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Edited by Mark H. Thiemens, University of California at San Diego, La Jolla, CA, and approved November 17, 2015 (received for review August 12, 2015)
Significance
Arctic ecosystems are major global sources of methane. We report that emissions during the cold season (September to May) contribute ≥50% of annual sources of methane from Alaskan tundra, based on fluxes obtained from eddy covariance sites and from regional fluxes calculated from aircraft data. The largest emissions were observed at the driest site (<5% inundation). Emissions of methane in the cold season are linked to the extended “zero curtain” period, where soil temperatures are poised near 0 °C, indicating that total emissions are very sensitive to soil climate and related factors, such as snow depth. The dominance of late season emissions, sensitivity to soil conditions, and importance of dry tundra are not currently simulated in most global climate models.
Abstract
Arctic terrestrial ecosystems are major global sources of methane (CH4); hence, it is important to understand the seasonal and climatic controls on CH4 emissions from these systems. Here, we report year-round CH4 emissions from Alaskan Arctic tundra eddy flux sites and regional fluxes derived from aircraft data. We find that emissions during the cold season (September to May) account for ≥50% of the annual CH4 flux, with the highest emissions from noninundated upland tundra. A major fraction of cold season emissions occur during the “zero curtain” period, when subsurface soil temperatures are poised near 0 °C. The zero curtain may persist longer than the growing season, and CH4 emissions are enhanced when the duration is extended by a deep thawed layer as can occur with thick snow cover. Regional scale fluxes of CH4 derived from aircraft data demonstrate the large spatial extent of late season CH4 emissions. Scaled to the circumpolar Arctic, cold season fluxes from tundra total 12 ± 5 (95% confidence interval) Tg CH4 y−1, ∼25% of global emissions from extratropical wetlands, or ∼6% of total global wetland methane emissions. The dominance of late-season emissions, sensitivity to soil environmental conditions, and importance of dry tundra are not currently simulated in most global climate models. Because Arctic warming disproportionally impacts the cold season, our results suggest that higher cold-season CH4 emissions will result from observed and predicted increases in snow thickness, active layer depth, and soil temperature, representing important positive feedbacks on climate warming.
Footnotes
- ↵1To whom correspondence should be addressed. Email: dzona{at}mail.sdsu.edu.
↵2D.Z. and B.G. contributed equally to this work.
Author contributions: D.Z., D.A.L., and W.C.O. designed research; D.Z., D.A.L., and W.C.O. performed research; R.C., J.L., S.C.W., C.E.M., S.J.D., C.S., A.K., R.Y.-W.C., and J.M.H. supported the collection and preparation of the Carbon in Arctic Reservoirs Vulnerability Experiment data; J.D.W. and J.S.K. contributed new reagents/analytic tools; D.Z., B.G., P.C.M., J.P.G., V.M., A.L., J.D.W., J.S.K., and W.C.O. analyzed data; R.C., J.L, and S.C.W. analyzed the aircraft data; and D.Z., B.G., R.C., S.C.W., C.E.M., S.J.D., S.D., C.S., A.K., R.Y.-W.C., J.M.H., P.C.M., A.L., J.D.W., J.S.K., D.A.L., and W.C.O. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Data deposition: The data reported in this paper have been deposited in the Oak Ridge National Laboratory Distributed Active Archive Center, Oak Ridge data repository (dx.doi.org/10.3334/ORNLDAAC/1300 and dx.doi.org/10.3334/CDIAC/hippo_010).
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1516017113/-/DCSupplemental.
Freely available online through the PNAS open access option.
Cold season Arctic tundra methane fluxes
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- Earth, Atmospheric, and Planetary Sciences
- Biological Sciences
- Environmental Sciences
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