Observational constraints on recent increases in the atmospheric CH₄ burden

Measurements of atmospheric CH₄ from air samples collected weekly at 46 remote surface sites show that, after a decade of near-zero growth, globally averaged atmospheric methane increased during 2007 and 2008. During 2007, CH₄ increased by 8.3 ± 0.6 ppb. CH₄ mole fractions averaged over polar northern latitudes and the Southern Hemisphere increased more than other zonally averaged regions. In 2008, globally averaged CH₄ increased by 4.4 ± 0.6 ppb; the largest increase was in the tropics, while polar northern latitudes did not increase. Satellite and in situ CO observations suggest only a minor contribution to increased CH4 from biomass burning. The most likely drivers of the CH₄ anomalies observed during 2007 and 2008 are anomalously high temperatures in the Arctic and greater than average precipitation in the tropics. Near-zero CH₄ growth in the Arctic during 2008 suggests we have not yet activated strong climate feedbacks from permafrost and CH₄ hydrates.