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Researchers trace carbon dioxide levels to prehistoric times

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CO2 levels now higher than any time in the last 23 million years

One of the most pressing messages that climate scientists attempt to convey to the public is how today’s CO2 levels compare to those of the Geologic past. Such comparisons can provide public context for current CO2 rise, as well as important information on the response of global temperatures to rising CO2. A new study published in Geology suggests that present-day CO2 levels (412 ppmv) are now likely higher than at any time in at least the last 23 million years!

In this newly published study, a team led by Brian Schubert, Associate Professor in the School of Geosciences at the University of Louisiana at Lafayette, used the remains of dead plants to produce a new record of atmospheric CO2 that spans 23 million years of uninterrupted Earth history. Their findings relied on the nearly continuous record of terrestrial photosynthesis provided by organic matter accumulated from partially decomposed plants.

“When plants grow, the relative amount of the two stable isotopes of carbon, carbon-12 and carbon-13, changes in response to the amount of CO2 in the atmosphere,” says Schubert. “One can therefore measure the relative amount of these two isotopes and calculate the CO2 concentration under which the plants grew.”

The remains of land plants can be used to calculate the amount of CO2 in Earth’s atmosphere. Photo credit: A. Hope Jahren

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A new way to measure past levels of carbon dioxide in the Earth’s atmosphere may help scientists better understand how climate has changed over time.

It may also help them better predict how anticipated increases in carbon dioxide will affect climate.

“Our new method uses organic matter from dead terrestrial plants, similar to the coal we burn for energy, that is preserved in the geologic record as fossils, soils, and sediments,” said Dr. Brian Schubert, an assistant professor in the University of Louisiana at Lafayette’s School of Geosciences.

Findings of research conducted by Schubert and Dr. A. Hope Jahren, a professor at the University of Hawaii at Manoa, are published in the May issue of  “Geology,” one of the most respected journals in that field.

Until now, scientists traced levels of carbon dioxide by studying ice-core records from Antarctica that date back 800,000 years.

But Schubert said the new method enables scientists to track carbon dioxide levels for more than 400 million years.

The key is what he describes as the “relative abundance” of two isotopes of carbon – carbon-12 and carbon-13 – that are used by plants during photosynthesis, which converts carbon dioxide and water into organic compounds. Those isotopes are affected by the carbon dioxide level in the atmosphere at the time plants are growing.

“Using this technique, we can better quantify carbon dioxide levels ever since the first forested ecosystems appeared on Earth and determine carbon dioxide levels during major mass extinction events, such as the extinction of dinosaurs,” he said.

Caption: Dr. Brian Schubert examines plant fossils under a microscope in the stable isotope laboratory at UL Lafayette. Measurement of carbon isotopes in fossils such as these can be used to determine past levels of carbon dioxide in the Earth’s atmosphere.