The positive net radiative greenhouse gas forcing of increasing methane emissions from a thawing boreal forest-wetland landscape

• Published in: Global Change Biology
• Main Authors: Helbig, Manuel, Chasmer, Laura E., Kljun, Natas Cha, Quinton, William L., Treat, Claire C., Sonnentag, Oliver
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2017
• Subjects: boreal forest; carbon dioxide; climate change; eddy covariance; methane; radiative forcing; wetland; Canada; Peat; permafrost; taiga; Taiga plains
• Online Access: https://pure.au.dk/portal/en/publications/2aef8bce-d875-414e-a436-7c8dd6ed6e1b; https://doi.org/10.1111/gcb.13520; http://www.scopus.com/inward/record.url?scp=84997693750&partnerID=8YFLogxK

At the southern margin of permafrost in North America, climate change causes widespread permafrost thaw. In boreal lowlands, thawing forested permafrost peat plateaus (‘forest’) lead to expansion of permafrost-free wetlands (‘wetland’). Expanding wetland area with saturated and warmer organic soils is expected to increase landscape methane (CH 4 ) emissions. Here, we quantify the thaw-induced increase in CH 4 emissions for a boreal forest-wetland landscape in the southern Taiga Plains, Canada, and evaluate its impact on net radiative forcing relative to potential long-term net carbon dioxide (CO 2 ) exchange. Using nested wetland and landscape eddy covariance net CH 4 flux measurements in combination with flux footprint modeling, we find that landscape CH 4 emissions increase with increasing wetland-to-forest ratio. Landscape CH 4 emissions are most sensitive to this ratio during peak emission periods, when wetland soils are up to 10 °C warmer than forest soils. The cumulative growing season (May–October) wetland CH 4 emission of ~13 g CH 4 m −2 is the dominating contribution to the landscape CH 4 emission of ~7 g CH 4 m −2 . In contrast, forest contributions to landscape CH 4 emissions appear to be negligible. The rapid wetland expansion of 0.26 ± 0.05% yr −1 in this region causes an estimated growing season increase of 0.034 ± 0.007 g CH 4 m −2 yr −1 in landscape CH 4 emissions. A long-term net CO 2 uptake of >200 g CO 2 m −2 yr −1 is required to offset the positive radiative forcing of increasing CH 4 emissions until the end of the 21st century as indicated by an atmospheric CH 4 and CO 2 concentration model. However, long-term apparent carbon accumulation rates in similar boreal forest-wetland landscapes and eddy covariance landscape net CO 2 flux measurements suggest a long-term net CO 2 uptake between 49 and 157 g CO 2 m −2 yr −1 . Thus, thaw-induced CH 4 emission increases likely exert a positive net radiative greenhouse gas forcing through the 21st century.