A global warming event of the ancient world previews our future

Recovering from abrupt global warming took hundreds of thousands of years.
Derya Ozdemir

A study of the ancient world has revealed something remarkable about the possible future of our planet.

Roughly 56 million years ago, a large release of greenhouse gases, likely spurred by volcanic activity, caused a period of sudden global warming known as the Paleocene-Eocene Thermal Maximum (PETM). The latest findings, which were published in the journal Science Advances, reveal that there was an extra transient rise in atmospheric CO2 immediately before the PETM, which resulted in a brief period of ocean acidification and warming.

Crucially, the amount of carbon released into the atmosphere during this precursor event was roughly equal to the current cumulative carbon emissions from fossil fuel combustion and other human activities.
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Uncovering the secrets in marine sediments

"The PETM is an important geologic climate event because it is one of best comparisons to current climate change and can help inform us how the Earth System will respond to current and future warming," explained lead author Tali Babila, Postdoctoral Research Associate currently at the University of Southampton.

The new findings are based on an investigation of marine sediments deposited in shallow waters along the United States' Atlantic coast. Since sea levels were higher during the PETM, and parts of Maryland, Delaware, and New Jersey were under water at the time, the researchers chose sediment cores drilled from this location for the study.

The PETM is identified in marine sediments by a dramatic shift in carbon isotope composition and other indicators of severe changes in ocean chemistry caused by the absorption of large amounts of CO2. Also, marine sediments contain the small shells of tiny sea organisms called foraminifera that lived in the ocean's surface waters, and the chemical composition of these shells documents the environmental conditions under which they formed.

The researchers were able to rebuild a precise record of ocean acidification by analyzing the boron isotope composition of individual foraminifera using novel analytical methods developed at the University of Southampton in the United Kingdom. This was part of a suite of geochemical analyses used to recreate environmental changes during the precursor and the main PETM event.

Preceded by a brief period of warming

The researchers concluded that the precursor signal in the Maryland parts implies a global event that likely lasted a few decades, if not millennia. The two carbon pulses resulted in dramatically different mechanisms and time scales for the Earth's carbon cycle and climate system recovery, with carbon emissions during the PETM surpassing the ocean's buffering capacity. It took tens of thousands of years for Earth's climate system to recover from the more extreme PETM. 

These two events offer unique insight into how Earth's current climate might react if the use of fossil fuels isn't stopped and carbon emissions continue to climb at their current rate. The short-lived precursor event shows what could happen if current emissions are quickly reduced, whereas the far more intense global warming of the PETM depicts the effects of continuing to emit carbon into the atmosphere at the current rate.

"Whilst natural geological processes such as rock weathering and carbon burial eventually meant Earth eventually recovered from the PETM, it took hundreds of thousands of years," Babila said. "So this is further proof that urgent action is needed today to rapidly cut the amount of carbon being release into the atmosphere to avoid long-lasting effects."

Study Abstract:

The Paleocene-Eocene Thermal Maximum (PETM) is recognized by a major negative carbon isotope (δ13C) excursion (CIE) signifying an injection of isotopically light carbon into exogenic reservoirs, the mass, source, and tempo of which continue to be debated. Evidence of a transient precursor carbon release(s) has been identified in a few localities, although it remains equivocal whether there is a global signal. Here, we present foraminiferal δ13C records from a marine continental margin section, which reveal a 1.0 to 1.5‰ negative pre-onset excursion (POE), and concomitant rise in sea surface temperature of at least 2°C and a decline in ocean pH. The recovery of both δ13C and pH before the CIE onset and apparent absence of a POE in deep-sea records suggests a rapid (< ocean mixing time scales) carbon release, followed by recovery driven by deep-sea mixing. Carbon released during the POE is therefore likely more similar to ongoing anthropogenic emissions in mass and rate than the main CIE.

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