Satellite Constraints on the Latitudinal Distribution and Temperature Sensitivity of Wetland Methane Emissions

Abstract

Wetland methane (CH₄) emissions comprise about one-third of the global CH₄ source. The latitudinal distribution and climate sensitivity of wetland CH₄ fluxes are the key determinants of the global CH₄-climate feedback. However, large differences exist between bottom-up estimates, informed by ground-based flux measurements, and top-down estimates derived from spaceborne total column CH₄. Despite the extensive coverage of satellite CH₄ concentration observations, challenges remain with using top-down estimates to test bottom-up models, mainly because of the uncertainties in the satellite retrievals, the model representation errors, the variable prior emissions, and the confounding role of the posterior error covariance structures. Here, we use satellite-based top-down CH₄ flux estimates (2010–2012) to test and refine 42 bottom-up estimates of wetland emissions that use a range of hypothesized wetland extents and process controls. Our comparison between bottom-up models and satellite-based fluxes innovatively accounts for cross-correlations and spatial uncertainties typically found in top-down inverse estimates, such that only the information from satellite observations and the atmospheric transport model is kept as a constraint. We present a satellite-constrained wetland CH₄ ensemble product derived from assembling the highest-performance bottom-up models, which estimates global wetland CH₄ emissions of 148 (117–189, 5th–95th percentile) Tg CH₄ yr⁻¹. We find that tropical wetland emissions contribute 72% (63%–85%) to the global wetland total. We also find that a lower-than-expected temperature sensitivity agrees better with atmospheric CH₄ measurements. Overall, our approach demonstrates the potential for using satellites to quantitatively refine bottom-up wetland CH₄ emission estimates, their latitudinal distributions, and their sensitivity to climate.