[9] We further consider two potentially important natural sources of atmospheric CH 4 in relation to the adopted scenarios: (1) Conversion of organic carbon to CH 4 , and release when permafrost thaws; and (2) release of CH 4 hydrates in marine sediments. Earlier studies have demonstrated that large releases of CH 4 from natural sources during warming events can have significant impacts on atmospheric CH 4 levels and may have potential synergistic effects leading to increased and/or sustained global warming. Observed surface temperatures in recent years show significant warming, indicating Arctic warming of more than a factor 2 greater than the global mean value [ Hansen et al. , 2007 ]. Permafrost thawing could be more extensive than previously predicted [ Camill , 2005 ; Osterkamp , 2005 ], with large potential for methane emission.

[8] In this study we use a global Chemical Transport Model (CTM), the Oslo CTM2 [ Isaksen et al. , 2005 ; Søvde et al. , 2008 ], to estimate the impact of additional CH 4 emissions on the atmospheric concentrations of the climate gases CH 4 , O 3 , stratospheric H 2 O, and CO 2 , and on RF from these forcing agents. The study covers a wide range of hypothetical methane emission scenarios, up to about 5 times the current emission rate. Although there is no evidence supporting the higher emission in this range, we include them in order to demonstrate the particularly strong positive feedback in the chemical system from large methane releases and the general impact on atmospheric composition and on climate forcing.

[7] Increased CH 4 emissions affect climate in several ways: Directly through increased CH 4 concentrations and indirectly through the chemical feedback on CH 4 levels and through production of O 3 and stratospheric H 2 O. Furthermore, CO 2 will increase since it is the end product of atmospheric CH 4 oxidation. In the current atmosphere the indirect RF is approximately the same as the direct methane RF, taking into account the effect on its own lifetime, on ozone, and on stratospheric water vapor [ Forster et al. , 2007 ].

[5] A small fraction is also removed by surface deposition. In the stratosphere, where water vapor is in the range of only a few ppm, CH 4 oxidation contributes to water vapor buildup. Since reaction (R1) also represents a significant loss path for OH, additional CH 4 emission will suppress OH and thereby increase the CH 4 lifetime, implying further increases in atmospheric CH 4 concentrations [ Isaksen and Hov , 1987 ; Prather et al. , 2001 ]. This represents a positive chemical feedback, with a feedback factor estimated to be about 1.4 (uncertainty range 1.3 to 1.7) for current atmospheric conditions [ Prather et al. , 2001 ]. The nonlinearity in the chemical system could result in a significantly enhanced feedback factor for large CH 4 emissions causing large perturbations [ Isaksen , 1988 ].

[2] Methane (CH 4 ) is an important greenhouse gas with a radiative forcing (RF) of 0.48 Wm −2 , due to anthropogenic activity since preindustrial time [ Forster et al. , 2007 ], being second only to CO 2 among the anthropogenic greenhouse gases. Its distribution and growth are well documented [ Forster et al. , 2007 ; Ramaswamy et al. , 2001 ] showing a significant increase in atmospheric concentrations since preindustrial times. Analyses of ice core data for the last 650,000 years show that atmospheric CH 4 concentrations varied from approximately 400 ppb during glacial periods to approximately 700 ppb during interglacial periods. The tropospheric average concentration is currently about 1,800 ppb, representing an approximate 2.5 increase since preindustrial time. The atmospheric concentrations in 2005 correspond to an atmospheric burden of 4,900 Tg CH 4 (1 Tg = 10 12 g). Observations since 1984, for which there are continuous measurements, show an increase in atmospheric abundances of CH 4 by about 10%. Growth rates have decreased significantly since the early 1990s, but with pronounced interannual variations [ Rigby et al. , 2008 ].

2. CH 4 Emissions From the Arctic Region

[12] We consider two major sources of CH 4 emissions from the warming Arctic: (1) Methane produced from microbial degradation of labile organic carbon that becomes bioavailable as permafrost thaws; and (2) methane released from gas hydrate deposits as they dissociate in response to climate warming. Thawing permafrost may also promote emissions from other methane sources in the Arctic, but the amount of methane that could potentially be produced by microbial processes in thawed soils or release of methane from gas hydrates far exceeds that associated with other Arctic sources. There is evidence that continuous permafrost is actively thawing in many circum‐Arctic regions, both onshore and in the shallow offshore continental shelves [Rachold et al., 2007].