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As a potent greenhouse gas, the amount of methane in the Earth’s atmosphere affects our climate. It also has bearing on the quality of air we breathe, as it influences the ability of the atmosphere to rid itself of pollutants (including other greenhouse gases) that could reach levels harmful to human health if allowed to accumulate. We therefore need to understand the causes of variations in the concentration of this key constituent, and an ability to account for past variations is a prerequisite for meaningful future predictions.
Air bubbles trapped in Antarctic ice (pictured) reveal large variations in the concentration of methane, [CH4], over the last 800,000 years that appear to broadly track changes in climate. The question at the heart of a recent study by Levine et al. (2011) is, why did [CH4] almost double from 360 parts per billion by volume (ppbv) at the so-called Last Glacial Maximum (LGM; around 21,000 years ago) to 700 ppbv in the relatively warm pre-industrial era (PI; about 200 years ago)? Did the amount of methane emitted from natural sources such as wetlands increase, or did the rate at which methane was removed from the atmosphere decrease, allowing those emissions to accumulate to higher concentrations? The relative contributions made by changes in methane sources and changes in methane ‘sinks’ have long been debated.
Early ‘bottom-up’ model estimates of the changes in methane sources could only explain around half the change in [CH4], appealing to a reduction in the main methane sink—oxidation by the hydroxyl radical (OH)—as the climate warmed; the warming (and increasingly wet) climate would have seen an increase in vegetation and an increase in the amount of volatile organic compounds emitted from vegetation (e.g. isoprene) that compete with methane for reaction with OH. However, the warming climate would have also been accompanied by an increase in humidity, fuelling greater OH production, and an increase in the reactivity that OH shows towards methane. So there were opposing influences on the rate of methane removal by OH.
Using a computer model of the Earth’s atmosphere, we find that the changes in emissions from vegetation would have had a significant influence on [CH4] of the sort previously proposed, but this would have been almost entirely negated by the accompanying changes in humidity and chemical reaction rates, the implication being the LGM-PI change in [CH4] was essentially entirely source driven. Meanwhile, estimates of the change in methane emissions from wetlands during this period have increased, and now include almost precisely that needed to explain the change in [CH4] without recourse to changes in the rate of methane removal. We therefore conclude that it is plausible the LGM-PI change in [CH4] was entirely source-driven, and the changes in methane sources and sinks between the LGM and the PI could be reconciled thus.
Levine, J. G.1, E. W. Wolff1, A. E. Jones1, L. C. Sime1, P. J. Valdes2, A. T. Archibald3,4, G. D. Carver3,4, N. J. Warwick3,4, and J. A. Pyle3,4 (2011), Reconciling the changes in atmospheric methane sources and sinks between the Last Glacial Maximum and the pre-industrial era, Geophys. Res. Lett., 38, L23804, doi:10.1029/2011GL049545.
1British Antarctic Survey, High Cross, Madingley Road, Cambridge, UK.
2School of Geographical Sciences, University of Bristol, Bristol, UK.
3Centre for Atmospheric Science, Department of Chemistry, University of Cambridge, Cambridge, UK.
4National Centre for Atmospheric Science, University of Cambridge, Cambridge, UK.