The Palaeocene-Eocene Thermal Maximum (PETM) is a rapid global warming event that occurred approximately 56 million years ago — rapid by geological standards. Oxygen isotope ratios and the carbon isotope record in deep-sea sediments show that global mean temperatures rose by 5 to 8°C over a period of 20,000 to 50,000 years. Even the polar regions hosted crocodiles and palm trees during this interval.

The trigger for the warming remains unresolved. Leading proposed mechanisms include large-scale volcanism in the North Atlantic (the Thulean volcanic province), the sudden dissociation of methane hydrates on the deep sea floor, and the combustion of organic carbon reservoirs. The carbon released into the atmosphere during the PETM is estimated at more than two thousand gigatons — a massive carbon pulse even by recent standards. However, the rate of release is estimated to have been far lower than present-day fossil fuel burning.

For the deep ocean the consequences were severe. Large volumes of CO₂ combined with seawater to produce carbonic acid that dissolved calcium carbonate shells; this ocean acidification wiped out 35–50 percent of deep-sea benthic foraminifera. The collapse of the carbonate compensation depth marks this layer as a readily identifiable 'carbonate dissolution' horizon in deep-sea sediment cores.

On land, the picture was different. Warm climates allowed tropical forests to extend northward to what is now Alaska. Mammals diversified rapidly during or immediately after the PETM and spread across the continents. Most critically: the oldest known true primate fossils (55-mya-first-primates) coincide with or closely follow this warming event. The mammalian dispersal of this period was facilitated by newly formed land bridges and expanding tropical forest corridors.

The PETM is used by present-day climate scientists as a critical reference point. As an event that demonstrates how a similar carbon pulse transformed ecosystems, disrupted ocean chemistry, and shaped biotic responses, it serves as an indispensable geological analogue for modelling the possible future consequences of rapid warming.