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Human carbon release rate is highest in the past 66 million years

Human carbon release rate is highest in the past 66 million years

New research published today in Nature Geoscience by Richard Zeebe, professor at the University of Hawai'i - M noa 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. Their findings suggest humans are releasing carbon about 10 times faster than during any event in the past 66 million years.

Carbon dioxide (CO2) is the primary greenhouse gas emitted through human activities. In 2013, CO2 accounted for about 82% of all U.S. greenhouse gas emissions from human activities. Since the Industrial Revolution began around 1750, human activities have contributed substantially to climate change by adding CO2 and other heat-trapping gases to the atmosphere.

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 for short, 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.

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.

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, says Candace Major, program director in the National Science Foundation (NSF)'s Division of Ocean Sciences, which funded the research.

Scientists like Zeebe 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.

Everyone is focused on what happens by 2100. But that's only two generations from today. It's like: If the world ends in 2100 we're probably OK! said Zeebe. But it's very clear that over a longer timescale there will be much bigger changes.

Human carbon release rate is highest in the past 66 million years