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

The ice core record reveals large variations in the concentration of atmospheric methane, [CH4], over the last 800 kyr. Amongst the most striking natural features are the large, rapid rises in [CH4], of 100-200 ppbv, on timescales of less than 100 years, at the beginning of Dansgaard-Oeschger (D-O) events during the last glacial period (21-110 kyr before present). Despite the potential insight they could offer into the likelihood of future rapid rises in [CH4], the relative roles of changes in methane sources and sinks during D-O events have been little explored. Here, we use a global atmospheric chemistry-transport model to explore-for the first time, in a process-based fashion-controls on the oxidizing capacity during an idealized D-O event that features a characteristically rapid rise in [CH4]. We find that the two controls previously identified in the literature as having had significant (though opposing) influences on the oxidizing capacity between glacial and interglacial periods-changes in air temperature and emissions of non-methane volatile organic compounds from vegetation-offset one another between idealized Heinrich stadial and Greenland interstadial states. The result is, the net change in oxidizing capacity is very small, implying the rapid rises in [CH4] at the beginning of D-O events were almost entirely source-driven. This poses a challenge to earth-system models-to generate a sufficiently large increase in methane emissions in response to a simulated D-O event, via a more realistic freshwater forcing impacting the strength of the Atlantic meridional overturning circulation or, possibly, other climate-change mechanisms. Citation: Levine, J. G., E. W. Wolff, P. O. Hopcroft, and P. J. Valdes (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.