| 英文摘要: | After approximately a decade of stability, the globally averaged concentration of methane in the atmosphere began to increase again from 2007 at a rate of 5.7 ± 1.2 ppb per year. The causes of this trend are important for our understanding of the impacts of climate change and the potential for positive feedbacks to enhance climatic changes. Euan Nisbet from Royal Holloway University, UK, and co-authors utilize data from the NOAA cooperative global air sampling network and Royal Holloway measurements at Alert, Canada, Ascension Island, equatorial Atlantic, and Cape Point, South Africa, to investigate the reasons for the rise. They develop a budget analysis of monthly average values of methane mole fraction and δ13CCH4 — a measure of the 13C/12C isotope ratio in methane — over four latitudinal zones. The sustained shift to more 13C-depleted values and the degree of interannual variability — indicative of biogenic methane sources — together with the tropical and Southern Hemisphere loci of methane growth indicate that fossil fuel emissions are unlikely to be the dominant driver of this trend. Although the data do not provide conclusive source diagnoses, the authors suggest that increased methane emissions from tropical wetlands and agriculture, linked to unusual climatic conditions, are the most likely culprits driving observed methane increases. |