Atmos. Chem. Phys. Discuss., 12, 18067-18105, 2012
www.atmos-chem-phys-discuss.net/12/18067/2012/
doi:10.5194/acpd-12-18067-2012
© Author(s) 2012. This work is distributed
under the Creative Commons Attribution 3.0 License.
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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.
Climate versus emission drivers of methane lifetime from 1860–2100
J. G. John1, A. M. Fiore1,*, V. Naik2, L. W. Horowitz1, and J. P. Dunne1
1Geophysical Fluid Dynamics Laboratory/NOAA, Princeton, NJ, USA
2UCAR/Geophysical Fluid Dynamics Laboratory, Princeton, NJ, USA
*now at: Department of Earth and Environmental Sciences, and Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, USA

Abstract. With a more-than-doubling in the atmospheric abundance of the potent greenhouse gas methane (CH4) since preindustrial times, and indications of renewed growth following a leveling off in recent years, questions arise as to future trends and resulting climate and public health impacts from continued growth without mitigation. Changes in atmospheric methane lifetime are determined by factors which regulate the abundance of OH, the primary methane removal mechanism, including changes in CH4 itself. We investigate the role of emissions of short-lived species and climate in determining the evolution of tropospheric methane lifetime in a suite of historical (1860–2005) and Representative Concentration Pathway (RCP) simulations (2006–2100), conducted with the Geophysical Fluid Dynamics Laboratory (GFDL) fully coupled chemistry-climate model (CM3). From preindustrial to present, CM3 simulates an overall 5% increase in CH4 lifetime due to a doubling of the methane burden which offsets coincident increases in nitrogen oxide (NOx) emissions. Over the last two decades, however, the methane lifetime declines steadily, coinciding with the most rapid climate warming and observed slow-down in CH4 growth rates, reflecting a possible negative feedback through the CH4 sink. The aerosol indirect effect plays a significant role in the CM3 climate and thus in the future evolution of the methane lifetime, due to the rapid projected decline of aerosols under all four RCPs. In all scenarios, the methane lifetime decreases (by 5–13%) except for the most extreme warming case (RCP8.5), where it increases by 4% due to the near-doubling of the CH4 abundance, reflecting a positive feedback on the climate system. In the RCP4.5 scenario changes in short-lived climate forcing agents reinforce climate warming and enhance OH, leading to a more-than-doubling of the decrease in methane lifetime from 2006 to 2100 relative to a simulation in which only well-mixed greenhouse gases are allowed to change along the RCP4.5 scenario (13% vs. 5%) Future work should include process-based studies to better understand and elucidate the individual mechanisms controlling methane lifetime.

Citation: John, J. G., Fiore, A. M., Naik, V., Horowitz, L. W., and Dunne, J. P.: Climate versus emission drivers of methane lifetime from 1860–2100, Atmos. Chem. Phys. Discuss., 12, 18067-18105, doi:10.5194/acpd-12-18067-2012, 2012.
 
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