variations in the shape of Earth’s orbit. The KGTM’s two warming phases coincided with maxima in the shorter, approximately 100,000-year eccentricity cycle and with high values of the longer eccentricity cycle. Orbital changes do not directly inject carbon dioxide into the atmosphere, but they can redistribute sunlight, influence ice-sheet growth and retreat, alter monsoon intensity, and modify the stability of carbon reservoirs. Under the right background conditions, such changes can push the climate system across a threshold where warming begins to reinforce itself.

The study suggests that this threshold was crossed between the initial and main phases of the KGTM. During the first phase, volcanic emissions likely caused atmospheric carbon dioxide to rise and temperatures to increase. Continued orbital forcing then may have weakened high-latitude ice sheets, changed weathering rates, disturbed ocean circulation, and released additional carbon from climate-sensitive reservoirs. These feedbacks could have transformed a limited volcanic perturbation into a much larger carbon release during the main phase. The resulting warming appears to have affected the oceans biologically as well as physically. Mercury and sedimentary evidence indicates the development of photic-zone euxinia, a condition in which the sunlit upper ocean becomes both oxygen-poor and rich in toxic hydrogen sulfide. Such waters are hostile to many marine organisms and can disrupt nutrient cycles at the base of the food web.

Fossil evidence records the ecological cost of the warming. Conodonts became smaller and less diverse, suggesting stress on marine food webs or changes in the environments where these animals lived. Reef ecosystems also shifted from communities dominated by metazoans, such as sponges and other animals, toward macroalgal systems. The combination of warming, oxygen loss, altered nutrient availability, and potentially expanding toxic waters would have placed pressure on marine organisms across broad regions. The researchers caution that the KGTM was not simply a temperature event; it was a coupled climate and ocean-chemistry disruption. That distinction is crucial for understanding the modern threat. Contemporary warming is already associated with declining oxygen in parts of the ocean, expanding dead zones, acidification, coral-reef deterioration, and biodiversity loss, although the speed and precise mechanisms of today’s changes differ from those of the Late Paleozoic.

The central warning from the reconstruction is that an icehouse climate may not be inherently stable once warming begins. Even a comparatively moderate initial rise in temperature can destabilize ice sheets and mobilize carbon stored in soils, sediments, wetlands, and permafrost. Once these feedbacks become strong enough, the system can move rapidly toward a new climate state. The KGTM took place over tens of thousands of years, far slower than modern warming, but its sequence of volcanic forcing, orbital amplification, carbon release, ocean deoxygenation, and ecological disruption demonstrates how interconnected the Earth system can be. The findings do not predict an identical future event, yet they show why early warming signals deserve close attention. The modern planet is adding greenhouse gases at a rate that has few close geological precedents, while high-latitude ice and carbon reservoirs are already responding. The ancient record indicates that the most dangerous consequences may emerge not only from the initial warming itself, but from the feedbacks it sets in motion.

Subject of Research: Not applicable

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

Keywords: Late Paleozoic Ice Age; Kasimovian-Gzhelian Thermal Maximum; global warming; paleoclimate; volcanic activity; orbital forcing; carbon dioxide; ocean acidification; photic-zone euxinia; climate–carbon cycle feedbacks

The PETM is characterized by a swift and dramatic spike in Earth’s surface temperatures, with estimates suggesting a global average temperature increase of 5 to 8 degrees Celsius within a few thousand years. This extraordinary warming phase is widely regarded as an analog for modern-day climate trajectories, driven predominantly by massive carbon injections into the atmosphere and oceans. The new study meticulously reconstructs the functional ecology of terrestrial plants during this interval, revealing a marked deterioration in vegetation roles that underpinned terrestrial ecosystem stability.

By integrating paleoecological proxies, isotope geochemistry, and advanced Earth system modeling, the researchers uncovered multifaceted disturbances in plant physiological processes. Photosynthesis, water regulation, and nutrient cycling — key functions that maintain ecosystem productivity and resilience — exhibited significant reductions. These functional impairments manifested as decreased carbon sequestration potential and altered hydrological cycles, providing crucial insights into how vegetation may respond to rapid climatic perturbations.

The team employed stomatal index analysis — a proxy derived from fossilized leaf structures — as a primary indicator of plant physiological stress during the PETM. They observed a consistent decline in stomatal density worldwide, suggesting that plants reduced gas exchange to conserve water under heightened thermal stress and increased atmospheric CO₂ levels. This physiological adjustment, while protective in the short term, compromised photosynthetic rates and dampened carbon uptake, which in turn exacerbated global carbon cycle feedbacks.

Moreover, isotopic signatures from paleosol carbonates and organic matter indicated shifts in plant community composition and productivity. There was a pronounced transition from woody gymnosperms to herbaceous angiosperms in many regions, reflecting both thermal tolerance limits and drought-induced stresses. Such vegetation turnover events fundamentally altered biome distributions, with tropical forests retreating and more arid-adapted ecosystems advancing, echoing patterns predicted for future climate scenarios.

The implications of these findings extend beyond paleobotany, illuminating cascading effects on ecosystem structure, biodiversity, and biogeochemical cycling. Loss of vegetation functionality during the PETM likely contributed to soil degradation, reduced habitat complexity, and nutrient imbalances, triggering feedback mechanisms that intensified climatic disruption. Understanding this interplay is pivotal for refining predictive models that aim to forecast ecosystem responses under contemporary warming.

Importantly, the research underscores the vulnerability of terrestrial ecosystems to swift temperature elevations, particularly when accompanied by increased CO₂ concentrations and hydrological stress. The PETM serves as a natural experiment demonstrating that even robust, ancient forest systems were susceptible to functional decline when pushed beyond ecological thresholds. This challenges previous assumptions that elevated CO₂ could universally promote vegetation growth, highlighting nuanced physiological constraints.

The study also details spatial heterogeneity in vegetation responses, noting that equatorial and mid-latitude biomes exhibited differential resilience patterns. Local climatic variables such as precipitation regimes and seasonal temperature extremes modulated the severity of functional losses. Such regional variability underscores the complexity of biological responses to climate perturbations and calls for high-resolution paleoenvironmental reconstructions to properly gauge ecosystem trajectories.

Beyond the terrestrial sphere, diminished vegetation functionality during the PETM likely altered atmospheric composition in ways that intensified global warming. Reduced net primary productivity decreased carbon sinks, prolonging atmospheric CO₂ residence times and amplifying the greenhouse effect. This feedback loop underscores vegetation’s critical role as both a driver and moderator of Earth’s climate system.

The research team also bridges geological data with modern plant physiological studies, identifying convergent patterns of stress response. For example, the stomatal conductance reductions observed during the PETM echo mechanisms seen in contemporary plants subjected to drought and heat stress. Such parallels validate the use of fossil proxies in reconstructing ancient physiological processes and enrich our understanding of plant adaptability limits.

In their discussion, the authors emphasize the urgency of integrating paleoecological insights into current climate impact assessments. The PETM, as an analogue for rapid warming, reveals thresholds beyond which vegetation degradation may become inevitable, with profound repercussions for ecosystem services such as carbon storage, water regulation, and soil stabilization.

The comprehensive dataset compiled for this study — spanning multiple continents and diverse paleoecosystems — represents a significant advancement in Earth system science. It highlights the necessity of multidisciplinary approaches combining paleoclimatology, paleoecology, and biogeochemistry to unravel the intricate feedbacks between vegetation and climate.

As anthropogenic warming accelerates in the 21st century, this research serves as a stark reminder of vulnerability intrinsic to terrestrial ecosystems. Despite physiological plasticity and evolutionary adaptation, the fundamental functions of vegetation can be compromised under sustained thermal and hydric stress, potentially triggering ecosystem collapse scenarios reminiscent of the PETM.

In sum, the paper authored by Rogger, Korasidis, Bowen, and colleagues provides a detailed reconstruction of vegetation functional losses during one of Earth’s most significant hyperthermal events. Their findings advance paleoclimatic science substantially, while simultaneously serving as a cautionary tale for contemporary climate futures. The interplay between rapid warming and terrestrial biosphere functions emerges as a critical nexus for research and conservation efforts.

The revelations from this study underscore the need to prioritize ecosystem resilience-building strategies, including conservation of genetic diversity and restoration of degraded landscapes. Understanding past vegetation responses enables better forecasting, guiding policy and management interventions to mitigate or avert similar functional collapses in modern ecosystems.

The paper’s integration of fossil record analysis with mechanistic models and physiological proxies provides a template for future paleoclimate research, encouraging a holistic perspective on how ancient biota navigated extreme environmental changes. Such frameworks will be invaluable as we confront an uncertain climatic horizon marked by unprecedented rates of change.

With this enhanced knowledge of how vegetation function faltered during the PETM, scientists and environmental stakeholders gain critical perspective on the fragility of Earth’s biosphere under rapid warming. The implications resonate across disciplines, reinforcing the indispensability of long-term ecological data in framing the future trajectory of life on our warming planet.

Subject of Research: Vegetation functional changes and ecosystem impacts during the Paleocene–Eocene Thermal Maximum (PETM).

Article Title: Loss of vegetation functions during the Paleocene–Eocene Thermal Maximum.

Article References:
Rogger, J., Korasidis, V.A., Bowen, G.J. et al. Loss of vegetation functions during the Paleocene–Eocene Thermal Maximum. Nat Commun (2025). https://doi.org/10.1038/s41467-025-66390-8

Image Credits: AI Generated