As we have noted previously, the second most important anthropogenic greenhouse gas (after carbon dioxide, or CO2), is methane (CH4), and although its concentration (only about 1.8 ppm) is far less than that of CO2 (at 390 ppm), it’s a stronger greenhouse gas on a per-molecule basis, and it ends up transformed into CO2 by atmospheric chemistry processes anyway. Also, more than half the atmospheric CH4 load is due to changes wrought by mankind. Atmospheric methane concentration had stabilized from about 1999 to 2007, but recently began rising again, as reported in Rigby et al (2008).
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