Cause-and-effect analysis of the photochemical interactions among CH₄ CO, O₃ and OH in the global troposphere

The Cause-and-Effect Analysis (CEA) technique is used to estimate changes in the equilibrium of a simplified system of interactions for global tropospheric methane due to changes in external forcing. This system consists of methane, carbon monoxide, tropospheric ozone, stratospheric ozone, total ozone, hydroxyl, water vapor and temperature. The CEA technique allows determination of all the loops in the system and the all pathways from the system's external parameters to all variables of the system. The values of the system's links are estimated for annually averaged conditions. It is found that the value of the net loop effect of the system, D, which is a measure of the system's sensitivity to changes in the external forcing, is less than unity and depends predominately on the interactions between temperature and water vapor, and on the links that define the influences of methane and carbon monoxide on hydroxyl. The anthropogenic sources of methane and carbon dioxide are considered as the external forcing to the system. It is shown how the system generates: (1) negative sensitivities of tropospheric methane and carbon monoxide to an increase in carbon dioxide and a positive sensitivity to an increase in the anthropogenic methane source, (2) negative sensitivity of tropospheric ozone to an increase in the carbon dioxide and a positive sensitivity to an increase in the anthropogenic methane source, and (3) negative sensitivity of hydroxyl to an increase in the anthropogenic methane source.