Reexamining assumptions about the role of atmospheric oxidation in recent methane growth

Since 2007, scientists have been searching to find the cause of a sudden and unexpected global rise in atmospheric methane, a potent greenhouse gas, following almost a decade in which concentrations had stayed relatively constant.

Recent studies have explored a range of possible causes. Suggestions have included a rise in oil and natural gas extraction, increased emissions from tropical wetlands or increases in emissions from growing East Asian economies.

However, a new 2017 paper by an international team of scientists in the Proceedings of the National Academy of Sciences (PNAS) investigates an alternative possibility: a rise and fall in the concentration of the substance that destroys methane in the atmosphere, the hydroxyl radical. The global concentration of the hydroxyl radical cannot be measured directly; therefore the concentration was determined by measuring the rate at which the solvent methyl chloroform, which is also destroyed by hydroxyl, was being removed from the atmosphere.

Lead author, Dr. Matt Rigby, from the University of Bristol notes that a change in the hydroxyl radical concentration is a viable explanation for the changes in methane that we’ve seen. It would mean that emissions would not have increased suddenly in 2007, but rather, have risen more gradually over the last couple of decades. However, while there is strong evidence in this study that hydroxyl radical changes are playing a significant role in the fluctuations in methane growth, uncertainties remain.

Co-author, Professor Ron Prinn, from the Massachusetts Institute of Technology, who leads the Advanced Global Atmospheric Gases Experiment (AGAGE), an international project that measures greenhouse gas concentrations, notes that the NASA-sponsored AGAGE program has been monitoring trends in methyl chloroform for nearly 40 years because of its role in depleting stratospheric ozone. Because methyl chloroform is now banned under the Montreal Protocol for the Protection of the Stratospheric Ozone Layer, its concentration has dropped very rapidly. This study examines how this rate of decline changes from one year to the next to infer the hydroxyl radical concentration.

Dr. Steve Montzka from the National Oceanic and Atmospheric Administration, who also co-authored the paper, and operates an independent measurement network for methyl chloroform, noted that this paper re-examines the assumptions that had previously been made in studies of hydroxyl radical and methyl chloroform and shows how they influence our understanding of methane’s atmospheric sink. One of the main findings is that objective analyses of two independent sets of methyl chloroform observations tells essentially the same story about the hydroxyl’s role in methane variability, even as it becomes more and more difficult to measure methyl chloroform given that its concentration is approaching zero.

The study also provided an unexpected finding: that emissions of methyl chloroform have not dropped to zero. Because its production is now banned globally, people were expecting to see no emissions of this substance at all. However, this study provides very strong evidence that emissions are continuing. The team is preparing a follow-up study that would determine from where these emissions are originating.

Related Article:

Role of atmospheric oxidation in recent methane growth. Rigby, M., S.A. Montzka, R.G. Prinn, J.W.C. White, D. Young, S. O’Doherty, M.F. Lunt, A.L. Ganesan, A.J. Manning, P.G. Simmonds, P.K. Salameh, C.M. Harth, J. Mühle, R.F. Weiss, P.J. Fraser, L.P. Steele, P.B. Krummel, A. McCulloch, and S. Park (2017). PNAS, 114(21): 5373-5377 (doi:10.1073/pnas.1616426114)