Influence of biomass burning during recent fluctuations in the slow growth of global tropospheric methane

Abstract

During the past 15 years the annual growth rate of tropospheric methane (CH 4 ) has shown striking changes over 2–3 year periods, varying from +1% yr −1 to slightly negative values (−0.2% yr −1 ). These fluctuations are superimposed on an overall slowdown of the CH 4 growth rate since the 1980s. Here we use our complementary measurement of other compounds (ethane, tetrachloroethene) to confirm the influence of biomass burning on large global CH 4 pulses in 1998 and 2002–2003. Methane growth rate fluctuations also track ENSO indices, most likely via the influence of ENSO activity on large‐scale biomass burning. We also report the seventh year of near‐zero growth of global CH 4 levels (Dec. 1998–Dec. 2005). The global CH 4 mixing ratio was 1772 ± 1 ppbv in 2005, and CH 4 increases of 118–376 ppbv between 2000–2020 (ten scenarios in the 2001 IPCC report, to levels around 1900+ ppbv by 2020, now appear quite unlikely.