A transient global warming event during Earth's penultimate icehouse.

Deep-time hyperthermal/transient warming events provide critical analogs for modern climatic warming and its impacts. However, all ancient hyperthermals documented to date have occurred under greenhouse climate states, limiting their relevance for understanding the trajectory of present-day climate change. Here, we present conodont oxygen isotope data and climate modeling for the Kasimovian-Gzhelian Thermal Maximum (KGTM) at ~304 Ma. This transient warming event is inferred to have been marked by a ~7.5 ± 1 °C rise in global mean surface temperature and an increase in atmospheric CO2 concentrations from ~[Formula: see text] to ~700 ± 100 ppm over ~175 kyr during the Late Paleozoic Ice Age. Carbon and mercury isotope records 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 carbon release and temperature increase potentially associated with enhanced volcanic activity at an orbital eccentricity maximum, which likely crossed a climatic tipping point and triggered massive carbon release and abrupt large-scale warming. These findings demonstrate that transient warming events and severe oceanic deterioration can be caused by a modest thermal perturbation within an icehouse climate state analogous to that of the modern Earth.