Implications of the recent fluctuations in the growth rate of tropospheric methane

Global measurements show that the mixing ratio of tropospheric methane (CH 4 ) increased by 1.1% (19.5 ± 1.7 ppbv) over the five‐year period 1996–2000, with striking fluctuations in its annual growth rate. Whereas the global CH 4 growth rate reached 15.9 ± 0.7 ppbv yr −1 in 1998, the growth rate was −2.1 ± 0.8 ppbv yr −1 in 2000. This is the first time in our 23‐year global monitoring program that we have measured a negative annual CH 4 growth rate. The CH 4 growth rate fluctuates in an unpredictable fashion, and we reemphasize that global CH 4 concentrations cannot be extrapolated into the future based on past trends. As a result, we suggest that the slowing of the CH 4 growth rate during much of the 1980s and 1990s cannot be used to imply that CH 4 will no longer be of concern in greenhouse gas studies during this century.