The Current and Future Environmental Role of Atmospheric Methane: Model Studies and Uncertainties

Abstract

Concern over increasing levels of methane in the atmosphere centers on its radiative and chemical properties. Methane absorbs terrestrial infrared radiation and contributes to the greenhouse effect. Effects on other greenhouse absorbers (e.g.,O₃, H₂O, and CO₂) as the result of its oxidation must also be considered. These indirect effects have made the quantification of the total climatic effects of chemically active gases, such as CH₄, much more difficult than if direct radiative effects are considered alone. The oxidation of methane also exerts a controlling influence on atmospheric OH levels and is a major source of carbon monoxide. The variations in OH induced by changing CH₄ levels feed back onto the lifetime of methane and the abundance of CO (Sze, 1977; Chameides et al., 1977). However, there is a shortage of intercompared model results documenting the effects of CH₄ and nonmethane hydrocarbon (NMHC) additions on tropospheric OH levels. Most analyses to date have relied on analyses of gas phase reaction sequences for methane oxidation (e.g., Crutzen, 1987, 1988), without considering the numerous feedbacks on atmospheric chemistry. More complete modeling studies are needed because OH levels also depend on the emissions of CO, NMHCs, and NOᵧ (NOₓ + NO₃ + 2N₂O₅ + CH₃CO₃NO₂ (PAN) + HNO₃ + HNO₄ + ClNO₃ + NO₃?), where NOₓ is NO + NO₂ and NOᵧ and Nₓ are interchangeable terms. Furthermore, analyses which simulate the role of climate in controlling CH₄ emissions from various natural sources (e.g., wetlands) are critical for attempting to predict the response of atmospheric methane levels to future climate change.