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Methane is a key atmospheric constituent, being both a potent greenhouse gas and an influence on the tropospheric oxidising capacity – the ability of the atmosphere to rid itself of pollutants. Future changes in the concentration of methane ( [CH4] ) thus have implications for both our climate and air quality, and an understanding of past changes in [CH4] is a prerequisite for meaningful future predictions. The study of past changes also offers valuable insights into interactions between composition and climate on timescales of anything up to 800,000 years - the length of time for which Antarctic ice provides a record of climate and the air bubbles trapped within it provide a record of [CH4].
The most striking natural features of this record are: the differences in [CH4] between glacial and interglacial periods, for instance almost doubling between the Last Glacial Maximum (LGM; 21,000 years ago) and the pre-industrial era (PI; 200 years ago); and rapid rises in [CH4], of up to half the glacial-interglacial difference in less than 100 years, accompanying northern-hemisphere warmings at the beginning of Dansgaard-Oeschger (D-O) events during the last glacial period (21,000-110,000 years ago). Whilst there has been much debate regarding the relative roles of changes in methane sources and sinks between the LGM and the PI (the dominant ones being wetlands and oxidation by the hydroxyl radical, OH, respectively), the rises in [CH4] at the beginning of D-O events have received less attention. Yet these could offer insights into the likelihood of future rapid rises in [CH4] in response to continued climate change.
Two controls on the oxidising capacity have been identified as having been influential between the LGM and the PI: emissions of non-methane volatile organic compounds (NMVOCs) from vegetation, with which methane competes for reaction with OH; and air temperatures, which affect the production of OH and the reactivity that OH shows towards methane (in addition to the kinetics of other chemical reactions). In a previous computer-modelling study, we found that the net effect of these two controls on the oxidising capacity was negligible between the LGM and the PI; their separate effects, though substantial, were roughly equal and opposite, implying the near-doubling of [CH4] during that period was essentially entirely source-driven. This conclusion, though subject to significant uncertainties, is consistent with the most recent estimates of the changes in methane sources between the LGM and the PI.
Here, we report a recent extension of that study (Levine et al., 2012) to a modelled D-O event featuring a characteristically rapid rise in [CH4] in response to a northern-hemisphere warming driven by a change in ocean circulation. We find that the influences of changes in NMVOC emissions and air temperatures across the rapid rise in [CH4] continue to offset each other, and the net effect on the oxidising capacity is again negligible. This suggests the rapid rises in [CH4] at the beginning of D-O events may have also been almost entirely source-driven, and the most striking natural features of the ice record of [CH4], over the last 110,000 years, may almost purely reflect changes in methane sources.
Levine, J. G.1, E. W. Wolff1, P. O. Hopcroft2, and P. J. Valdes2 (2012), Controls on the tropospheric oxidizing capacity during an idealized Dansgaard-Oeschger event, and their implications for the rapid rises in atmospheric methane during the last glacial period , Geophys. Res. Lett., 39, L12805, doi:10.1029/2012GL051866.
1British Antarctic Survey, High Cross, Madingley Road, Cambridge, UK
2Bristol Research Initiative for the Dynamic Global Environment, University of Bristol, Bristol, UK