HR: 16:30h
AN: A14C-02    [Abstracts]
TI: Assessing our Understanding of Recent Trends in Atmospheric Methane
AU: * Dlugokencky, E
EM: ed.dlugokencky@noaa.gov
AF: NOAA ESRL, 325 Broadway, Boulder, CO 80305, United States
AU: Masarie, K
EM: kenneth.masarie@noaa.gov
AF: NOAA ESRL, 325 Broadway, Boulder, CO 80305, United States
AU: Lang, P
EM: Patricia.M.Lang@noaa.gov
AF: NOAA ESRL, 325 Broadway, Boulder, CO 80305, United States
AU: Houweling, S
EM: s.houweling@phys.uu.nl
AF: Netherlands Institute for Space Research, Princetonplein 5, Utrecht, 3584 CC, Netherlands
AB: Direct and indirect components to anthropogenic radiative forcing by atmospheric CH₄ are estimated to be 0.7 W m^-2, or about 1/2 the contribution of CO₂. Through its chemistry, methane also affects the abundances of tropospheric ozone, a strong oxidant and greenhouse gas that impacts human health and agricultural crop yields, and OH, a radical whose concentration determines the lifetimes of many greenhouse gases. Methane has been identified as a greenhouse gas to target for short-term stabilization in radiative forcing because it has a relatively short life time and because reductions in its emissions from many sources are cost effective. But large uncertainties in the CH₄ budget still exist, so it is not possible to predict the future atmospheric burden of CH₄ and it potential impact on climate or atmospheric chemistry. NOAA has been monitoring atmospheric CH4 since 1983. At the start of our program, the rate of increase in atmospheric methane was ~15 ppb yr^-1, but since 1999, the growth rate has been near zero. Through 1990, the monotonic decrease in global growth rate was consistent with a system approaching steady state with constant global emissions and a lifetime of ~10 yr. Had this trend toward steady state continued, the rate of increase would have slowly approached zero. Interannual variability in the growth rate makes it difficult to say whether the atmospheric burden is currently increasing, stable, or decreasing, but we have observed net decreases in globally averaged CH₄ in 4 of the last 7 years. It would be surprising if atmospheric methane were decreasing, because CH₄ emissions are not regulated and scenarios of emissions such as those used by IPCC suggest that improved living standards in the developing world and increased energy demand should result in increasing emissions, which would result in an increase in the atmospheric CH₄ burden. We examine current ideas on why atmospheric CH₄ has stabilized in light of the NOAA ESRL CH₄ observations. We conclude that significantly more in situ measurements and improved modeling capability are needed before a detailed understanding of the global CH₄ budget will be achieved.
DE: 0315 Biosphere/atmosphere interactions (0426, 1610)
DE: 0322 Constituent sources and sinks
DE: 0365 Troposphere: composition and chemistry
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting