ANN ARBOR—The rate at which carbon emissions warmed Earth's climate almost 56 million years ago resembles modern, human-caused global warming much more than previously believed but involved two pulses of carbon to the atmosphere, researchers at the University of Utah, the University of Michigan and three other universities found.
The findings mean the so-called Paleocene-Eocene thermal maximum, or PETM, can provide clues to the future of modern climate change. The good news: Earth and most species survived. The bad news: It took millennia to recover from the episode, when temperatures rose by 9-to-15 degrees Fahrenheit (5-to-8 degrees Celsius).
"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," said University of Utah geochemist Gabe Bowen, lead author of the study published today in the journal Nature Geoscience. "However, in this event it took almost 200,000 years before things got back to normal."
U-M's Philip Gingerich is a co-author of the paper and a principal investigator of the National Science Foundation grant sponsoring the Wyoming coring project that led to the paper. First author Bowen was an undergraduate in the U-M Department of Earth and Environmental Sciences. Another of the paper's authors, William Clyde of the University of New Hampshire, received his doctorate under Gingerich at U-M. Two of Gingerich's sons, now U-M undergraduates, worked on the project as well.
"It is surprising to find that the onset of the PETM is characterized by not one but two successive carbon release events, separated by a recovery to background levels," said Gingerich, a professor of earth and environmental sciences, ecology and evolutionary biology, and anthropology, and a curator at the U-M Museum of Paleontology.
"This two-step onset is a working hypothesis based on evidence not seen elsewhere that will now be tested further and probed to understand its implications for the cause of PETM greenhouse warming."
Bowen, Gingerich and their colleagues report that carbonate or limestone nodules in Wyoming sediment cores show the global warming episode 55.5 million-to-55.3 million years ago involved the average annual release of a minimum of 0.9 petagrams (1.98 trillion pounds) of carbon to the atmosphere, and probably much more over shorter periods.
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," the researchers wrote. Since 1900, human burning of fossil fuels emitted an average of 3 petagrams per year—even closer to the rate 55.5 million years ago.
Each pulse of carbon emissions lasted no more than 1,500 years. Previous conflicting evidence indicated the carbon release lasted anywhere from less than a year to tens of thousands of years. 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 ruled as unlikely some theorized causes of the warming episode, including an asteroid impact, slow melting of permafrost, burning of organic-rich soil or drying out of a major seaway. Instead, the findings suggest, in terms of timing, that more likely causes included melting of seafloor methane ices known as clathrates, or volcanism heating organic-rich rocks and releasing methane.
"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," said Bowen, an associate professor of geology and geophysics at the University of Utah.
"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."
Bowen cautioned, however, that global climate already was much warmer than today's when the Paleocene-Eocene warming began, and there were no ice caps, "so this played out on a different playing field than what we have today."
The Paleocene-Eocene thermal maximum was first recognized by its effect on the evolution of life documented in the fossil record. On land, the PETM is a faunal change event marking the first appearance of the earliest ancestors of cows, sheep, horses and primates in the fossil record.
Gingerich's U-M research team was responsible for this discovery, starting in the 1970s and early 1980s, working on the Polecat Bench Paleocene-Eocene stratigraphic section in northwestern Wyoming—the same stratigraphic section analyzed in great detail in the new Nature Geoscience paper.
At around the same time, Ellen Thomas at Yale University documented an important benthic microfossil extinction event in the world's oceans. The two sections, continental and marine, were tied together in a paper by Paul Koch, James Zachos and Gingerich in a widely cited study published in Nature in 1992.
Gingerich continues to study details of the Paleocene-Eocene faunal transition at Polecat Bench as a way to understand implications of present-day greenhouse warming.
Scott Wing, co-author of the Nature Geoscience paper and a paleobiologist at the Smithsonian Institution, said the study "gives us the best idea yet of how quickly this vast amount of carbon was released at the beginning of the global warming event we call the Paleocene-Eocene thermal maximum. 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."
Bowen, Gingerich, Wing and Clyde conducted the study with University of Utah geology and geophysics master's graduate Bianca Maibauer and technician Amy Steimke; Mary Kraus of the University of Colorado; and Ursula Rohl and Thomas Westerhold of the University of Bremen, Germany. The study was funded by the National Science Foundation and the German Research Foundation.
- Read the paper at: http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2316.html