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U.S. Dept. of Commerce / NOAA / OAR / CMDL / CCGG

Continuing decline in the growth rate of the atmospheric methane burden

E.J. Dlugokencky, K.A. Masarie, P.M. Lang, and P.P. Tans
NOAA CMDL Carbon Cycle-Greenhouse Gases

Nature
June 4, 1998

edlugokencky@cmdl.noaa.gov

Abbreviated Abstract
The amount of methane in Earth's atmosphere has more than doubled since preindustrial times. This increase is responsible for about 20% of the estimated change in direct radiative forcing due to anthropogenic greenhouse gas emissions. Recent measurements indicate that although methane continues to increase, the rate of increase has slowed. Carbon Cycle-Greenhouse Gases analysis of methane measurements made during 1984- 1996 suggests that methane emissions and global OH have been nearly constant during this time period, and the decreasing growth rate of methane reflects an appraoch to steady-state on a time scale comparable to methane's lifetime (about 9 years). If global emissions and OH remain constant, globally averaged methane will increase from 1730 ppb in 1996 to a maximum of about 1800 ppb over the next few decades, with little change in the contribution of methane to the greenhouse effect.

Top Panel: Globally averaged atmospheric methane values (solid circles) are derived from high-precision measurements of methane in air samples collected weekly at 43 sites of the NOAA CMDL cooperative air sampling network. A mathematical model (dot-dashed line) that accounts for methane emissions and loss in the atmosphere accurately estimates the methane lifetime, and it gives a good fit to the global averages. Assuming that emissions do not change, methane will slowly increase to a maximum value of less than 1800 parts per billion over the next few decades.

Bottom Panel: Total annual methane emissions (circles), calculated from the methane global averages (top panel) and a methane lifetime of 8.9 years, are nearly constant (except for interannual variations). Methane loss (triangles) by chemical reaction depends on the amount of methane in the atmosphere, and thus increases as atmospheric methane increases. The difference between emissions and loss is the net annual atmospheric methane increase, which is slowly approaching zero.

• Postscript version of figure.
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• Postscript version of figure with white background.