Arctic cold season methane emissions much higher than predicted

  • New study finds amount of methane escaping the Arctic tundra is higher than estimated by current climate models
  • Methane is a potent greenhouse gas which accelerates atmospheric warming

The Arctic tundra is releasing at least as much methane during its cold season as it does during the summer, a new study has found.

The findings by an international team of researchers, including scientists from the University of Sheffield, show that contrary to previous assumptions, methane gas is escaping and entering the atmosphere during the cold months, which run from September through to May, when the soil surface is frozen.

Scientists previously thought that the methane contribution during the cold season was nearly zero.

Methane is the second most prevalent greenhouse gas that intensifies atmospheric warming and is 25 times more potent per molecule than carbon dioxide over a 100-year period.

Dr Donatella Zona, from the University of Sheffield’s Department of Animal and Plant Sciences, said: "The overwhelming majority of measurements for methane emissions in the Arctic over the last decades have been performed during the two to three summer months, while the cold period accounts for nearly 80 per cent of the year.

"Virtually all the climate models assume there’s no or very little emission of methane when the ground is frozen. Our study shows that assumption is incorrect."

The study published today in the Proceeding of the National Academy of Sciences shows that methane emissions during the cold period in the Arctic could account for 50 per cent of the entire annual methane loss into the atmosphere.

The water trapped in the permafrost does not freeze completely at zero degrees Celsius. The study shows that the top layer of the ground, known as the active layer, thaws in the summer and refreezes in the winter, and it experiences a kind of sandwiching effect as it freezes.

When temperatures are right around zero degrees Celsius - the so-called "zero curtain" - the top and bottom of the active layer begin to freeze, while the middle remains insulated. Microorganisms in this unfrozen middle layer around 0C continue to break down organic matter and emit methane many months into the Arctic’s cold period each year.

To find out how much methane is emitted during the Arctic winter Dr Zona and her colleagues oversaw the upgrading of five sampling towers to operate continuously year round above the Arctic Circle in Alaska.

The researchers recorded methane emissions from these sites over two summer-fall-winter cycles between June 2013 and January 2015. A major portion of methane emissions during the cold season were observed when temperatures hovered near the zero curtain.

"This is extremely relevant for the Arctic ecosystem, as the zero curtain period continues from September until the end of December, lasting as long or longer than the entire summer season," said Dr Zona.

"These results are opposite of what modelers have been assuming, which is that all of the methane emissions occur during the warm summer months and the cold-season methane contribution is nearly zero."

The new study also found that the highest emissions of methane occur from non-inundated upland tundra - areas which until now have been assumed to be a negligible contributor to the methane budget.

Dr Zona, who also holds a position at the Department of Biology at San Diego State University in America, added: "Non-inundated sites can be high methane emitters, but we also found that these drier sites presented the highest relative contribution of cold season methane emissions of the annual methane budget."

The Arctic cold season is traditionally ignored by researchers due to the challenges faced during that time including the need for specialised instruments and the expense of travel costs.

To test whether the study’s site-specific sampling was representative of methane emissions across the Arctic, the researchers compared their results to measurements recorded during aircraft flights over the region made by NASA’s Carbon in Arctic Reservoirs Vulnerability Experiment (CARVE).

"The site-level data were well aligned with the larger-scale aircraft measurements," said Dr Zona.

Steve Wofsy from Harvard University, one of the authors of the study, added: "Now that we know how important the winter is to the methane budget, we are working to determine the long-term trends in greenhouse emissions from tundra and their sensitivity to winter warming."

Additional information

For a copy of the research paper email clare.parkin [a] (p) uk

The University of Sheffield; San Diego State University; NASA JPL; Harvard University; the University of Montana; the Italian National Research Council (CNR), Biometeorology Institute; the University of Helsinki; the University of Colorado, Boulder; Atmospheric and Environmental Research, Lexington; the University of Alaska, Fairbanks; Dalhousie University; and Open University in Milton Keynes all contributed to the study.

The research was funded by the Office of Polar Programs of the National Science Foundation (NSF).

University of Sheffield
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