Paleocene-Eocene Thermal Maximum: Earth's Ancient Global Warming

The Paleocene-Eocene Thermal Maximum (PETM) was an episode of rapid global warming about 56 million years ago, when a huge release of carbon into the oceans and atmosphere raised average temperatures by roughly 5 to 8 degrees Celsius. It acidified the oceans, wiped out many deep-sea species and helped launch the spread of modern mammal groups. Because it is the best-studied natural example of a sudden carbon release, scientists use it to understand present-day climate change.

Background: an already warm world

By the late Paleocene, about 10 million years after the Chicxulub asteroid impact, Earth was already much warmer than today. There were no large permanent ice sheets, forests grew at high latitudes and sea levels were higher.

Mammals were diversifying but most modern groups had not yet appeared. In northern South America, giant reptiles such as the snake Titanoboa lived in tropical rainforests.

The continents were in roughly similar positions to today, but the North Atlantic was still opening. As Greenland and Europe drifted apart, great volumes of magma rose in what geologists call the North Atlantic Igneous Province, a fact that later became central to explaining the PETM.

What happened

The clearest signature of the PETM is a sharp shift in carbon isotopes, called the carbon isotope excursion. Carbon comes in a lighter and heavier form, and organic carbon from living things, methane and many volcanic sources is rich in the lighter form. In sediments from around the world, the ratio suddenly shifts towards light carbon, showing that a vast amount of it entered the oceans and atmosphere.

Estimates of the carbon released range widely, from about 2,000 to more than 10,000 billion tonnes, depending on the source assumed. Most studies suggest the main release took place over a few thousand to perhaps 20,000 years, extremely fast by geological standards.

Global temperatures rose by roughly 5 to 8 degrees Celsius on top of an already warm baseline. Sea-surface temperatures in the Arctic Ocean may have reached around 23 degrees Celsius, while tropical waters became extremely hot. The full event, including recovery, lasted about 150,000 to 200,000 years.

Possible causes

Scientists agree on the effect but still debate the source of the carbon. Several explanations, which are not mutually exclusive, have been proposed.

One leading idea links the PETM to the North Atlantic Igneous Province. Magma forced into carbon-rich sediments beneath the sea floor off Norway would have baked out large amounts of methane and carbon dioxide, and hundreds of vent structures from this activity have been mapped.

Another idea is that initial warming destabilised methane hydrates, ice-like deposits of methane trapped in seabed sediments, releasing them in a feedback loop. Thawing permafrost and burning or decaying peat on land may also have contributed. A comet impact has been suggested, but it has little support.

Effects on life

In the oceans

Carbon dioxide dissolving in seawater makes it more acidic and harder for organisms to build calcium carbonate shells. In 2005 James Zachos and colleagues showed from cores drilled on the Walvis Ridge in the South Atlantic that the depth below which carbonate dissolves rose by more than 2 kilometres during the PETM.

Deep-sea benthic foraminifera, tiny shelled organisms living on the sea floor, suffered the worst. Roughly 30 to 50 percent of their species disappeared, the largest extinction in their history over the last 90 million years or so. Surface plankton changed dramatically, and the warm-water dinoflagellate Apectodinium spread worldwide, even into the Arctic.

Low oxygen levels developed in parts of the ocean, and coral reef systems were disrupted. Yet many marine groups survived with migrations rather than extinctions, showing that the effects varied greatly from place to place.

On land and among mammals

On land, plant communities shifted towards the poles, and rainfall patterns changed. In the Bighorn Basin of Wyoming, river deposits record intense seasonal rain and drought, and fossil leaves show increased insect damage.

The PETM coincides with the sudden appearance in North America, Europe and Asia of three groups of mammals that still dominate today: primates, even-toed hoofed animals (the ancestors of deer, pigs and cattle) and odd-toed hoofed animals (the ancestors of horses, rhinos and tapirs). Warm conditions opened high-latitude land bridges that allowed them to spread rapidly between continents.

Some mammals also shrank. Fossils of the early horse Sifrhippus from Wyoming suggest that its body size fell by about 30 percent during the warming and grew again afterwards, possibly because of heat and reduced food quality.

How it was discovered

In 1991 the geologists James Kennett and Lowell Stott published evidence from an Ocean Drilling Program core taken at Maud Rise, off Antarctica. They described abrupt deep-sea warming and a wave of extinctions among benthic foraminifera at the end of the Paleocene.

At first the event was called the Late Paleocene Thermal Maximum. As research expanded, including studies of coastal cliffs such as those at Zumaia in the Basque Country, it became the PETM. In 2003 the official boundary between the Paleocene and Eocene was defined at Dababiya in Egypt, at the start of the carbon isotope excursion.

Aftermath and later warming events

The PETM did not end the warm conditions of the early Eocene. Instead it was the largest of several short warming pulses, often called hyperthermals, that punctuated a longer warming trend. A smaller event about 54 million years ago, known as the Eocene Thermal Maximum 2, left a similar but weaker carbon isotope signature.

The long-term warmth peaked roughly 53 to 49 million years ago in the Early Eocene Climatic Optimum, when crocodile relatives and palm trees lived within the Arctic Circle. The exceptionally preserved forest life of Germany’s Messel Pit dates from the end of this greenhouse phase. Only later in the Eocene did a slow cooling begin that eventually led to the growth of ice on Antarctica.

By the numbers

Timeline

• c. 56 million years ago: A rapid carbon release begins the PETM at the end of the Paleocene.
• c. 56-55.8 million years ago: Peak warmth; ocean acidification and the spread of new mammal groups.
• 1991: James Kennett and Lowell Stott publish evidence of abrupt deep-sea warming and extinctions.
• 2003: The Paleocene-Eocene boundary is formally defined at Dababiya, Egypt.
• 2004: Researchers link volcanic vent structures off Norway to a possible carbon source.
• 2005: Zachos and colleagues document rapid ocean acidification in South Atlantic cores.

Lessons for today

The PETM is often compared with modern climate change, and the comparison is useful but imperfect. Most estimates suggest that humans are releasing carbon roughly ten times faster than it was released during the PETM. This means oceans and ecosystems have even less time to adjust.

The event also shows how long recovery takes. Natural processes, especially the slow weathering of rocks and burial of organic carbon, needed well over 100,000 years to draw the excess carbon back out of the atmosphere. Research on the PETM has informed the work of bodies such as the Intergovernmental Panel on Climate Change.

Why it matters

The PETM is the clearest natural experiment in what happens when a large amount of carbon enters the climate system quickly. It reshaped ocean life, helped spread the ancestors of many modern mammals and offers a long-term perspective on the consequences of warming that scientists cannot gain from modern records alone.