FLUXNET-CH4 synthesis activity: Objectives, observations, and future directions

This paper describes the formation of, and initial results for, a new FLUXNET coordination network for ecosystem-scale methane (CH₄) measurements at 60 sites globally, organized by the Global Carbon Project in partnership with other initiatives and regional flux tower networks. The objectives of the effort are presented along with an overview of the coverage of eddy covariance (EC) CH₄ flux measurements globally, initial results comparing CH₄ fluxes across the sites, and future research directions and needs. Annual estimates of net CH₄ fluxes across sites ranged from −0.2 ± 0.02 g C m^–2 yr^–1 for an upland forest site to 114.9 ± 13.4 g C m^–2 yr^–1 for an estuarine freshwater marsh, with fluxes exceeding 40 g C m^–2 yr^–1 at multiple sites. Average annual soil and air temperatures were found to be the strongest predictor of annual CH₄ flux across wetland sites globally. Water table position was positively correlated with annual CH₄ emissions, although only for wetland sites that were not consistently inundated throughout the year. The ratio of annual CH₄ fluxes to ecosystem respiration increased significantly with mean site temperature. Uncertainties in annual CH₄ estimates due to gap-filling and random errors were on average ±1.6 g C m^–2 yr^–1 at 95% confidence, with the relative error decreasing exponentially with increasing flux magnitude across sites. Through the analysis and synthesis of a growing EC CH₄ flux database, the controls on ecosystem CH₄ fluxes can be better understood, used to inform and validate Earth system models, and reconcile differences between land surface model- and atmospheric-based estimates of CH₄ emissions.