Methane fluxes in the high northern latitudes for 2005-2013 estimated using a Bayesian atmospheric inversion

We present methane (CH₄) flux estimates for 2005 to 2013 from a Bayesian inversion focusing on the high northern latitudes (north of 50°N). Our inversion is based on atmospheric transport modelled by the Lagrangian particle dispersion model FLEXPART and CH₄ observations from 17 in situ and five discrete flask-sampling sites distributed over northern North America and Eurasia. CH₄ fluxes are determined at monthly temporal resolution and on a variable grid with maximum resolution of 1° × 1°. Our inversion finds a CH₄ source from the high northern latitudes of 82 to 84Tgyr^-1, constituting ∼15% of the global total, compared to 64 to 68Tgyr^-1 (∼12%) in the prior estimates. For northern North America, we estimate a mean source of 16.6 to 17.9Tgyr^-1, which is dominated by fluxes in the Hudson Bay Lowlands (HBL) and western Canada, specifically the province of Alberta. Our estimate for the HBL, of 2.7 to 3.4Tgyr^-1, is close to the prior estimate (which includes wetland fluxes from the land surface model, LPX-Bern) and to other independent inversion estimates. However, our estimate for Alberta, of 5.0 to 5.8Tgyr^-1, is significantly higher than the prior (which also includes anthropogenic sources from the EDGAR-4.2FT2010 inventory). Since the fluxes from this region persist throughout the winter, this may signify that the anthropogenic emissions are underestimated. For northern Eurasia, we find a mean source of 52.2 to 55.5Tgyr^-1, with a strong contribution from fluxes in the Western Siberian Lowlands (WSL) for which we estimate a source of 19.3 to 19.9Tgyr^-1. Over the 9-year inversion period, we find significant year-to-year variations in the fluxes, which in North America, and specifically in the HBL, appear to be driven at least in part by soil temperature, while in the WSL, the variability is more dependent on soil moisture. Moreover, we find significant positive trends in the CH₄ fluxes in North America of 0.38 to 0.57Tgyr^-2, and northern Eurasia of 0.76 to 1.09Tgyr^-2. In North America, this could be due to an increase in soil temperature, while in North Eurasia, specifically Russia, the trend is likely due, at least in part, to an increase in anthropogenic sources.