Estimation of methane fluxes in the high northern latitudes from a Bayesian atmospheric inversion
Abstract:Methane (CH4) is the second most important anthropogenic greenhouse gas after CO2. Globally, atmospheric CH4 concentrations have increased since direct measurements began, in the early 1980s, but then stabilized from the mid 1990s to the mid 2000s. Since 2007, the atmospheric CH4 growth rate has become positive again causing concern that it may be the response to climate feedbacks, especially in the Arctic, where there is a potential for a large release of CH4 to the atmosphere under warmer conditions. Such feedbacks include high latitude wetlands, permafrost and the destabilization of methane hydrates.
We present CH4 emission estimates for the high northern latitudes (i.e., north of 50°N) from 2005 to 2012 using atmospheric mole fraction observations in a Bayesian inversion framework. This framework is based on the Lagrangian Particle Dispersion model, FLEXPART, run with ECMWF meteorological analyses. Emissions were optimized monthly and on a spatial grid of variable resolution. Background mixing ratios were found by coupling FLEXPART to output from the optimized Eulerian chemistry transport model, TM5. Meteorological models tend to overestimate the boundary layer heights (BLH) in the Arctic, especially in winter, which can lead to biases in the estimated fluxes. Therefore, we explored a number of data selection criteria to avoid assimilating data at times when significant errors in BLH were likely. Overall, the inversion found higher emissions in Northern Eurasia and North America compared to the prior in both summer and winter. The inversion fluxes also displayed considerable inter-annual variability, which appears to be related to climate variability.