Changes in the global methane budget since 2000
Abstract:Methane is the second anthropogenic greenhouse gas after carbon dioxide. Atmospheric methane has contributed about 20% of the climate forcing by long-lived greenhouse gases since pre-industrial times but also influences the oxidizing capacity of the atmosphere. With a lifetime of 9-10 years in the atmosphere, 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. After the loss of SCIAMACHY (2002-2012), two space missions are currently producing methane weighted-columns, GOSAT (since 2009) and METOP-IASI (since 2007).
Although sources and sinks of methane are identified, large uncertainties remain in their spatio-temporal quantification. Here, we present a synthesis of 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. The results of an ensemble of atmospheric inversions (top-down) and of process-based models (bottom-up) are presented and compared in order to propose scenarios for the increase of atmospheric methane since 2007, after almost a decade of stagnation. Global and regional methane budgets and their changes are presented and discussed for the period 2000-2012. Focus is placed in comparing methane inversions constrained by satellite data with those constrained by surface data.