A transient burst of global warming around 300 million years ago may offer one of the clearest deep-time warnings yet about how an icehouse planet can be pushed toward abrupt climate change. A research team led by Professor Yao Le of the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences has reconstructed a major warming episode that unfolded during the Late Paleozoic Ice Age, Earth’s penultimate known icehouse interval. The study, published in the Proceedings of the National Academy of Sciences, identifies a previously underappreciated event called the Kasimovian-Gzhelian Thermal Maximum, or KGTM. According to the researchers, the event began with volcanic activity and was later intensified by changes in Earth’s orbit, which helped activate powerful climate–carbon cycle feedbacks. Global mean surface temperature eventually rose by approximately 7.5 degrees Celsius, while atmospheric carbon dioxide concentrations increased from roughly 300 parts per million to about 700 parts per million.
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
