Although the Earth's environment is constantly changing, there have been a few unusual episodes over the last c. 200 Ma when change was extreme in terms of its rapidity, severity, long-lasting consequences and unpredictability. The geochemical and biotic records for two of these episodes, the Palaeocene–Eocene thermal maximum and the Toarcian Oceanic Anoxic Event (Early Jurassic), possess many significant similarities. Each event was associated with a major carbon isotope excursion, significant levels of biotic extinctions, severe global warming, an enhanced hydrological cycle, and evidence for widespread seawater anoxia. Both carbon isotope excursions can be subdivided into distinct stages with broadly similar characteristics and durations; based on a detailed comparison, the Palaeocene–Eocene thermal maximum may have been an incipient Oceanic Anoxic Event. The geochemical and biotic changes during these two events are most readily explained by the abrupt, large-scale dissociation of methane hydrate that followed a period of more gradual environmental change linked to the emplacement of a large igneous province. Carbon release rates at those times were of the same order of magnitude as the current anthropogenic release rate of carbon to the atmosphere, indicating that ancient events such as these may usefully serve as analogues for present-day environmental change.