Paleocene-Eocene Thermal Maximum

The Paleocene-Eocene Thermal Maximum, or PETM, was a sudden burst of global warming about 56 million years ago. In a geological instant, perhaps several thousand years, a huge amount of carbon entered the atmosphere and oceans, and global temperatures rose by roughly 5 to 8 degrees Celsius. The hot period lasted around 170,000 years before the climate slowly returned to normal. Palm trees and crocodile relatives lived near the Arctic, and Arctic Ocean surface waters reached around 23 degrees Celsius.

The sources of the carbon are still debated. Leading suggestions include magma from the opening North Atlantic heating organic-rich rocks, the release of frozen methane from the seafloor, and the thawing and burning of peat and permafrost, with feedbacks adding more. The event happened as Pangaea's break-up and heavy volcanism continued to reshape the planet's plumbing. In the oceans, extra carbon dioxide made the water more acidic, and many deep-sea single-celled organisms called foraminifera died out. Deep waters lost oxygen.

On land, mammals reorganized. Many groups appeared or spread across the northern continents at this time, including early horses, even-toed hoofed animals and the first true primates, perhaps helped by land bridges and warm, wet forests reaching far north. Some mammals shrank in body size as the world warmed. The PETM matters today because it is the best-studied natural example of a rapid carbon release. Scientists estimate that humans are currently releasing carbon far faster than happened then, so the event is closely watched for what it says about long-term climate effects.