Simulation of the Effect of an Increase in Methane on Air Temperature

The infrared radiative effect of methane was analyzed using the 2D,interactive chemical dynamical radiative SOCRATES model of the NationalCenter for Atmospheric Research. Then, a sensitivity experiment, with themethane volume mixing ratio increased by 10%, was carried out to studythe influence of an increase of methane on air temperature. The resultsshowed that methane has a heating effect through the infrared radiativeprocess in the troposphere and a cooling effect in the stratosphere.However, the cooling effect of the methane is much smaller than that ofwater vapor in the stratosphere and is negligible in the mesosphere. Thesimulation results also showed that when methane concentration is increasedby 10%, the air temperature lowers in the stratosphere and mesosphere andincreases in the troposphere. The cooling can reach 0.2 K at the stratopauseand can vary from 0.2--0.4 K in the mesosphere, and the temperature risevaries by around 0.001--0.002 K in the troposphere. The cooling results fromthe increase of the infrared radiative cooling rate caused by increasedwater vapor and O₃ concentration, which are stimulated by the increasein methane in most of the stratosphere. The infrared radiation cooling ofmethane itself is minor. The depletion of O₃ stimulated by the methaneincrease results indirectly in a decrease in the rate of solar radiationheating, producing cooling in the stratopause and mesosphere. Thetropospheric warming is mainly caused by the increase of methane, whichproduces infrared radiative heating. The increase in H₂O and O₃ caused by the methane increase also contributes to a rise in temperature in the troposphere.