A hyperthermal event initiated by modest warming in a modern-like icehouse

Deep-time hyperthermal events provide critical analogues for modern climatic warming and its impacts1. However, all ancient hyperthermals documented to date occurredunder greenhouseclimate states2–4, limiting their relevance for understanding the trajectory of present-day climate change.Here, we present conodont oxygen isotope dataand climate modeling for the Kasimovian-Gzhelian Thermal Maximum (KGTM)at ~304 Ma. This event was marked by a ~7.5℃rise in global mean surface temperatureand an increase in atmospheric CO2 concentrationsfrom ~300 to ~750 ppm over ~175 kyr, yielding an Earth system sensitivityof ~5.5 ℃ during the Late Palaeozoic Ice Age. Carbon and mercury isotoperecords indicate large-scale carbon release and the development of photic-zone euxinia during the KGTM, coincident with a marine biocrisis.The KGTM was initiated by a modest temperature increase associated with enhanced volcanic activity at an orbital eccentricity maximum, which crossed a climatic tipping point and triggered massive carbon release and abruptlarge scale warming. These findings demonstrate thathyperthermalevents and severe oceanic deterioration can be triggered by modest warming within an icehouse climatestate analogous to thatof the modern Earth.