Untangling variations in the global methane budget

Abstract

The drivers of trends in methane concentrations in the atmosphere over past decades are still poorly understood. Simulations from a chemistry transport model and box model show that human activity is the main driver of a complex system.The observational record for the global atmospheric burden of methane over the past 40 years tells a complex story of competing influences. Methane, a strong greenhouse gas with a mean lifetime in the atmosphere of around a decade, is emitted via an array of natural and anthropogenic sources. Over the past few decades, its rate of accumulation in the atmosphere has varied significantly due to the changing magnitudes of various source sectors and the main atmospheric sink. However, our understanding of methane emissions from diverse sources such as wetlands, fossil fuels, waste, cattle, forest fires, and even termites remains incomplete, leaving a number of enduring questions regarding the causes of the observed changes. Now, Ragnhild Skeie and colleagues 1 , writing in Communications Earth & Environment, have shown how the changing influence of three of the most uncertain aspects of the methane budget-anthropogenic emissions, wetlands and chemical loss-have contributed to the observed changes in its growth rate.Atmospheric methane (CH 4 ) rose quickly in the early 1980s, mainly due to the increasing global reliance on fossil fuels. The rate of increase slowed to near-zero in the early 2000s, but since 2007 it has once again been rising, at a pace that has increased over time. Debate continues amongst scientists as to whether this renewed growth is again due to the increasing use of fossil fuels or is a consequence of increased agricultural production, climate change-related feedback affecting wetland emissions, variations in the atmospheric sink, or a combination of factors [2][3][4][5][6][7][8][9][10] .Measuring isotopes of methane in the atmosphere provides additional information regarding the changing mixture of sources and sinks, which have different individual isotopic 'signatures'. The atmospheric ratio of methane molecules containing carbon-13 ( 13 CH 4 ) to those containing carbon-12 ( 12 CH 4 ) has been decreasing since around 2007, the same year that total CH 4 began to increase. This evidence indicates that increasing fossil fuel use, which would increase the 13 CH 4 / 12 CH 4 ratio, cannot alone be responsible for the renewed CH 4 growth.Ragnhild Skeie and colleagues tackle this problem using a simple box model of the atmosphere to quantify the contributions of anthropogenic emissions, wetland fluxes and the main atmospheric CH 4 sink to the observed growth since the 1980s. In addition, a similar model was used to simulate the atmospheric 13 CH 4 / 12 CH 4 ratio. Box models treat the atmosphere as a single reservoir, which allows computationally efficient calculation of individual source sector contributions and their uncertainties over this long time period, whilst using state-of-the-art inventories and models for the input flux parameters. The results show that anthro...

Unraveling the dynamics of atmospheric methane: the impact of anthropogenic and natural emissions

“…Moreover, the urgent need for an explanation of methane concentration variability is heightened by the consecutive record-breaking annual increases observed since the 2020s [9,10]. Only through a comprehensive understanding of past variations can efforts be targeted to reduce atmospheric CH 4 concentrations [11].…”