A climatic optimum that occurred approximately 55 Ma ago and lasted for less than 100 000 years, at the boundary between the Palaeocene and Eocene epochs. At the commencement of the PETM the global mean temperature rose rapidly by 5–8 °C and the oceans warmed throughout their depths. A second, briefer rise in temperature occurred about 2 million years later. The cause of the rise in temperature is not known.