Past Global Warming Event Comparable To Today’s Climate Conditions

Modern climate change is not exactly unprecedented, according to a new study published in Nature Geoscience, which found similarities between current conditions and the rate at which carbon emissions warmed the planet’s climate nearly 56 million years ago.
However, University of Utah geochemist Gabe Bowen and his colleagues report that the past event, known as the Paleocene-Eocene thermal maximum (PETM), involved not one but two pulses of carbon to the atmosphere. While the Earth and most of its species were able to survive the PETM, it took millennia to fully recover from the episode in which temperatures increased by 5 to 8 degrees Celsius (9 to 15 degrees Fahrenheit).
“There is a positive note in that the world persisted, it did not go down in flames, it has a way of self-correcting and righting itself,” Bowen, lead author of the new study and a member of the university’s Global Change and Sustainability Center, said in a statement Monday. “However, in this event it took almost 200,000 years before things got back to normal.”
He and colleagues from the University of Colorado, the University of Bremen, the University of Michigan, the Smithsonian Institution and the University of New Hampshire report that carbonate or limestone nodules in Wyoming sediment cores show that the PETM global warming episode involved the average annual release of a minimum of 0.9 petagrams (1.98 trillion pounds) of carbon to the atmosphere, likely over shorter periods of time.
That is “within an order of magnitude of, and may have approached, the 9.5 petagrams [20.9 trillion pounds] per year associated with modern anthropogenic carbon emissions,” Bowen’s team reported in their paper. They added that, since the start of the 20th century, the human burning of fossil fuels emitted an average of 3 petagrams per year (even closer to the PETM levels), with each pulse of carbon emissions lasted a maximum of 1,500 years.
“Previous conflicting evidence indicated the carbon release lasted anywhere from less than a year to tens of thousands of years,” the National Science Foundation (NSF), which helped fund the study, explained. “The new research shows atmospheric carbon levels returned to normal within a few thousand years after the first pulse, probably as carbon dissolved in the ocean. It took up to 200,000 years for conditions to normalize after the second pulse.”
The new study also determined it was unlikely that some theorized causes of the warming episode, such as an asteroid impact, slow melting of permafrost, drying out of a major waterway or the burning of organic-rich soil, were actually responsible for the event. Instead, the authors suggest that melting of seafloor methane ices known as clathrates or volcanic activity heating organic-rich rocks and releasing methane were more likely causes.
“The Paleocene-Eocene thermal maximum has stood out as a striking, but contested, example of how 21st-century-style atmospheric carbon dioxide buildup can affect climate, environments and ecosystems worldwide,” Bowen said. “This new study tightens the link. Carbon release back then looked a lot like human fossil-fuel emissions today, so we might learn a lot about the future from changes in climate, plants, and animal communities 55.5 million years ago.”
However, he warned that the global climate was already far warmer than modern condition at the start of the PETM than it is today, and there were no icecaps, meaning that past and present conditions were not identical. Even so, co-author and Smithsonian Institution paleobiologist Scott Wing said that the research “gives us the best idea yet of how quickly this vast amount of carbon was released at the beginning” of the PETM global warming event.
“The answer is just a few thousands of years or less. That’s important because it means the ancient event happened at a rate more like human-caused global warming than we ever realized,” Wing noted. Previous research had indicated that there was “enhanced storminess” in some regions and “increased aridity” in others during the PETM, Bowen added, with only “a little bit of extinction” in some single-celled, deep-sea organisms called foraminifera.
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