Trace gas trends and their potential role in climate change
This study examines the potential climatic effects of the radiatively active trace gases that have been detected in the atmosphere including chlorofluorocarbons, chlorocarbons, hydrocarbons, fluorinated and brominated species, and other compounds of nitrogen and sulfur, in addition to CO2 and O3. A one-dimensional radiative-convective model is used to estimate trace gas effects on atmospheric and surface temperatures for three cases: (1) modern day (1980) observed concentrations are adopted and their present trends are extrapolated 50 years into the future. These projections are based on analyses of observed trends and atmospheric residence times; (2) the preindustrial to present increase in CO2 and other trace gases are inferred from available observations; (3) a hypothetical increase of 0–1 ppbv is considered to provide insights into the radiative processes. Trace gases other than CO2 are shown to be potentially as important as CO2 for long-term climate trends. The relative importance of the 30 or so trace gases included in this study depends on the problem under consideration. The inferred CO2 increase from preindustrial to the present causes an equilibrium warming of the model surface by 0.5 K, which is amplified by 50% by CH4, CFCl3 (F11), CF2Cl2 (F12), and tropospheric ozone. For the projected increase from year 1980 to 2030, the other trace gases amplify the estimated CO2 warming of 0.7 K by about 110%: CFCl3, CF2Cl2, ozone, and CH4 each contribute in the 0.1–0.2 K range followed by N2O, CHClF2 (F22), CH3CCl3, and CCl4 in the 0.03–0.1 K range. Finally, on a per ppb basis, about 12 trace gases are identified to be important: CBrF3, C2F6 (F116), CHF3, and CF3Cl (F13) have greenhouse effects comparable to those of CFCl3 (F11) and CF2Cl2 (F12). The narrow-band overlap treatment and the accurate spectral and angular integration techniques employed in the present radiation model enable quantitative interpretation of the differences between various published estimates for the greenhouse effects of CFCl3 and CF2Cl2. For the projected trace gas increase, we compute the stratospheric O3 change by employing a photochemical model coupled to the radiative-convective model. The O3 change cools the stratosphere and the magnitude of the cooling is as large as that due to the projected CO2 increase. Because of the O3-induced stratospheric cooling and the surface warming due to the greenhouse effect, the trace gas effects on climate are virtually indistinguishable from those of CO2.