The Global Methane Budget: the knowns and unknowns

After carbon dioxide, atmospheric methane is the second most impactful anthropogenic greenhouse gas for global warming. Observations of atmospheric methane in ambient air began in 1978, and now include a wide range of in-situ and remote-sensed observations from the surface, aircraft or from space. Those observations have shown that methane mixing ratio have been multiplied by 2.6 since pr-industrial time and the recent period has experienced record methane growth rate in the atmosphere. This is a well-known and established fact. Questions arise when it comes to methane sources and sinks and the causes of such an increase, sustained by at different rate over time. Different approaches are used to estimates methane sources and sinks: atmospheric inversions use atmospheric mixing ratios measurements to infer methane emissions and sinks (top-down approaches), land-surface models simulate the processes that emit methane at the surface (e.g. wetland and freshwater emissions) or remove methane from the atmosphere (e.g. OH radicals), and inventories estimates anthropogenic emissions based on socio-economic statistics (bottom-up approaches).

Despite significant efforts over the last decades, there are still significant uncertainties in the spatial and temporal quantification of methane sources and sinks. The Global Methane Budget (GMB), under the umbrella of the Global Carbon Project, aims to releases regular synthesis of the methane budget at global and region scales.

This presentation will present the well-known facts, the quite-knowns sources and sinks and their uncertainties, the remaining large uncertainties on the methane budget and its changes over the past decades based on the latest Global Methane Budget activities, and will discussion the not-well knowns and unknows in the methane biogeochemical cycles, including the question of the contribution of the forest ecosystem.