Phanerozoic evolution of atmospheric methane

Abstract

A simple geochemical box model for the global cycle of methane (CH4) has been developed and applied to reconstruct the evolution of atmospheric CH4 over the entire Phanerozoic. According to the model, the partial pressure of atmospheric CH4 (pCH4) 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 CH4. Through its radiative forcing, CH4 heated the average global surface temperature by up to 1°C. The elevated pCH4 values during the Permian‐Carboniferous cold period may have moderated the temperature decline caused by the coeval drawdown of atmospheric CO2. Additional runs with a global carbon model indicate that the heating induced by elevated pCH4 favored the drawdown of atmospheric pCO2 via enhanced rates of silicate weathering. Simulations with a state‐of‐the‐art climate model reveal that the effects of atmospheric CH4 on average global surface temperature also depend on the partial pressures of CO2. The CH4 climate effect is amplified by high background levels of atmospheric CO2 such that a coeval increase in the partial pressure of both greenhouse gases has a much stronger climate effect than previously anticipated.