Professor Richard Zeebe (Hawaii) and co-authors Professor Andy Ridgwell (Bristol) and Professor James Zachos (California) looked at changes in the Earth’s temperature and atmospheric carbon dioxide (CO2) since the end of the age of the dinosaurs.
They were able to determine the duration of the onset of an important past climate event, the Paleocene-Eocene Thermal Maximum (PETM), 56 million years ago.
The researchers combined analyses of chemical properties of PETM sediment cores with numerical simulations of Earth’s climate and carbon cycle. Their new method allowed 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 the 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.
Professor Zeebe said: “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.”
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 our CO2 emissions) is a complex process involving a whole suite of components.
Professor Zeebe said: “If you kick a system very fast, it usually responds differently than if you nudge it slowly but steadily. Also, it is rather likely that future disruptions of ecosystems will exceed the relatively limited extinctions observed at the PETM.”
“In studying one of the most dramatic episodes of global change since the end of the age of the dinosaurs, these scientists show that we are currently in uncharted territory in the rate carbon is being released into the atmosphere and oceans,” said Candace Major, program director in the National Science Foundation (NSF)’s Division of Ocean Sciences, which funded the research.
Scientists also study the PETM to better understand long-term changes in Earth’s future climate. Most of the current climate debate concentrates only on this century but the PETM suggests that the consequences of our massive fossil fuel burning will have a much, much longer tail.
The team continue their work on the PETM to study other aspects of the event, for example, determining how severe ocean acidification was during the PETM and what impact it had on calcifying organisms in the ocean. This may provide insight about what to expect in the future as Earth’s climate continues to warm and oceans keep acidifying.
Paper
‘Anthropogenic carbon release rate unprecedented during the past 66 million years’ by Richard E. Zeebe, Andy Ridgwell and James C. Zachos in Nature Geoscience
Funding for this research was provided by the U.S. National Science Foundation and the European Union.