There is a cautionary tale buried in Earth’s past. Some 56 million years ago, about 10 million years after the dinosaurs went extinct, a massive amount of carbon surged into the atmosphere, triggering a rise in temperature of 5°C. Scientists often look to the so-called Paleocene-Eocene thermal maximum (PETM) as an analog for today’s rising temperatures, because the magnitude of that ancient carbon injection is thought to be comparable to what humans will release if fossil fuel emissions continue unabated for a few more centuries. But just how quickly the injection happened has been debated. Now, a new study suggests that the carbon was vented over 4000 years—making the overall carbon injection rate about 10 times slower than today’s rate. That’s worrisome, the study’s authors say, because the emission rate matters, particularly when it comes to the adaptive response of plants and animals.
“Ecosystems need time to adjust,” says Richard Zeebe, an oceanographer from the University of Hawaii, Manoa, who led the study. “We’re doing it faster and most likely the consequences are going to be more severe.”
To estimate the rate of those ancient carbon inputs, Zeebe and his colleagues used data from a 24-centimeter piece of sediment core retrieved from a borehole in New Jersey. The section, a mixture of sands, clays, and silts that were deposited in a shallow offshore sea, is thought to span the PETM carbon injection period. But traditional radiometric dating techniques aren’t precise enough to determine the exact ages of the sediments and therefore how fast carbon was being released.
So the researchers developed an alternative way to estimate that input rate, based on measurements of carbon and oxygen isotopes at dozens of different spots containing carbonate within the sediment core. Carbon has two stable (nonradiogenic) isotopes: abundant carbon-12 atoms and trace amounts of carbon-13. Living things have a preference for the lighter carbon-12, and so carbon emissions from organic sources like fossil fuels are relatively depleted in carbon-13. Similarly, ocean temperatures modulate which oxygen isotope—lighter oxygen-16 or heavier oxygen-18—creatures will use to build their carbonate shells. Ultimately, the carbon isotopes in the sediments point to organic carbon inputs that lead to increasing atmospheric temperatures, whereas the oxygen isotopes are a proxy for changing ocean temperatures.
Carbon added to the atmosphere doesn’t immediately translate to warmer temperatures in the ocean; there is a time-lag of hundreds of years between the injection of atmospheric carbon and the later warming of the ocean. But Zeebe and his colleagues didn’t find such a lag between the carbon and the oxygen isotopes. The isotopes moved in sync with each other, suggesting that enough time had passed between each isotope measurement for the carbon injection and ocean warming to be in equilibrium. Based on climate and carbon cycle modeling, they arrive at an estimate of 4000 years, the team reports online today in Nature Geoscience. “These two systems go hand in hand,” Zeebe says. “You can only do this if you stretch the carbon release over a relatively long period of time.”
Those findings are in contrast to an earlier study that relied on the same data from the same core, but came to a far more radical conclusion. That study, published in 2013 in the Proceedings of the National Academy of Sciences (PNAS), suggested that the PETM carbon was injected in just 13 years—more or less all at once. The PNAS study’s authors found layers in the sediments that they interpreted as seasonal. But Zeebe and others dismiss the layers as artifacts created by the drilling process.
The leader of the PNAS study, paleo-oceanographer James Wright of Rutgers University, New Brunswick, in New Jersey, acknowledges the criticism about the rhythmic layers. But Wright says the new study has problems, too. The core sediments originally came from a shallow water site where ocean temperatures would have responded to atmospheric warming from carbon inputs almost right away, he says. “There’s nothing in the data right now that rejects an instantaneous input.”
Other PETM experts say the new study’s 4000-year injection period sits well with previous estimates. Jerry Dickens, an oceanographer at Rice University in Houston, Texas, says the work reinforces the idea that the PETM “is not a fast event relative to what we’re doing as humans.” But he cautions that the new study’s model contains an inherent assumption: that temperature rose in response to an input of carbon. Dickens says many scientists think it was the opposite. “Temperature is driving the carbon input, not the other way around,” he says.
Gabriel Bowen, a paleoclimatologist at the University of Utah in Salt Lake City, says the new study offers strong evidence that the PETM was not an instantaneous event. But he has concerns with the way that the new model assumes that carbon and oxygen isotopes should move together so precisely, when so many things could cause them to vary year to year: phytoplankton blooms, local oceanographic conditions, or the injection of freshwater.
A lingering question is what caused the massive injection of carbon in the first place. Scientists have struggled to find possible culprits that could have produced sufficient carbon. Because of his interpretation of the New Jersey core, Wright believes it was a sudden event, and has suggested a comet impact could do the trick. Comets contain lots of carbon, not to mention the rock vaporized by the comet. “Most people think we’re nuts, and maybe we are,” he says. But, as he announced to colleagues at a workshop in Arizona in January, he has recently found glassy spherules in three different sediment cores in the layers corresponding in time to the beginning of the PETM carbon injection. These spherules, he says, represent possible evidence of a comet impact at that time.
However, most scientists believe that a cascade of events in the PETM caused the carbon injection. It may have begun with bouts of volcanism in the oceans, which would have heated up organic carbon in surrounding rocks that then wound up in the atmosphere. Rising atmospheric temperatures would trigger other processes, such as the release of methane buried in the sea floor or carbon trapped in permafrost. The idea that warming temperatures in the coming centuries could trigger new sources of carbon to burst onto the scene, Dickens says, is “fascinating and troublesome.”