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Earth's Atmospheric Methane Concentration: 2008 Update
Reference
Schnell, R.C. and Dlugokencky, E. 2008. Methane. In: Levinson, D.H. and Lawrimore, J.H., Eds. State of the Climate in 2007. Special Supplement to the Bulletin of the American Meteorological Society 89: S27.

What was done
The authors provide an update through 2007 of atmospheric methane concentrations as determined from weekly discrete samples collected on a regular basis since 1983 at the NOAA/ESRL Mauna Loa Observatory.

What was learned
Our adaptation of the graphical rendition of the data provided by the authors is presented in the figure below.

Figure 1. Trace gas mole fractions of methane (CH4) as measured at Mauna Loa, Hawaii. Adapted from Schnell and Dlugokencky (2008).

What it means
In commenting on the data contained in the figure above, Schnell and Dlugokencky state that "atmospheric CH4 has remained nearly constant since the late 1990s." This is a most important finding, because, as they also note, "methane's contribution to anthropogenic radiative forcing, including direct and indirect effects, is about 0.7 W m-2, about half that of CO2." In addition, they say that "the increase in methane since the preindustrial era is responsible for approximately one-half the estimated increase in background tropospheric O3 during that time."

In light of these finding, it can be appreciated that over the past decade there have been essentially no increases in these two negative impacts of methane emissions to the atmosphere (i.e., global warming and tropospheric ozone production), and that the leveling out of the atmosphere's methane concentration -- the exact causes of which, in the words of Schnell and Dlugokencky, "are still unclear" -- has resulted in a full one-third reduction in the combined radiative forcing that would otherwise have been produced by a continuation of the prior rates-of-rise of the concentrations of the two atmospheric greenhouse gases.

Reviewed 15 October 2008