Abstract
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 methane lifetime against loss by tropospheric OH, (τCH4_OH), in a suite of historical (1860–2005) and future 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 τCH4_OH due to a doubling of the methane burden which offsets coincident increases in nitrogen oxide (NOx emissions. Over the last two decades, however, the τCH4_OH 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. Sensitivity simulations with CM3 suggest that the aerosol indirect effect (aerosol-cloud interactions) plays a significant role in cooling the CM3 climate. The projected decline in aerosols under all RCPs contributes to climate warming over the 21st century, which influences the future evolution of OH concentration and τCH4_OH. Projected changes in τCH4_OH from 2006 to 2100 range from −13% to +4%. The only projected increase occurs in the most extreme warming case (RCP8.5) due to the near-doubling of the CH4 abundance, reflecting a positive feedback on the climate system. The largest decrease occurs in the RCP4.5 scenario due to changes in short-lived climate forcing agents which reinforce climate warming and enhance OH. This decrease is more-than-halved in a sensitivity simulation in which only well-mixed greenhouse gas radiative forcing changes along the RCP4.5 scenario (5% vs. 13%).
Highlights
Atmospheric methane (CH4) is the second most important anthropogenic greenhouse gas after carbon dioxide (CO2), reflecting its stronger heat-trapping efficiency (100-yr global warming potential of 25) and more-than-doubling in abundance since pre-industrial times (Forster et al, 2007)
We investigate here the relative importance of changes in climate factors (temperature, OH, water vapor (H2O), lightning nitrogen oxide (NOx) (LNOx), photolysis rates) versus anthropogenic emissions (CH4 abundance, carbon monoxide (CO) and nitrous oxide (NOx)) in contributing to changes in the τCH4 OH from 1860 to 2100, in the context of the new set of historical and future emission scenarios (Lamarque et al, 2010; Meinshausen et al, 2011; van Vuuren et al, 2011) developed for the fifth phase of the Coupled Model Intercomparison Project (CMIP5) (Taylor et al, 2012), in support of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment (AR5)
OH is reduced as a result of the lower atmospheric humidity, as well as decreases in lightning NOx (Fig. 4a); with minima in temperature, OH, water vapor and Lightning NOx (LNOx) coinciding with the peak methane lifetimes occurring in 1884, 1965, 1983 and 1992
Summary
Atmospheric methane (CH4) is the second most important anthropogenic greenhouse gas after carbon dioxide (CO2), reflecting its stronger heat-trapping efficiency (100-yr global warming potential of 25) and more-than-doubling in abundance since pre-industrial times (Forster et al, 2007). Method ice core measurements of CH2O 3-D model Multi 1-D model 2-D model 3-D model 3-D model 3-D model 3-D model 3-D model 3-D model without non-methane hydrocarbon (NMHC) 3-D model 3-D model 3-D model 3-D model 3-D model 3-D model on the role of changes in anthropogenic emissions (CH4 and gas-phase OH precursors) versus climate (temperature, water vapor, photolysis rates, lightning NOx) on the evolution of τCH4 OH on decadal-to-century time scales in a suite of historical simulations and future scenarios This new tool allows investigation of a broader suite of chemistry-climate interactions and is intended to complement multi-model comparisons conducted for specific decadal time periods such as those occurring under the Atmospheric Chemistry and Climate Model Intercomparison Project (ACCMIP) in support of IPCC AR5
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