The event is known as the Palaeocene-Eocene Thermal Maximum (PETM) but the cause of the warming is still debated. Huge amounts of carbon entered into the atmosphere at the time along with methane.

Allow us to refresh your memory: it was the Paleocene-Eocene Thermal Maximum, a 200,000-year period of rapid carbon release and global warming that turned Earth's oceans acidic and saw terrestrial ...