Phanerozoic evolution of atmospheric methane

A simple geochemical box model for the global cycle of methane (CH 4 ) has been developed and applied to reconstruct the evolution of atmospheric CH 4 over the entire Phanerozoic. According to the model, the partial pressure of atmospheric CH 4 ( p CH 4 ) increased up to approximately 10 ppmv during the Carboniferous coal swamp era. This implies a maximum radiative forcing of about 3.5 W m −2 via CH 4 . Through its radiative forcing, CH 4 heated the average global surface temperature by up to 1°C. The elevated p CH 4 values during the Permian‐Carboniferous cold period may have moderated the temperature decline caused by the coeval drawdown of atmospheric CO 2 . Additional runs with a global carbon model indicate that the heating induced by elevated p CH 4 favored the drawdown of atmospheric p CO 2 via enhanced rates of silicate weathering. Simulations with a state‐of‐the‐art climate model reveal that the effects of atmospheric CH 4 on average global surface temperature also depend on the partial pressures of CO 2 . The CH 4 climate effect is amplified by high background levels of atmospheric CO 2 such that a coeval increase in the partial pressure of both greenhouse gases has a much stronger climate effect than previously anticipated.