By studying deep sea sediments, a team of researchers has discovered that humans are releasing carbon 10 times faster than during any event in the past 66 million years.
The research by Richard Zeebe at the University of Hawai'i - Manoa School of Ocean and Earth Science and Technology (SOEST) and colleagues looks at changes of Earth's temperature and atmospheric carbon dioxide (CO2) since the end of the age of the dinosaurs.
The research team developed a new approach and was able to determine the duration of the onset of an important past climate event, the Paleocene-Eocene Thermal Maximum (PETM), 56 million years ago.
"As far as we know, the PETM has the largest carbon release during the past 66 million years," said Zeebe.
Zeebe and co-authors Andy Ridgwell (University of Bristol/ University of California) and James Zachos (University of California) combined analyses of chemical properties of PETM sediment cores with numerical simulations of Earth's climate and carbon cycle.
Their new method allows them to extract rates of change from a sediment record without the need for an actual sediment age model. Applied to the PETM, they calculated how fast the carbon was released, how fast Earth's surface warmed, and constrained the time scale of the onset, which was at least 4,000 years.
The rate of carbon release during the PETM was determined 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 CO2.
The researchers estimated the maximum sustained carbon release rate during the PETM had to be less than 4 billion metric tons of CO2 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.
The study is published in Nature Geoscience.
