Assessing Bottom-Up and Top-Down Methane Emission Estimates in Northern High Latitude Regions (2018–2021)

The northern high latitudes (NHLs) are undergoing rapid environmental changes with global warming, which may trigger feedback mechanisms that amplify natural methane emissions from wetlands and increase contributions from wildfires. Studying year-to-year variations in these emissions can provide understanding of the key factors driving natural methane fluxes. In addition, the NHLs produce substantial methane emissions from fossil fuel production. However, the spatial heterogeneity and overlap of methane sources in the region complicates the attribution of emissions to specific sources. 

This study presents an intercomparison of methane emissions estimates across four NHL regions—Russia, Canada, Alaska, and Norway-Sweden-Finland—between 2018 and 2021, focusing on the magnitude and seasonality of emissions. Emissions are compared using a combination of bottom-up and top-down estimates. Bottom-up estimates for key sectors, including anthropogenic activities, biomass burning, and wetlands, are produced by inventories and process models. Top-down estimates are derived from an ensemble of atmospheric inversions that separately optimise anthropogenic and biospheric emissions. The inversions, derived from the CarbonTracker Europe-CH4 model, incorporate a range of prior estimates, uncertainties, and atmospheric methane measurements from in-situ surface stations and satellite observations from TROPOMI and GOSAT.  

Preliminary findings indicate that for all four regions, posterior natural emissions are strongly influenced by the choice of prior emissions in shaping both their seasonality and magnitude. The CarbonTracker Europe-CH4 ensemble produces posterior emissions estimates consistent with the Global Carbon Project ensemble, which utilised different inversion models.  

By integrating a wide range of emissions estimates, this study aims to improve our understanding of the NHL methane budget. The findings contribute to ongoing methane emission assessments under the Eye-CLIMA, IM4CA (Investigating Methane for Climate Action), ESA SMART-CH4 (Satellite Monitoring of Atmospheric Methane) projects and ESA-AMPAC (Arctic Methane and Permafrost Challenge).