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Methane researchers will all be accustomed to the old jokes about flatulent cows, but fewer will be familiar with the debate about the possible role of methane emissions from other large bodied herbivores in an earlier era.
Prior to the arrival of humans, around 12,500 years ago, a vast population of now extinct megafauna (mammoths, camelids, giant ground sloths, to name but a few) roamed the Americas. The megafauna’s dramatic demise is considered to be the earliest catastrophic event attributable to our own species.
A Nature Geoscience article published last year by Smith et al. (1) suggested that the drop in atmospheric methane concentrations observed in the ice core record at the onset of the Younger Dryas, ~12,800 years ago, occurred at an unusually rapid rate, and could be explained, at least in part, by the loss of methane emissions from megafauna. Using estimates of the amount of methane emitted by the 114 species of herbivores known to have become extinct by the end of the Pleistocene epoch, Smith and co-authors calculated that the loss of megafauna could account for between 12.5 and 100% of the observed methane decrease. They also suggested that the onset of the ‘Anthropocene’ should be recalibrated to 13,400 years before present, when humans first started migrating on a large scale into the Americas.
More recently in Nature Geoscience, doubt has been cast on some of these claims (2). Brook and Severinghaus assert they are inconsistent with constraints imposed by the ice core record, noting that the drop in concentration of atmospheric methane discussed by Smith and co-authors and more usually attributed to decreased wetland emissions, is not in fact uniquely rapid in rate, and that furthermore it is likely that the megafauna extinctions occurred too early to be the cause of the observed methane decrease.
The debate about changes in atmospheric methane concentration (causes, timings and rates) in the atmosphere over the past 125,000 years is doubtless not over yet. The ‘Early Anthropocene’ hypothesis, originally proposed by William Ruddiman (3) in the context of the onset of human-related activities such forest clearance (8000 years ago) and rice agriculture (5000 years ago) is also still a live topic, as discussed elsewhere on this website in the news item ‘Early Anthropocene doubt’. Watch this space.
(1) Smith, F.A., Elliot, S.M. and Lyons, S.K. (2010) Methane emissions from extinct megafauna. Nature Geoscience, 3, 374-375.
(2) Brook , E.J. and Severinghaus, J.P. (2011) Methane and megafauna. Nature Geoscience, 4, 271-272.
(3) Ruddiman, W. (2003) The anthropogenic greenhouse era began thousands of years ago. Climate Change. 61, 261-293.