Paleocene–Eocene Thermal Maximum instance of climate change
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 time period with a more than 5–8 °C global average temperature rise across the event. This climate event occurred at the time boundary of the Paleocene and Eocene geological epochs. The exact age and duration of the event is uncertain but it is estimated to have occurred around 55.5 million years ago (Ma).The associated period of massive carbon release into the atmosphere has been estimated to have lasted from 20,000 to 50,000 years. The entire warm period lasted for about 200,000 years. Global temperatures increased by 5–8 °C.The onset of the Paleocene–Eocene thermal maximum has been linked to volcanism and uplift associated with the North Atlantic Igneous Province, causing extreme changes in Earth's carbon cycle and a significant temperature rise. The period is marked by a prominent negative excursion in carbon stable isotope (δ13C) records from around the globe; more specifically, there was a large decrease in 13C/12C ratio of marine and terrestrial carbonates and organic carbon. Paired δ13C, δ11B, and δ18O data suggest that ~12000 Gt of carbon (at least 44000 Gt CO2e) were released over 50,000 years, averaging 0.24 Gt per year.
Stratigraphic sections of rock from this period reveal numerous other changes. Fossil records for many organisms show major turnovers. For example, in the marine realm, a mass extinction of benthic foraminifera, a global expansion of subtropical dinoflagellates, and an appearance of excursion, planktic foraminifera and calcareous nanofossils all occurred during the beginning stages of PETM. On land, modern mammal orders (including primates) suddenly appear in Europe and in North America.Recovery
Climate proxies, such as ocean sediments (depositional rates) indicate a duration of ∼83 ka, with ∼33 ka in the early rapid phase and ∼50 ka in a subsequent gradual phase.The most likely method of recovery involves an increase in biological productivity, transporting carbon to the deep ocean. This would be assisted by higher global temperatures and CO2 levels, as well as an increased nutrient supply (which would result from higher continental weathering due to higher temperatures and rainfall; volcanoes may have provided further nutrients). Evidence for higher biological productivity comes in the form of bio-concentrated barium. However, this proxy may instead reflect the addition of barium dissolved in methane. Diversifications suggest that productivity increased in near-shore environments, which would have been warm and fertilized by run-off, outweighing the reduction in productivity in the deep oceans. Another pulse of NAIP volcanic activity may have also played a role in terminating the hyperthermal via a volcanic winter.Comparison with today's climate change
Since at least 1997, the Paleocene–Eocene thermal maximum has been investigated in geoscience as an analog to understand the effects of global warming and of massive carbon inputs to the ocean and atmosphere, including ocean acidification. Humans today emit about 10 Gt of carbon (about 37 Gt CO2e) per year, and will have released a comparable amount in about 1,000 years at that rate. A main difference is that during the Paleocene–Eocene thermal maximum, the planet was ice-free, as the Drake Passage had not yet opened and the Central American Seaway had not yet closed. Although the PETM is now commonly held to be a "case study" for global warming and massive carbon emission, the cause, details, and overall significance of the event remain uncertain.Model simulations of peak carbon addition to the ocean–atmosphere system during the PETM give a probable range of 0.3–1.7 petagrams of carbon per year (Pg C/yr), which is much slower than the currently observed rate of carbon emissions. It has been suggested that today's methane emission regime from the ocean floor is potentially similar to that during the PETM. (One petagram of carbon = 1 gigaton of carbon, GtC; the current rate of carbon injection into the atmosphere is over 10 GtC/yr, much larger than the carbon injection rate that occurred during the PETM.)
Professor of Earth and planetary sciences James Zachos notes that IPCC projections for 2300 in the 'business-as-usual' scenario could "potentially bring global temperature to a level the planet has not seen in 50 million years" – during the early Eocene. Some have described the PETM as arguably the best ancient analog of modern climate change. Scientists have investigated effects of climate change on chemistry of the oceans by exploring oceanic changes during the PETM.A study found that the PETM shows that substantial climate-shifting tipping points in the Earth system exist, which "can trigger release of additional carbon reservoirs and drive Earth's climate into a hotter state".Whether climate sensitivity was lower or higher during the PETM than today remains under debate. A 2022 study found that the Eurasian Epicontinental Sea acted as a major carbon sink during the PETM due to its high biological productivity and helped to slow and mitigate the warming, and that the existence of many large epicontinental seas at that time made the Earth's climate less sensitive to forcing by greenhouse gases relative to today, when much fewer epicontinental seas exist. Other research, however, suggests that climate sensitivity was higher during the PETM than today, meaning that sensitivity to greenhouse gas release increases the higher their concentration in the atmosphere.
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