August 9, 2013 | Levels of carbon dioxide (CO₂) in the atmosphere are increasingly fluctuating between seasons, a new study concludes. The research, based on a multi-year airborne survey of atmospheric chemistry called HIAPER Pole-to-Pole Observations (HIPPO), may indicate widespread changes in northern ecosystems.

CO₂ levels in the atmosphere naturally rise and fall over the course of a year, with plants taking up more of the greenhouse gas as they grow during the spring and summer and releasing it as leaves die in the fall and winter.

But the new study shows that the range is increasing significantly as more CO₂ is emitted from the burning of fossil fuels and other human activities.

The international study—led by the Scripps Institution of Oceanography, University of California San Diego, with co-authors including National Center for Atmospheric Research (NCAR) scientist Britton Stephens—relied on observations of atmospheric CO₂ made by the NSF/NCAR Gulfstream V research jet (HIAPER) aircraft at altitudes between 3 and 6 kilometers (10,000-20,000 feet). The researchers also used measurements from a network of aircraft sampling managed by the National Oceanic and Atmospheric Administration (NOAA).

The amplitude of atmospheric CO₂ increased by roughly 50 percent across high latitude regions north of 45° N, in comparison to previous aircraft observations from the late 1950s and early 1960s.

This means that more carbon is accumulating in forests and other vegetation in the Northern Hemisphere during the summer, and more carbon is being released in the fall and winter, said study lead author Heather Graven, a postdoctoral researcher at Scripps. While it is not yet understood why the seasonal amplitude has increased, it may be associated in part with forests expanding or seasons changing as temperatures warm.

“This study documents a profound shift in seasonal fluctuations that are tied to photosynthesis and respiration on land,” Stephens says. “I anticipate great interest in the broader research community to explain its cause and to translate that new knowledge into better predictions of climate change and its impacts.”

The study, “Enhanced seasonal exchange of CO₂ by northern ecosystems since 1960,” appeared this week in Science Express and will be in print editions of the journal Science on Aug. 30. It was funded by the National Science Foundation, NOAA, the Department of Energy, and the Office of Naval Research.

For more, see the Scripps news release.

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