The Paleocene–Eocene thermal maximum (PETM), alternatively "Eocene thermal maximum 1 (ETM1)" and formerly known as the "Initial Eocene" or "Late Paleocene thermal maximum", was a geologically brief time interval characterized by a 5–8 °C (9–14 °F) global average temperature rise and massive input of carbon into the ocean and atmosphere. The event began, now formally codified, at the precise time boundary between the Paleocene and Eocene geological epochs. The exact age and duration of the PETM remain uncertain, but it occurred around 55.8 million years ago (Ma) and lasted about 200 thousand years (Ka).
The PETM arguably represents our best past analogue for which to understand how global warming and the carbon cycle operate in a greenhouse world. The time interval is marked by a prominent negative excursion in carbon stable isotope (δC) records from around the globe; more specifically, a large decrease in the C/C ratio of marine and terrestrial carbonates and organic carbon has been found and correlated across hundreds of locations. The magnitude and timing of the PETM (δC) excursion, which attest to the massive past carbon release to our ocean and atmosphere, and the source of this carbon remain topics of considerable current geoscience research.
