these sources are actually influenced to some degree by changes in land use. Recent measurements have suggested that plants and crops may also emit trace amounts of CH4 (Keppler et al., 2006), although the size of this source has been questioned (Dueck et al., 2007).
The atmospheric concentration of CH4 rose sharply through the late 1970s before starting to level off, ultimately reaching a relatively steady concentration of around 1775 ppb—which is more than two-and-a-half times its average preindustrial concentration—from 1999 to 2006 (Figure 6.5). There have been several theories proposed for the apparent leveling off of CH4 concentrations, including a decline in industrial emissions during the 1990s and a slowdown of natural wetland-related emissions (Dlugokencky et al., 2003). As discussed at the end of the chapter, there are also concerns that warming temperatures could lead to renewed rise in CH4 levels as a result of melting permafrost across the Arctic (Schuur et al., 2009) or, less likely, the destabilization of methane hydrates3 on the seafloor (Archer and Buffet, 2005; Overpeck and Cole, 2006). The causes of the recent uptick in concentrations in 2007 and 2008 are currently being studied (Dlugokencky et al., 2009).
Unlike CO2, which is only removed slowly from the atmosphere by processes at the land surface, the atmospheric concentration of CH4 is limited mainly by a chemical reaction in the atmosphere that yields CO2 and water vapor. As a result, molecules of CH4 spend on average less than 10 years in the atmosphere. However, CH4 is a much