Paleocene–Eocene Thermal Maximum

The PETM is recognized in sedimentary records by a prominent negative carbon-isotope excursion: the relative abundance of carbon-13 falls, indicating that isotopically light carbon entered the atmosphere–ocean system. Records from marine sediments and fossils also document environmental change. A study of shallow-marine foraminifera from the Maryland Coastal Plain found evidence for a precursor excursion associated at that site with at least 2°C of sea-surface warming and a decline in ocean pH before the main isotope excursion. Whether this precursor was globally expressed remains a subject of research, and such records should not be taken as a complete global chronology.

The PETM occurred roughly 56 million years ago and involved a substantial rise in global temperature. Estimates vary with the proxy records, geographic coverage, and methods used; one synthesis describes a global mean surface warming of about 4–5°C, while NOAA Climate.gov summarizes estimates of roughly 5–8°C. Geological studies infer that a large amount of carbon entered the climate system, but published totals and rates remain uncertain. Proposed sources and triggers include volcanic carbon, methane, and feedbacks involving the carbon cycle. The relative contributions and exact sequence remain debated, so no single mechanism should be treated as definitively established.

The additional carbon altered the ocean–atmosphere system. Geological and geochemical evidence links the PETM to ocean warming and acidification, and studies also discuss changes such as reduced oxygen in some marine settings and altered hydrology. Fossils record ecological responses on land and in the sea, including shifts in species distributions. Because different regions preserve different conditions and intervals, the evidence is assembled from multiple types of proxy rather than from a single complete record.

The PETM onset was geologically rapid, but its exact duration is difficult to determine and remains sensitive to the archive and dating method. An astrochronological study of a Maryland shelf record estimated an onset of about 6,000 years and argued that astronomical forcing may have helped set conditions for the event; this is a site-based result, not a universally settled duration. The event's recovery phase was much longer than its initial onset. Carbonate dissolution, sedimentation changes, and later carbon-cycle feedbacks complicate reconstruction of the full sequence.

The PETM offers a deep-time test of climate and carbon-cycle processes under strong warming, but it is not a direct duplicate of modern climate change. Its background geography, ecosystems, and carbon reservoirs differed from those of today, and estimates of ancient emissions carry significant uncertainties. Comparisons are most useful for examining mechanisms—such as warming following carbon addition, ocean chemistry changes, and ecosystem responses—while respecting differences in pace, context, and evidence.