Scientists create road map for improving carbon estimates

Earth’s terrestrial ecosystems remove about one quarter of the carbon dioxide emitted into the atmosphere by fossil fuel burning, but much remains unknown about this important process that serves as a buffer against rising levels of heat-trapping gases, according to scientists.

Penn State researchers found that a common tool used to understand carbon dioxide fluxes, or how the gas moves between the atmosphere and ecosystems, may be overconfident because of uncertainties in the release of carbon dioxide by the combustion of fossil fuels.

“The uncertainty in fossil fuel emissions is a key factor in the uncertainty surrounding terrestrial flux estimates,” said Sha Feng, assistant research professor in the Department of Meteorology and Atmospheric Science at Penn State and lead author on the study. “When we use atmospheric inversions to estimate fluxes, we assume we know fossil fuel emissions very well. This research challenges that assumption.”

Atmospheric inversions models are a tool that allow scientists to use atmospheric carbon dioxide concentrations to estimate an ecosystems’ carbon sink, or how much carbon things like trees and other organic matter remove from the atmosphere and store.

This approach is an important tool — it is the primary reason we know that ecosystems are removing carbon dioxide from the atmosphere — but uncertainties in its estimates can lead to divergent projections of future climate and mitigation strategies, according to the researchers.

This led the Penn State team to develop the first ensemble-based model to quantify uncertainties at a continental scale to inform inversion models. They reported their findings in the journal Geophysical Researcher Letters.

“We found uncertainty in fossil fuel emissions actually causes a significant amount of variability in what we would expect to measure in the atmosphere at any place in time across North America,” said Ken Davis, professor of atmospheric and climate science at Penn State. “And that was a surprise.”

The researchers said the findings do not mean we lack an understanding about how much carbon dioxide in total enters the atmosphere from the burning of fossil fuels.

“You shouldn’t conclude that we don’t know how much coal we burn,” Davis said. “But if I make a measurement over North Carolina in the middle of the summer or in the middle of winter in a certain weather system, could my modeled CO2 be different from my measurements? And that answer is yes.”

Uncertainties in atmospheric transport and continental boundary inflow also impact inversions, the researchers found.

The study offers targeted sampling and analysis strategies to reduce these uncertainties. Additional observations around the United States East Coast, for example, could address fossil fuel emission uncertainties, the scientists said.

This study was funded through a five-year, $30 million mission led by Penn State to improve the quantification of present-day carbon-related greenhouse gas sources and sinks.

The Atmospheric Carbon and Transport — America (ACT-America) project recently concluded field campaigns that included flights to measure atmospheric concentrations of carbon dioxide, methane and other gases and atmospheric properties within weather systems across the eastern U.S.

Klaus Keller, professor of geosciences, Penn State, participated in this study.

Also collaborating were Thomas Lauvaux, research scientist at the French National Center for Scientific Research, Peter Rayner, professor at the University of Melbourne, Tomohiro Oda, research scientist at the NASA Goddard Space Flight Center and Kevin Gurney, professor at Northern Arizona University.

NASA funded the research through the ACT-America program.

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