Atmos. Chem. Phys. Discuss., 12, 20931-20974, 2012
© Author(s) 2012. This work is distributed
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This discussion paper has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP.
Future methane, hydroxyl, and their uncertainties: key climate and emission parameters for future predictions
C. D. Holmes1, M. J. Prather1, O. A. Søvde2, and G. Myhre2
1Department of Earth System Science, University of California, Irvine, CA 92697-3100, USA
2Center for International Climate and Environmental Research (CICERO), Oslo, Norway

Abstract. Accurate prediction of future methane abundances following a climate scenario requires understanding the lifetime changes driven by anthropogenic emissions, meteorological factors, and chemistry-climate feedbacks. Uncertainty in any of these influences or the underlying processes implies uncertainty in future abundance and radiative forcing. We simulate methane lifetime in multiple models over the period 1997–2009, adding sensitivity tests to determine key variables that drive the year-to-year variability. Across three atmospheric chemistry and transport models – UCI CTM, GEOS-Chem, and Oslo CTM3 – we find that temperature, water vapor, ozone column, biomass burning and lightning NOx are the dominant sources of interannual changes in methane lifetime. We also evaluate the model responses to forcings that have impacts on decadal time scales, such as methane feedback, and anthropogenic NOx emissions. In general, these different CTMs show similar sensitivities to the driving variables. We construct a parametric model that reproduces most of the interannual variability of each CTM and use it to predict methane lifetime from 1980 through 2100 following a specified emissions and climate scenario (RCP 8.5). The parametric model propagates uncertainties through all steps and provides a foundation for predicting methane abundances in any climate scenario. Our sensitivity tests also enable a new estimate of the methane global warming potential (GWP), accounting for stratospheric ozone effects, including those mediated by water vapor. We estimate the 100-yr GWP to be 32.

Citation: Holmes, C. D., Prather, M. J., Søvde, O. A., and Myhre, G.: Future methane, hydroxyl, and their uncertainties: key climate and emission parameters for future predictions, Atmos. Chem. Phys. Discuss., 12, 20931-20974, doi:10.5194/acpd-12-20931-2012, 2012.
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