Permafrost Region Greenhouse Gas Budgets Suggest a Weak CO<sub>2</sub> Sink and CH<sub>4</sub> and N<sub>2</sub>O Sources, But Magnitudes Differ Between Top‐Down and Bottom‐Up Methods
Gustaf Hugelius, Justine Ramage, Eleanor Burke, Abhishek Chatterjee, T. Luke Smallman, Tuula Aalto, Ana Bastos, Christina Biasi, Josep G. Canadell, Naveen Chandra, Frédéric Chevallier, Philippe Ciais, Jinfeng Chang, Liang Feng, Matthew W. Jones, Thomas Kleinen, McKenzie A. Kuhn, Ronny Lauerwald, Junjie Liu, Efrèn López‐Blanco, Ingrid T. Luijkx, Maija E. Marushchak, Susan M. Natali, Yosuke Niwa, David Olefeldt, Paul I. Palmer, Prabir K. Patra, Wouter Peters, Stefano Potter, Benjamin Poulter, Brendan M. Rogers, W. J. Riley, Marielle Saunois, Edward A. G. Schuur, Rona L. Thompson, Claire C. Treat, Aki Tsuruta, Merritt R. Turetsky, Anna- Maria Virkkala, Carolina Voigt, Jennifer D. Watts, Qing Zhu, Bo Zheng
Abstract
Abstract Large stocks of soil carbon (C) and nitrogen (N) in northern permafrost soils are vulnerable to remobilization under climate change. However, there are large uncertainties in present‐day greenhouse gas (GHG) budgets. We compare bottom‐up (data‐driven upscaling and process‐based models) and top‐down (atmospheric inversion models) budgets of carbon dioxide (CO 2 ), methane (CH 4 ) and nitrous oxide (N 2 O) as well as lateral fluxes of C and N across the region over 2000–2020. Bottom‐up approaches estimate higher land‐to‐atmosphere fluxes for all GHGs. Both bottom‐up and top‐down approaches show a sink of CO 2 in natural ecosystems (bottom‐up: −29 (−709, 455), top‐down: −587 (−862, −312) Tg CO 2 ‐C yr −1 ) and sources of CH 4 (bottom‐up: 38 (22, 53), top‐down: 15 (11, 18) Tg CH 4 ‐C yr −1 ) and N 2 O (bottom‐up: 0.7 (0.1, 1.3), top‐down: 0.09 (−0.19, 0.37) Tg N 2 O‐N yr −1 ). The combined global warming potential of all three gases (GWP‐100) cannot be distinguished from neutral. Over shorter timescales (GWP‐20), the region is a net GHG source because CH 4 dominates the total forcing. The net CO 2 sink in Boreal forests and wetlands is largely offset by fires and inland water CO 2 emissions as well as CH 4 emissions from wetlands and inland waters, with a smaller contribution from N 2 O emissions. Priorities for future research include the representation of inland waters in process‐based models and the compilation of process‐model ensembles for CH 4 and N 2 O. Discrepancies between bottom‐up and top‐down methods call for analyses of how prior flux ensembles impact inversion budgets, more and well‐distributed in situ GHG measurements and improved resolution in upscaling techniques.