The rate of carbon emissions during a period of abrupt warming 56 million years ago was ten times slower than current emissions from the burning of fossil fuels and other human activities, according to a new study published in Nature Geoscience. The study shows that the current rate at which carbon is being released into the atmosphere is unprecedented in at least the past 66 million years.
Evidence found in sediment cores from the seafloor shows that 56 million years ago, massive emissions of greenhouse gases, possibly triggered by volcanism, caused a global warming of at least 5 degrees Celsius. Climate scientists have considered this episode, known as the Paleocene-Eocene Thermal Maximum (PETM), the closest analogue to current climate change. But the actual magnitude and rate of carbon emissions during the PETM have been difficult to determine.
"The challenge we've faced is constraining the rate of the carbon emission and warming, which is limited by the fidelity of the sediment records," said coauthor James Zachos, professor of Earth and planetary sciences at UC Santa Cruz.
New approach
For the new study, Zachos, first author Richard Zeebe of the University of Hawaii, Manoa, and coauthor Andy Ridgwell of the University of Bristol, U.K., developed a new approach that enables them to extract rates of change from a sediment record. Carbon release and climate change are recorded by different chemical signatures in the sediments, and the lag between the carbon release and the climate response reflects the rate of change.
The researchers compared the timing of the onset of climate change in the PETM with the timing of carbon emissions as recorded by marine sediments, and found that they occurred at essentially the same time. Using a climate and carbon cycle model, they showed that such a near-concurrent change suggests the PETM carbon emissions occurred over a period of at least 4,000 years.
"With a fast rate of forcing, there's a thermal lag in warming the oceans, in particular, because they are cold and large," Zachos explained. "If there's no observable lag, the rate of change could not have been any faster than a few thousand years."
The rate of carbon release during the PETM was found to be much smaller than the current input of carbon to the atmosphere from human activities. Carbon release rates from human sources reached a record high in 2014 of about 37 billion metric tons of carbon dioxide. The researchers estimated the maximum sustained carbon release rate during the PETM had to be less than 4 billion metric tons of carbon dioxide per year, about one-tenth the current rate.
"Because our carbon release rate is unprecedented over such a long time period in Earth's history, it also means that we have effectively entered a 'no-analogue' state. This represents a big challenge for projecting future climate changes because we have no good comparison from the past," said Zeebe.
Whereas large climate transitions in the past may have been relatively smooth, there is no guarantee for the future. The climate system is non-linear, which means its response to a forcing (such as carbon dioxide emissions) is a complex process involving a whole suite of components.
The findings have serious implications for the ability of modern ecosystems to adapt to climate change. In a commentary accompanying the paper in Nature Geoscience, Peter Stassen wrote that during the PETM, "marine ecosystems may have had sufficient time to adapt to environmental changes through migration or evolution. It therefore remains possible that the current rates of change might exceed the adaptive capacity of modern marine ecosystems and their constituents."
