Changes in the global methane budget since 2000

Abstract:

Atmospheric methane is the second anthropogenic greenhouse gas after carbon dioxide, with a 20% contribution to climate forcing since pre-industrial times. With a lifetime around 10 years in the atmosphere and a diversity of emission types, methane is an important target for climate change mitigation. Observations of atmospheric methane began in 1978, reached global coverage after 1983, and now include a large variety of in-situ and remote-sensed observations from the surface or from space. These data are assimilated in atmospheric inversion to infer methane emissions and sinks (top-down). In parallel, a large international effort is conducted to model processes (bottom-up) emitting methane at the surface (e.g. wetland emissions) or destroying methane in the atmosphere (e.g. OH radicals).

Although sources and sinks of methane are identified, large uncertainties remain in their spatio-temporal quantification. Here, we present a synthesis of global and regional methane emissions and sinks since 2000 using an integrated approach to combine: atmospheric measurements, chemistry-transport models, ecosystem models, emission inventories, and climate-chemistry models. Robust and not robust emission estimates are extracted and presented from an ensemble of atmospheric inversions and of process-based models. The three most striking results imply :

- a probable overestimation of Chinese methane emission and trend since 2000,

- a mostly tropical origin (75%) of emission changes from 2005 to 2010,

- a balanced (but still uncertain) partition of emission changes, between 2005 and 2010, between natural (wetlands) and anthropogenic (agriculture & waste, coal, biomass burning) emissions.