The recent addition of anthropogenic sources of methane, as for CO2, has disturbed the pre-industrial atmospheric flux equilibrium. Currently, the amount of methane emitted into the atmosphere is greater than the total removed, and observed atmospheric increase of methane is currently 37Mt/a (2.5Mt/a). Consequently, atmospheric methane concentrations are increasing. Figure 6.5 shows how methane has increased in the atmosphere during the last two hundred years. A similar exponential growth to that seen in the CO2 record is clearly noticeable.
Average global atmospheric methane concentrations have increased from about 730ppbv to 1,700ppbv, representing a 130% increase (Machida et al., 1994). A hemispheric gradient exists, with concentrations in the Northern Hemisphere being about 6% higher than those south of the equator, since most of the principal (land-based) sources are within the Northern Hemisphere (Nakazawa et al., 1993).