A transient thermal maximum occurred near the Paleocene-Eocene boundary about 56 million years ago and was associated with a rapid increase in atmospheric carbon dioxide concentrations, possibly caused by massive outgassing and oxidation of methane hydrates from the ocean floor.
Notes on verification
Well-supported by peer-reviewed literature and scientific consensus on the PETM event; the methane hydrate outgassing mechanism is a leading but still debated hypothesis, which the claim appropriately frames as 'possible'.
Sources
- Climate change - Abrupt Changes, Earth History, Impacts (britannica.com)
- https://www.pnas.org/doi/10.1073/pnas.1603348113 (pnas.org)
- https://www.realclimate.org/index.php/archives/2009/08/petm-weirdness/ (realclimate.org)