Why would CH4 (methane) production be concentrated in the Northern hemisphere?
Atmospheric concentration of CO (Carbon Monoxide): Carbon Monoxide is also an anthropogenic greenhouse gas. http://www.cmdl.noaa.gov/ccgg/gallery/index.php?pageType=folder&currDir=./Data_Figures Notice the similar (to CO2 and CH4) relationship with the concentration of CO in the marine boundary layer in the Northern versus Southern Hemispheres.