Amount varies geographically Most abundant greenhouse gas, not most efficient absorber
	Yearly variations due to biological effects in spring/summer huge quantities are removed by photosynthesis (fixed carbon) fall/winter fixed carbon released during biological decay of plant material
	The concentration of CO2 in the atmosphere is sufficient to almost completely absorb the IR radiation in several spectral regions Additional CO2 does not remove much more IR radiation Methane absorptions occur in spectral regions where little IR absorption is occurring Each new methane molecule provides can absorb IR radiation