Background
The authors note that "methane changes have both a direct radiative impact on climate and an indirect impact via their effect on ozone and OH," and that "emissions of the ozone precursors NOX, CO and volatile organic compounds (VOCs, which here exclude methane) affect both tropospheric ozone and methane."  Hence, it is no mean chore to correctly calculate the role played by methane in driving global warming.  Nevertheless, Shindell et al. attempt to do just that.

What was done
In the words of the four NASA scientists, "we calculate radiative forcing due to anthropogenic emissions of methane and tropospheric ozone precursors using a fully coupled chemistry-aerosol-climate general circulation model including methane chemistry and prescribed emissions," in contrast to currently employed "abundance-based assessments" that look at methane and other greenhouse gas concentrations after these complex chemical interactions have occurred and altered the original relative proportions of their initial emissions to the atmosphere.

What was learned
Shindell et al. calculate that eliminating current anthropogenic methane emissions would eliminate a radiative forcing of 0.79 W m^-2, which is equivalent to 54% of the radiative forcing due to the CO2 increase experienced from preindustrial times to the present day.  This result is to be compared with a radiative forcing reduction of 0.48 W m^-2 derived from abundance-based assessments of well-mixed atmospheric methane concentrations.  In addition, they note that if the forcing due to production of stratospheric H2O from methane were included in the calculation, eliminating anthropogenic methane emissions would eliminate 61% of the radiative forcing due to the CO2 increase experienced since preindustrial times.

What it means
The authors say their new evaluation of the global warming potential of methane "provides useful insight for policy-making."  What might that be?  For one thing, it suggests that focusing on reducing methane emissions may be a much more powerful approach to slowing global warming than people had previously thought.  For another, it suggests that earth's biosphere may already be employing this approach without any help from man, for as indicated by several of the items archived under the general heading of Methane in our Subject Index, the rate of rise of the atmosphere's methane concentration has been decelerating over the past several years, to the point where the methane content of the air is no longer rising, which may be due in part to biological consequences of the ongoing rise in the air's CO2 content.  If this is indeed the primary reason for the leveling out of the atmosphere's methane concentration, it is clear that the computational problem of assessing methane's global warming potential is even more complex than Shindell et al. have considered it to be, and that the biosphere may be exerting its own CO2-induced negative influence on atmospheric methane concentration, the cooling effect of which may be quite substantial.