Trends in the seasonal amplitude of atmospheric methane.

Methane is an important greenhouse gas and its atmospheric concentration has almost tripled since pre-industrial times. Atmospheric methane mixing ratios vary seasonally, with the seasonal cycle amplitude (SCA) having decreased in northern high latitudes and increased in the subtropics and tropics since the 1980s. These opposing SCA trends can help understanding of long-term changes in the global methane budget, as methane emissions and sinks have opposing effects on the SCA. However, trends in the methane SCA have not yet been explored in detail. Here we use a suite of atmospheric transport model simulations and attribute the observed trends in the seasonal amplitude of methane to changes in emissions and the atmospheric sink from reaction with the hydroxyl radical (OH). We find that the decreasing amplitude in the northern high latitudes is mainly caused by an increase in natural emissions (such as wetlands) owing to a warmer climate, adding evidence to previous studies suggesting a positive climate feedback. In contrast, the enhanced methane amplitude in the subtropics and tropics is mainly attributed to strengthened OH oxidation. Our results provide independent evidence for an increase in tropospheric OH concentration of 10 ± 1% since 1984, which together with an increasing atmospheric methane concentration suggests a 21 ± 1% increase in the atmospheric methane sink.