The Paleocene–Eocene thermal maximum (PETM), also called "Eocene thermal maximum 1 (ETM1)" and previously known as the "Initial Eocene" or "Late Paleocene thermal maximum," was a short period in Earth's history marked by a global temperature increase of 5–8 °C (9–14 °F) and a large release of carbon into the ocean and atmosphere. This event began exactly at the boundary between the Paleocene and Eocene geological time periods. Scientists are still unsure of the exact age and length of the PETM, but it is believed to have occurred about 55.8 million years ago and lasted approximately 200,000 years.
The PETM is considered the best example from Earth's past for studying how global warming and the carbon cycle work in a greenhouse climate. During this time, carbon isotope (δC) records from around the world show a significant drop in the ratio of carbon isotopes in marine and land-based carbonates and organic matter. This change, along with the timing of the event, indicates a massive release of carbon into the ocean and atmosphere. Scientists continue to research the source of this carbon.
Over the past few decades, studies of rock layers from the PETM have revealed changes beyond temperature rise and carbon emissions. Fossil records show major shifts in life forms, consistent with the start of a new geological epoch. In the ocean, many species of benthic foraminifera (tiny marine organisms) went extinct, subtropical dinoflagellates (a type of algae) spread globally, and new species, such as certain planktic foraminifera and calcareous nannofossils, appeared. On land, many modern mammal groups, including primates, suddenly appeared in Europe and North America.
Setting
During the early Paleogene, the arrangement of oceans and continents was somewhat different compared to today. The Panama Isthmus had not yet formed, which connected North America and South America, allowing direct movement of warm ocean currents between the Pacific and Atlantic Oceans. The Drake Passage, which now separates South America and Antarctica, was closed at this time, possibly preventing Antarctica from becoming isolated and cold. The Arctic Ocean was also smaller and more limited in size. Although different scientific methods used to estimate past atmospheric carbon dioxide levels during the Cenozoic era do not agree exactly, all suggest that CO₂ levels were much higher during the early Paleogene than they are today. Regardless of these differences, there were no large ice sheets on land or significant sea ice during the late Paleocene and early Eocene.
Earth surface temperatures slowly rose by about 6 degrees Celsius (11 degrees Fahrenheit) from the late Paleocene through the early Eocene. This long-term warming included at least three (and likely more) short but intense warming events called "hyperthermals." These events lasted less than 200,000 years and were marked by rapid global temperature increases, major environmental changes, and large amounts of carbon released into the atmosphere. The Paleocene-Eocene Thermal Maximum (PETM) was the most extreme of these hyperthermals and caused major changes in rock layers, living organisms, and chemical compositions found in sediment records worldwide. Other hyperthermal events occurred around 53.7 million years ago (called ETM-2 or H-1, also known as the Elmo event), 53.6 million years ago (H-2), 53.3 million years ago (I-1), 53.2 million years ago (I-2), and 52.8 million years ago (informally named K, X, or ETM-3). Scientists are still studying how many of these events occurred, their exact ages, and their global effects. It is not yet clear whether these events only happened during the long-term warming or if they are connected to similar events in older parts of Earth's history, such as the Toarcian turnover in the Jurassic period.
Global warming
A study from 2020 estimated the global mean surface temperature (GMST) with 66% confidence during the latest Paleocene (about 57 million years ago) as 22.3–28.3 °C (72.1–82.9 °F), during the Paleocene-Eocene Thermal Maximum (PETM) (56 million years ago) as 27.2–34.5 °C (81.0–94.1 °F), and during the Early Eocene Climatic Optimum (EECO) (53.3 to 49.1 million years ago) as 23.2–29.7 °C (73.8–85.5 °F). Scientists think the temperature rose by about 3 to 6 degrees Celsius at the start of the PETM, or possibly between 5 and 8 degrees Celsius. This warming happened on top of a long-term warming trend in the early Paleogene. Evidence includes a large drop in oxygen isotope ratios (δ O) in foraminifera shells from both surface and deep ocean waters. Since there was little or no polar ice during the early Paleogene, this drop likely means ocean temperatures increased. Other evidence includes the spread of warm-loving species to higher latitudes, changes in plant leaf shapes and sizes, Mg/Ca ratios in foraminifera, and organic compound ratios like TEX 86. Models also suggest the climate became more uniform, with a 5 °C decrease in the range of annual temperatures across most continents.
Proxy data from Esplugafereda in northeastern Spain shows a rapid 8 °C (14 °F) temperature rise, matching other regional records. In the Fushun Basin, temperatures rose from 15.6 to 19.7 °C (60.1 to 67.5 °F). Southern California had an average annual temperature of about 17 °C, with a range of ±4.4 °C. In Antarctica, at least part of the year had minimum temperatures of 15 °C.
TEX 86 values suggest that tropical sea surface temperatures (SST) reached over 36 °C (97 °F) during the PETM, enough to stress even heat-resistant organisms like dinoflagellates, some of which went extinct. Oxygen isotope ratios from Tanzania suggest tropical SSTs may have been even higher, over 40 °C (104 °F). Ocean Drilling Program Site 1209 in the tropical western Pacific shows SSTs increased from 34 °C (93 °F) before the PETM to about 40 °C. In the eastern Tethys, SSTs rose by 3 to 5 °C (5 to 9 °F). Low-latitude Indian Ocean Mg/Ca records show seawater warmed by about 4–5 °C (7–9 °F) at all depths. In the Pacific Ocean, tropical SSTs increased by about 4–5 °C. TEX 86 values from New Zealand deposits, then located between 50°S and 60°S, indicate SSTs of 26–28 °C (79–82 °F), an increase of over 10 °C (18 °F) from an average of 13–16 °C (55–61 °F) at the boundary between the Selandian and Thanetian. The extreme warmth in the southwestern Pacific extended into the Australo-Antarctic Gulf. Sediment cores from the East Tasman Plateau, then at a paleolatitude of ~65°S, show SSTs increased from about 26 to 33 °C (79 to 91 °F) during the PETM. In the North Sea, SSTs rose by 10 °C (18 °F), reaching ~33 °C, while in the West Siberian Sea, SSTs climbed to ~27 °C.
The central Arctic Ocean was ice-free before, during, and after the PETM, as shown by sediment cores from the Arctic Coring Expedition (ACEX) at 87°N on the Lomonosov Ridge. Temperatures during the PETM increased, as shown by the presence of subtropical dinoflagellates (Apectodinium spp.) and higher TEX 86 values. This record suggests a 6 °C (11 °F) rise from ~17 °C (63 °F) before the PETM to ~23 °C (73 °F) during the PETM. If TEX 86 reflects summer temperatures, it still means the North Pole was much warmer than today, but not significantly warmer than other regions.
These findings are important because, in many global warming models, high-latitude temperatures increase more at the poles due to ice-albedo feedback. However, during the PETM, this feedback may not have occurred because there was little polar ice, so temperatures at the Equator and poles rose similarly. There is no evidence of greater warming in polar regions compared to other areas, suggesting no ice-albedo feedback and no sea or land ice in the late Paleocene.
Scientists still debate the exact global temperature rise during the PETM and whether it varied by latitude. Oxygen isotope and Mg/Ca ratios in ocean carbonate shells are common tools for estimating past temperatures, but these can be unreliable at low latitudes due to recrystallization of carbonate on the seafloor, which lowers recorded values. At high latitudes, these and other proxies like TEX 86 are also affected by seasonality, as they tend to reflect summer temperatures rather than annual averages.
Carbon cycle disturbance
Strong evidence shows that a large amount of carbon with low carbon levels was added to Earth’s systems at the start of the PETM. This is supported by two key findings. First, many locations around the world, including more than 130 places in different environments, show a clear drop in the carbon isotope levels (δC) in materials that contain carbon. Second, layers of rock from the deep ocean show signs of carbonate dissolving, which is linked to the PETM.
Scientists still debate how much carbon was added to the ocean and atmosphere during the PETM. In theory, this can be estimated by looking at the size of the drop in carbon isotope levels (called a CIE), the amount of carbonate dissolving on the ocean floor, or both. However, the change in δC values varies depending on where and in what type of material the measurements are taken. For example, some records of bulk carbonate show a change of about 2‰, while others from land-based carbonate or organic matter show changes greater than 6‰. Similarly, the amount of carbonate dissolving also differs across ocean regions. It was very high in parts of the North and Central Atlantic Ocean but much lower in the Pacific Ocean. Based on current data, the estimated total carbon added during the PETM ranges from about 2,000 to 7,000 gigatons.
Timing of carbon addition and warming
The timing of the PETM δC excursion is very important. This is because the total length of the CIE, from the quick drop in δC to the return to original levels, helps scientists understand key parts of Earth's global carbon cycle. It also gives clues about the source of carbon-depleted CO₂.
Scientists estimate the total duration of the CIE using several methods. A key sediment layer for studying and dating the PETM was collected in 1987 by the Ocean Drilling Program at Hole 690B near Maud Rise in the South Atlantic Ocean. At this location, the PETM CIE spans about 2 meters (6.6 feet). Long-term age estimates, using methods like biostratigraphy and magnetostratigraphy, show that sediment layers formed at an average rate of about 1.23 centimeters (0.48 inches) every 1,000 years. If this rate was constant, the entire event—from start to end—lasted about 200,000 years. Later, scientists noticed the CIE covered 10 or 11 subtle cycles in sediment properties, such as iron content. If these cycles match precession, a slightly longer time of about 200,000 years was calculated by Rohl et al. in 2000. If a large amount of carbon-depleted CO₂ is quickly added to Earth's ocean or atmosphere, a CIE lasting about 200,000 years would occur because the carbon would slowly move through steady processes like weathering, volcanism, and the removal of carbon by carbonate and organic matter. Another study, using updated orbital data and sediment cores from the South Atlantic and Southern Ocean, found the event lasted about 170,000 years.
Models of Earth's global carbon cycle also estimate the CIE to last about 200,000 years.
Age estimates from several deep-sea locations have been checked using helium (He) content, assuming the production rate of this cosmogenic nuclide is steady over short times. This method suggests the PETM CIE began quickly (in less than 20,000 years). However, helium records show the return to near-original conditions happened faster (in less than 100,000 years) than predicted by steady processes like weathering and carbon removal.
Other evidence shows warming began about 3,000 years before the δC excursion.
Some scientists think the size of the CIE might be smaller than it appears because local processes at many sites caused large amounts of non-local sediments to mix with local sediments, changing isotopic values. Microbial breakdown of organic matter has also been linked to changes in carbon isotope ratios in bulk organic material.
Effects
During the PETM, the climate became much wetter, especially in the tropics, where evaporation rates were highest. Studies using special types of water showed more moisture moving to the Arctic. Warm weather extended as far north as the Polar basin. Fossils of Azolla floating ferns found in polar regions suggest that temperatures there were similar to subtropical areas. East Asia had more rain during the PETM. In Central China, dense subtropical forests grew because of increased rainfall. Average temperatures there were between 21 and 24 °C (70 and 75 °F), and yearly rainfall ranged from 1,396 to 1,997 mm (4.580 to 6.552 ft). In the Jiangshan Basin, hot and humid conditions led to more sediment buildup. Similarly, Central Asia became wetter as rainfall from early monsoons reached farther inland. Thick layers of plant material in India’s Cambay Shale Formation show that heavy rainfall caused more soil erosion and organic matter burial. In the Arctic, increased rainfall caused more sediment to settle. Rainfall also rose sharply in the North Sea. In Cap d'Ailly, now in Normandy, a short dry period occurred before the PETM, after which the area became wetter, changing from a marsh to a swamp with frequent algae growth. Rainfall patterns became unpredictable along the New Jersey Shelf, while the Gulf Coast in eastern Texas had more rain. In the Rocky Mountain Interior, rainfall decreased as the center of North America became drier with more seasonal changes. Along California’s central coast, conditions became drier overall, though summer rains increased. The drying of western North America happened because weather patterns shifted northward. East Africa had dry periods interrupted by strong rainfall, showing the PETM climate was not universally wet. Coastal areas of the western Tethys Ocean became drier. Evidence from Forada, Italy, suggests that dry and wet periods alternated during the PETM, linked to Earth’s orbital cycles, and overall, rainfall in the central-western Tethys Ocean decreased.
More freshwater entered the Arctic Ocean due to increased rainfall in the Northern Hemisphere, driven by storm patterns moving toward the poles under global warming. Freshwater input into the oceans rose sharply during the PETM and continued after the event ended.
The PETM caused the only major oxygen-poor event in the Cenozoic era. Oxygen levels dropped because of warmer seawater, layers of water separating, and methane released from underwater ice. Nitrogen removal increased. In some ocean areas, like the North Atlantic, no mixing of ocean floor sediments occurred, possibly due to oxygen-poor water or changes in ocean currents. However, many ocean regions remained mixed. Ratios of iodine to calcium suggest oxygen-poor zones in the ocean expanded. Oxygen-poor and sulfur-rich conditions were most common in enclosed ocean areas, like the Arctic and Tethys Oceans. These conditions also affected the North Sea Basin, shown by higher levels of uranium, molybdenum, sulfur, and pyrite in sediments, along with sulfur-bound isorenieratane. The Gulf Coastal Plain and Atlantic Coastal Plain also had oxygen-poor conditions. The Tasman Sea saw lower oxygen levels, as shown by chemical ratios. In contrast, tropical ocean surfaces remained oxygen-rich throughout the PETM.
Early in the PETM, oxygen-poor conditions may have slowed warming by trapping carbon in buried organic matter. Increased weathering and erosion, shown by lithium isotope changes, led to more organic carbon burial, which acted as a cooling effect.
Without ice, rising ocean temperatures caused sea levels to rise. Evidence from Arctic Ocean fossils shows more marine organic material than land material, indicating a large ocean spread. In the Indian Subcontinent, sea levels rose by 20–50 meters.
At the start of the PETM, ocean currents changed rapidly in less than 5,000 years. Currents reversed direction as ocean circulation shifted from the Southern to Northern Hemisphere. This change lasted 40,000 years and moved warm water to the deep ocean, increasing warming. A major change in deep-sea life, seen in foraminifera, supports this.
Ocean acidification occurred during the PETM, with seawater pH dropping by about 0.46 units. This caused the depth where calcium carbonate dissolves (the lysocline) to rise. Today, this depth is around 4 km (2.5 mi), but during the PETM, it rose in some areas. In the North Atlantic, acidification was strongest, as shown by sudden changes in ocean sediment layers from gray carbonate to red clay. Acidic water from the North Atlantic may have spread to other ocean regions. Models suggest acidic water built up in the deep North Atlantic at the start of the PETM.
Acidification explains differences in calcium carbonate dissolution across the ocean. In parts of the southeast Atlantic, the lysocline rose by 2 km in a few thousand years. In the tropical Pacific, the lysocline rose by about 500 m. Acidification may have increased the movement of surface water into the deep ocean, slowing the buildup of atmospheric carbon dioxide. Reduced shell formation by marine life also affected ocean chemistry, causing an overproduction of calcium carbonate once normal levels returned, helping restore ocean conditions. Enhanced runoff from rain led to more coccolithophorid blooms, which removed carbonate from seawater, easing acidification.
Tiny magnetite particles (Fe₃O₄) were found in PETM-age ocean sediments. A 2008 study found these particles had long, prism-like shapes and spearhead forms.
Possible causes
It is hard to tell which cause was responsible for the PETM. Temperatures were rising globally at a steady pace, and a mechanism must be found to explain a sudden, sharp increase in temperature that may have been made worse by feedback loops. The best way to study this is by looking at the balance of carbon isotopes. Scientists know that the carbon in the oceans and atmosphere changed by about −0.2% to −0.3% in δ C. By studying the isotopic signatures of other carbon sources, they can estimate how much carbon from those sources would be needed to cause this change. This method assumes that the amount of carbon in the oceans and atmosphere during the Paleogene was the same as it is today, but this is hard to confirm.
The initial warming is thought to have been caused by a large amount of carbon, such as CO₂ or CH₄, being released into the atmosphere. One possible source is a large group of kimberlite pipes in northern Canada that formed about 56 million years ago. These pipes may have released magmatic CO₂, which could have caused the early warming. Scientists estimate that about 900 to 1,100 gigatons of carbon would have been needed to raise ocean temperatures by about 3°C during the PETM. This amount could have been released when these kimberlite pipes formed. Warm surface water sinking to deeper parts of the ocean may have caused methane hydrates on the seafloor to break apart, releasing isotopically light carbon that explains the carbon isotope change. Other kimberlite clusters and warming events from the same time suggest that CO₂ released during kimberlite eruptions could be a possible cause of these sudden warming events.
One major idea is that volcanic activity linked to the North Atlantic Igneous Province (NAIP) caused the PETM. This activity is believed to have released over 10,000 gigatons of carbon, based on the isotopic makeup of the carbon added. Evidence of heavy volcanic activity during the PETM includes mercury and tellurium anomalies, as well as increased levels of osmium isotopes in Arctic Ocean sediments. These findings support the idea that volcanic eruptions caused the warming.
Hot magma from volcanoes might have heated carbon-rich sediments, releasing large amounts of isotopically light methane, which could have caused global warming and the observed isotope change. Evidence for this includes intrusive rock formations and hydrothermal vents found in sedimentary basins near Norway and Shetland. These vents were shallow, allowing gases to escape into the atmosphere and affect the climate. Large volcanic eruptions can influence climate by blocking sunlight, lowering temperatures, and changing wind patterns. Sulfur gases from eruptions form sulfate aerosols that stay in the stratosphere for years. Volcanic activity may also have triggered the release of methane clathrates, creating feedback loops. NAIP volcanism not only added greenhouse gases but also changed the shape of the North Atlantic, limiting ocean currents through the Faroe-Shetland Basin and English Channel.
Later volcanic activity from NAIP may have caused other warming events in the Early Eocene, such as ETM2. Some scientists think volcanic activity near the Caribbean may have disrupted ocean currents, making climate changes more extreme.
Smaller global warming events, like ETM2, suggest that these events might happen regularly due to changes in Earth's orbit. Studies of sediment layers in Maryland suggest the PETM was linked to extreme changes in Earth's orbit. Current warming is expected to last about 50,000 years because of a minimum in Earth's orbital eccentricity. However, a study found the PETM occurred during a minimum in a 400,000-year orbital cycle, which contradicts the idea that orbital changes caused the warming.
One idea is that a comet rich in carbon hit Earth, causing the warming. A comet impact near the Paleocene-Eocene boundary could explain features like the iridium layer in Spain and a sudden layer of clay with magnetic particles. A comet impact would cause immediate changes in the atmosphere and ocean, followed by effects in deeper waters. This would require at least 100 gigatons of extraterrestrial carbon. A thick clay layer in New Jersey had unusual magnetic particles, but these were found to be from bacteria, not a comet. Recent studies found non-biological magnetic particles in the layer, suggesting it may not be from a comet.
A 2016 study found impact debris in Atlantic sediments, including glass-like particles called microtektites and microkrystites, supporting the idea of a comet impact during the PETM.
Some scientists once thought that burning large amounts of peat caused the PETM, as there may have been more carbon in plants during the Paleocene. However, this idea was rejected because burning 90% of Earth's biomass would be needed to cause the observed isotope change. Peat was common during the Paleocene, but no evidence of burned organic matter, like soot, has been found. Studies of polycyclic aromatic hydrocarbons during the PETM have not confirmed this theory.
Recovery
Climate clues, such as ocean sediments (how fast they build up), suggest a time span of about 83,000 years. This includes about 33,000 years during a fast early period and about 50,000 years during a slower later period.
The most likely way the climate recovered was through increased biological activity, which moved carbon into the deep ocean. This process would be helped by higher global temperatures, higher carbon dioxide levels, and more nutrients. More nutrients could come from increased erosion of rocks on land due to warmer temperatures and more rain, as well as possible contributions from volcanic eruptions. Evidence of higher biological activity includes high levels of barium found in ocean life. However, this clue might instead show barium from methane dissolving in water. Studies suggest that productivity increased in areas near the shore, which were warm and rich in nutrients from runoff, even though productivity in deep oceans decreased. Large amounts of the aquatic fern Azolla found in the Arctic Ocean floor during the middle Eocene (known as the "Azolla Event") may have helped end the PETM by storing carbon in buried, decayed Azolla. Another period of volcanic activity from the North Atlantic Igneous Province (NAIP) may also have contributed to ending the warm period by causing a volcanic winter.
Comparison with today's climate change
Since at least 1997, scientists have studied the PETM to learn how global warming and large amounts of carbon added to the ocean and atmosphere might affect Earth. One major difference between the PETM and today is that during the PETM, Earth had no ice because the Drake Passage was still closed and the Central American Seaway had not yet formed. While the PETM is often used as an example to study global warming and carbon emissions, the exact cause, details, and importance of the event are still not fully understood.
During the PETM, carbon was released into the atmosphere and ocean more slowly than today’s human-caused emissions. Models suggest that the highest rate of carbon added to the ocean and atmosphere during the PETM was between 0.3 and 1.7 petagrams of carbon per year (Pg C/yr). This is much slower than the current rate of carbon emissions, which is over 10 gigatons of carbon per year (GtC/yr). Scientists have suggested that today’s methane emissions from the ocean floor may be similar to those during the PETM. Because modern carbon emissions are faster than those during the PETM, some researchers believe a PETM-like situation could be the best-case outcome of human-caused global warming, while a mass extinction similar to the Cretaceous-Palaeogene event might be the worst-case result.
Professor James Zachos, an expert in Earth and planetary sciences, has noted that if current trends continue, global temperatures by 2300 could reach levels not seen in 50 million years, during the early Eocene. Some scientists consider the PETM to be the best ancient example of modern climate change. Researchers have studied ocean chemistry changes during the PETM to understand how climate change affects the ocean.
A study found that the PETM shows that Earth has tipping points that can cause additional carbon to be released, leading to a warmer climate. Whether Earth’s climate was more or less sensitive to carbon changes during the PETM than it is today is still debated. A 2022 study found that the Eurasian Epicontinental Sea helped absorb a large amount of carbon during the PETM because of its high biological activity, which may have slowed warming. At that time, many large epicontinental seas existed, making Earth’s climate less sensitive to greenhouse gases compared to today, when fewer such seas are present. Other research, however, suggests that Earth’s climate was more sensitive to greenhouse gases during the PETM than it is today, meaning that higher concentrations of greenhouse gases in the atmosphere may increase climate sensitivity.