The Paleocene-Eocene Thermal Maximum (PETM) was a hyperthermal event that dramatically reshaped Earth’s climate and ecosystems. Occurring at the boundary between the Paleocene and Eocene epochs, this global warming episode represents one of the most abrupt and intense climate shifts in the geological record of the last 66 million years. Scientists study the PETM to understand how Earth systems respond to rapid greenhouse gas increases.
Defining the Paleocene-Eocene Thermal Maximum
The PETM occurred approximately 55.8 million years ago, marking a sharp transition in Earth’s history. This period of extreme warmth lasted for a geologically short time, estimated to be between 100,000 and 200,000 years in total duration. The global average temperature spiked significantly, rising by an estimated 5°C to 8°C above the already warm background climate of the late Paleocene.
The primary evidence identifying the PETM globally is the Carbon Isotope Excursion (CIE). This geological marker is recorded as a shift in the ratio of stable carbon isotopes (\(\delta^{13}C\)) found in sediments worldwide. This drop indicates a massive, rapid input of carbon into the ocean and atmosphere from a reservoir depleted in the heavier carbon-13 isotope. The excursion serves as a precise time horizon used to correlate environmental and biological changes.
The Source of Rapid Carbon Release
The trigger for the massive carbon injection, estimated to be up to 10,000 gigatons of carbon, remains a subject of scientific debate. The scale of the input required a source of carbon that was isotopically light. One prominent hypothesis centers on the dissociation of methane hydrates, or clathrates, which are ice-like compounds holding methane gas trapped in seafloor sediments. A warming ocean could have destabilized these deposits, releasing methane, a potent greenhouse gas, that would then oxidize into carbon dioxide in the atmosphere.
Another leading theory involves massive volcanic activity, specifically the emplacement of the North Atlantic Igneous Province (NAIP). This volcanism could have triggered the event by injecting \(\text{CO}_2\) directly or by heating organic-rich sediments. The heat from the magma could have “cooked” these sediments, generating thermogenic methane and carbon dioxide that vented into the atmosphere.
Recent research suggests that the total carbon release may not have been a single event but possibly two distinct pulses lasting thousands of years each. The initial warming may have begun with one mechanism, which then crossed a threshold that triggered a self-reinforcing feedback loop, such as methane clathrate dissociation. Regardless of the exact source, the release rate was high, occurring over a period of perhaps a few thousand years.
Global Climate and Ocean Changes
The rapid release of carbon dioxide and methane drove the planet into a state of extreme warmth. Global mean temperatures rose by several degrees, but the warming was amplified at the poles, where sea surface temperatures may have reached as high as 23°C in the Arctic. This heat caused significant changes to the deep ocean, where temperatures rose by 4°C to 5°C, fundamentally altering circulation patterns.
The increased atmospheric \(\text{CO}_2\) was absorbed by the oceans, leading to widespread deep-ocean acidification. This reduced the ocean’s pH and lowered the concentration of carbonate ions, resulting in a widespread carbonate dissolution event. In deep-sea sediments, this dissolution is visible as a layer where calcium carbonate shells of marine organisms were completely dissolved.
On land, the hydrological cycle intensified, leading to increased aridity in some regions and more intense rainfall and flooding in others. The warmer temperatures also caused seawater to expand, contributing to a global rise in sea level. This combination of heat, ocean acidification, and altered water cycles created a stressed global environment.
Biological Response and Evolutionary Shifts
Life on Earth responded to the extreme environmental changes with significant migrations and evolutionary adaptations. The warming allowed tropical and subtropical flora to expand their range toward the poles. This shift in vegetation zones created new habitats and facilitated the dispersal of land mammals across continents, including the earliest ancestors of modern groups like primates and horses.
In both marine and terrestrial environments, a phenomenon known as “dwarfing” occurred, where some species experienced a reduction in body size. The most significant biological impact was in the deep sea, where the combination of warming and ocean acidification caused a major extinction event among benthic foraminifera, a group of single-celled organisms that live on the seafloor.
Between 35% and 50% of deep-sea benthic foraminifera species went extinct, representing the only deep-sea extinction event in the last 90 million years. While the PETM did not cause a mass extinction on the scale of other geological events, it prompted a massive turnover and diversification in many groups, particularly among mammals.
Why the PETM Matters Today
The Paleocene-Eocene Thermal Maximum is an ancient analogue for understanding the consequences of rapid, human-driven carbon emissions. It demonstrates how carbon release can destabilize the global climate system. By studying the PETM, scientists can better constrain Earth’s “climate sensitivity”—the amount of warming that results from a given increase in atmospheric \(\text{CO}_2\).
While the total amount of carbon released during the PETM was similar to what would be emitted if all known fossil fuel reserves were burned, the rate of carbon release today is estimated to be at least 10 times faster. This difference in speed suggests that the current warming event may push Earth systems past environmental thresholds much more quickly than occurred during the PETM.
The recovery period of the PETM, which took tens of thousands of years, illustrates the slow timeline required for Earth’s natural feedback mechanisms to draw down excess carbon and restore pre-event conditions. The PETM therefore provides a geological precedent, offering insights into the long-term, sustained impacts of carbon perturbation on global temperature, ocean chemistry, and ecosystem stability.