25 ppmv of CO2 equivalent (see Figure 2.1). From about 1998 to 2007, methane concentrations remained nearly constant (Forster et al., 2007). However, methane began to increase after 2007. In the absence of mitigation, methane is expected to continue to make significant contributions to climate change during the 21st century (see Section 2.2).
FIGURE 2.1 (left) Best estimates and very likely uncertainty ranges for aerosols and gas contributions to CO2-equivalent concentrations for 2005, based on the radiative forcing given in Forster et al. (2007). All major gases contributing more than 0.15 W m–2 are shown. Halocarbons including chlorofluorocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, and perfluorocarbons have been grouped. Direct effects of all aerosols have been grouped together with their indirect effects on clouds. (right) Total CO2-equivalent concentrations in 2005 for CO2 only, for CO2 plus all gases, and for CO2 plus gases plus aerosols.