The Paleocene-Eocene Transition: When Earth's Climate Suddenly Changed

Unprecedented global warming

56 million years ago, Earth experienced one of its greatest climate shocks. The Paleocene-Eocene Thermal Maximum (PETM) saw global temperatures rise by 5 to 8 °C in less than 20,000 years. This warming, extremely rapid on a geological scale, profoundly altered carbon cycles, ocean currents, and biodiversity. Marine sediments from this era bear witness to massive ocean acidification, making life difficult for organisms with calcareous shells, such as foraminifera (unicellular marine organisms whose shell is an excellent climate indicator).

Causes of the climatic tipping point

The exact cause of the PETM is still debated, but the most accepted scenario involves a sudden and massive release of carbon into the Earth system. Several sources are considered:

• Volcanism of the North Atlantic Igneous Province: The opening of the Atlantic Ocean caused colossal volcanic eruptions, burning vast coal and oil shale deposits, thereby releasing phenomenal amounts of \(CO_2\) and methane.
• Melting of methane hydrates: The initial warming, possibly of volcanic origin, could have destabilized methane hydrates (solid compounds of water and methane, stable at high pressure and low temperature) present in seafloor sediments, in turn releasing this potent greenhouse gas.
• Permafrost thawing: Warming could also have thawed vast areas of permafrost (permanently frozen ground), releasing methane and \(CO_2\) trapped for millennia.

Biological and environmental consequences

The effects of the PETM were catastrophic for many species, but also acted as a selective force and driver of diversification for others.

In the oceans

The massive absorption of \(CO_2\) by the oceans caused unprecedented acidification, dissolving the shells of planktonic organisms. Benthic foraminifera, living in the sediments, suffered a mass extinction, with disappearance rates reaching 50%. This disappearance was accompanied by anoxia (oxygen depletion) in deep waters, creating widespread "dead zones."

On the continents

On land, warming allowed mammals to migrate to higher latitudes. It was at this time that the first primates and ungulates appeared in Europe and North America. A phenomenon of dwarfism was also observed in some mammals, with body size decreasing in response to heat stress and lower nutrient availability.

The PETM: a warning for the future

Studying the PETM is crucial because it constitutes the best geological analogue for current climate change. The observed consequences (ocean acidification, mass extinction, ecosystem disruption) offer a glimpse into what a world warmer by several degrees might look like. But this analogue has its limits, as shown in the next chapter.

A lesson on climate thresholds

The PETM showed that the Earth system can abruptly shift from one climate state to another once a certain warming threshold is crossed. The most recent estimates place the amount of carbon accumulated during the initial phase of the PETM between 2,500 and 4,500 gigatons, a magnitude that our fossil fuel reserves could match or exceed over the coming centuries. Understanding this threshold is essential for anticipating tipping points in the climate system, such as permafrost thawing or ice sheet melting.

An unprecedented emission rate: the PETM compared to current warming

While the PETM remains the best natural analogue for modern climate change, an important nuance applies: the rate at which carbon is released today is incomparable to that of the PETM. A landmark study by Richard Zeebe, Andy Ridgwell, and James Zachos, published in 2016 in Nature Geoscience, reconstructed the carbon release rate at the onset of the PETM from sediments on the New Jersey continental shelf. Their results show that the initial carbon injection lasted at least 4,000 years, with a maximum sustained rate of less than approximately 1.1 gigatons of carbon per year.

A rate ten times faster today

However, human carbon emissions reached approximately 10 gigatons per year by the mid-2010s and continue to rise. In other words, humanity is currently releasing carbon into the atmosphere at a rate about ten times faster than during the most intense warming episode known in the last 66 million years. The study's authors speak of a "no-analogue state": no event in the geological record allows us to predict with certainty how the climate system will respond to such a rapid perturbation. This difference in rate is crucial, because a system pushed abruptly can react more sharply and less predictably than one subjected to a slow perturbation, even if the total amount of carbon involved is of the same order of magnitude.

Returning to balance: the slow healing of the Earth system

Another valuable lesson from the PETM: the rapidity with which excess carbon can be injected into the climate system contrasts sharply with the slowness of its removal. Once the warming peak was reached, Earth's climate took approximately 150,000 to 200,000 years to return to conditions close to those before the PETM. This recovery occurred primarily through the weathering of continental silicates, a slow geochemical process that consumes atmospheric \(CO_2\) over hundreds of thousands of years, as well as through the gradual burial of organic carbon in marine sediments.

Slow recovery of the oceans

Ocean sediment cores also show that after the peak of acidification, the carbonate compensation depth (the limit below which calcareous shells dissolve in the ocean) experienced a temporary overshoot before gradually stabilizing, indicating that ocean chemistry took a very long time to return to a stable equilibrium. This lag between the speed of the perturbation (a few millennia) and that of the recovery (several hundred thousand years) is one of the most important messages the PETM sends to the present: even if carbon emissions were to stop quickly today, they would commit Earth's climate to timescales far exceeding those of human societies.

Key figures in the discovery

Understanding the PETM is the result of work by numerous paleoclimatologists and geochemists. Among them, James Zachos from the University of California, Santa Cruz, whose work on carbon and oxygen isotopes was fundamental in quantifying the extent of warming and acidification. Philip Gingerich (born 1946) from the University of Michigan provided major evidence on mammal migrations and evolution during this event. More recently, Richard Zeebe from the University of Hawaii at Mānoa highlighted the unprecedented nature of the current carbon emission rate compared to the PETM.