Enigma of the recent methane budget

摘要

There has been a recent unexpected variation in the atmospheric concentration of methane (CH_4) - the second most important anthropogenically modified greenhouse gas after carbon dioxide. Its growth rate has declined almost to a standstill in the past three decades, despite more than doubling since pre-industrial times; signs of renewed growth are only now starting to appear. What has caused the puzzling behaviour of this atmospheric trace gas? In potentially contradictory studies on pages 194 and 198 of this issue, Kai et al.~1 and Aydin et al.~2 attempt to answer this question. The growth rate of the atmospheric methane concentration effectively reflects the global methane budget (Fig. 1), which at present delicately balances large sinks and sources. Global sinks arise primarily from the activity of the hydroxyl radical (OH'), which is involved in photochemical oxidation reactions in the atmosphere. Although sinks are quite well understood, the relative contributions of the various processes that produce methane are uncertain.