Researchers reveal the process and mechanism of an ancient icehouse hyperthermal event yielding insights on the trajectory of modern climate change

In a study published in the prestigious international journal Proceedings of the National Academy of Sciences (PNAS) on Aug. 11, researchers led by Associate Professor YAO Le from the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences (NIGPAS), together with colleaguesat domestic and international institutions, revealed the process and mechanism of a hyperthermal event during the Earth’s penultimate icehouse, known as the Late Paleozoic Ice Age (LPIA).

This study documents that the Earth experienced a transient global warming event triggered by volcanic activity and orbital pacing during an icehouse climate condition about 300 million years ago. This transient warming event was marked by a ~7.5 ℃ rise in global mean surface temperature (GMST) and an increase in atmospheric CO₂ concentrations (pCO₂) from ~300 to ~700 ppm.

The modern world is experiencing unprecedented warming within an icehouse climate state, resulting in ocean acidification and deoxygenation and widespread deterioration of marine ecosystems. During geological times, multiple hyperthermal events occurred, however, reported hyperthermals during greenhouse climate states are inadequate as an alogues for modern climate warming given their dissimilar boundary conditions (i.e., occurrence during a greenhouse instead of an icehouse climate state).

The LPIA is the only deep-time icehouse interval known to have atmospheric pCO₂levels comparable to those of the Quaternary (∼300 ppm). Conodont and brachiopod oxygen isotope (δ¹⁸O) recordsfrom North America and the Donets Basin indicate climate warming near the Kasimovian-Gzhelian Boundary (KGB) during the LPIA. However, due to low resolution of the available datasets, the magnitude and temporal expression of these δ¹⁸O excursions are inconsistent.

The research team conducted secondary ion mass spectrometry (SIMS) oxygen isotopeanalysis (δ¹⁸O_apatite) of conodonts from the Naqing and Narao sections in South China and the Usolka section in the southern Urals of Russia. Large negative δ¹⁸O_apatiteshifts at the KGB record a pronounced warming event, herein named the Kasimovian-Gzhelian Thermal Maximum (KGTM).

The KGTM consists of two warming phases, the initial and main warming phases, which were characterized by sea-surface temperature (SST) rises of ~2.0 ℃ and ~3.5 ℃, respectively. Cyclostratigraphy at Naqing constrains the duration of the initial and main phases to~60 kyr and ~35 kyr, respectively. Comparison of SSTs from Usolka with Community Earth System Model (CESM) simulations of pCO₂ indicates a rise of atmospheric pCO₂ from~300 to ~700 ppm. These results reveal that the KGTM spans the modern climatic state (~300 to 400 ppm), although present-day rates of temperature rise are far higher (~10-15 ℃/kyr) than estimates for the KGTM (~0.04 to ~0.10 ℃/kyr).

Mercury isotoperecords (∆¹⁹⁹Hg and δ²⁰²Hg) indicate the development of photic-zone euxinia (PZE)during the main phase of the KGTM. Marine ecosystems changed at that time, as reflected in decreases in conodont size and diversity and shifts from metazoanto macroalgal reef ecosystems. The coherence of ∆¹⁹⁹Hg values (0 to +0.04‰) during the initial phase of the KGTM across South China and the southern Urals indicates a contemporaneous increase in local to regional volcanic activity, which triggered the initial climatic warming. On the other hand, a distinct negative shift in ∆¹⁹⁹Hg valuesduring the main phase implies no volcanic activity during that interval.

The KGB was marked by coupled and rhythmic variations in the δ¹⁸O_apatite and Δ¹³C (=δ¹³C_carb – δ¹³C_org) records of the Naqing section that conform to 405-kyr long-eccentricity cycles, supporting regulation of atmospheric pCO₂ by orbital forcing. During the KGTM event, the initial and main warming phases coincided with maxima of the 100-kyr short-eccentricity cycle and high values of the 405-kyr long-eccentricity cycle.

In conclusion, YAO and his colleagues inferred that the KGTM was initiated by volcanic activity with superimposedorbital forcing, which induced carbon emissions and rapid temperature increases. Based on atmospheric CO₂ accumulation during the initial phase, sustained orbital forcing is inferred to have triggered positive climate-carbon cycle feedbacks across a critical climatic tipping point, leading to massive carbon release and abrupt large-scale warming during the main phase of the KGTM.

A major implication of this study is that humanity needs to pay attention to modest warming events during the modern icehouse climate, which have the potential to lead to accelerated melting of high-latitude icesheets and carbon emissions from permafrost. Through positive climate-carboncycle feedbacks, the Earth system could potentially cross a climatic tipping pointand trigger massive carbon release and abrupt large-scale warming, leading to a major expansion of oceanic anoxia and biodiversity collapse.

This study was funded by the National Natural Science Foundation of China, the Youth Innovation Promotion Association of the CAS, and the State Key Laboratory of Paleobiology and Stratigraphy of Nanjing Institute of Geology and Paleontology, CAS.

Reference: Le Yao*, Thomas J. Algeo, Qiulai Wang*, Wang Zheng, Qiang Wei*, Yu-ping Qi, Guzel M. Sungatullina, Genming Luo, Ganqing Jiang, Guoqiang Tang, Jian Zhang, Hui Wang, Yaqiu Zhao, Xing Huang, Qiu-Li Li, Xiang-dong Wang, Shucheng Xie, and Xian-Hua Li, 2026. A transient global warming event during Earth’s penultimate icehouse. Proceedings of the National Academy of Sciences of the United States of America. https://doi.org/10.1073/pnas.2601643123.

Fig.1 Schematic diagram shows the process and mechanism of the Kasimovian-Gzhelian Thermal Maximum (KGTM) event during the Pennsylvanian Subperiod.

Fig. 2 Isotopic and biotic records from the Kasimovian-Gzhelian carbonate successions at the Naqing and Narao sections in South China, and the Usolkasection in southern Urals.The light orange and dark pink bars indicate the initial and main phases of the Kasimovian-Gzhelian Thermal Maximum (KGTM), respectively.

Fig.3 Mercury isotope and concentration records from the Kasimovian-Gzhelian carbonate successions at the Naqing, Narao and Usolkasections.The light orange and dark pink bars represent the initial and main phases of the KGTM, respectively.

Fig.4 Geochemical, biotic and orbital records for the Kasimovian-Gzheliantransitionat Naqing. The light pink and light blue bars indicate warming and cooling stages of the background interval during the Kasimovian-Gzhelian transition, respectively. The light orange and dark pinkbars represent the initial and main phases of the KGTM, respectively.

Fig.5 Comparison of sea-surface temperature (SST) obtained from conodont SIMS oxygen isotopes (δ¹⁸O_apatite) and the CESM simulations, andglobal distribution of SST under various atmospheric pCO₂ conditions.