Trace gas trends and their potential role in climate change

Abstract

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 CO 2 and O 3. 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 CO 2 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 CO 2 are shown to be potentially as important as CO 2 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 CO 2 increase from preindustrial to the present causes an equilibrium warming of the model surface by 0.5 K, which is amplified by 50% by CH 4, CFCl 3 (F11), CF 2 Cl 2 (F12), and tropospheric ozone. For the projected increase from year 1980 to 2030, the other trace gases amplify the estimated CO 2 warming of 0.7 K by about 110%: CFCl 3, CF 2 Cl 2, ozone, and CH 4 each contribute in the 0.1–0.2 K range followed by N 2 O, CHClF 2 (F22), CH 3 CCl 3, and CCl 4 in the 0.03–0.1 K range. Finally, on a per ppb basis, about 12 trace gases are identified to be important: CBrF 3, C 2 F 6 (F116), CHF 3, and CF 3 Cl (F13) have greenhouse effects comparable to those of CFCl 3 (F11) and CF 2 Cl 2 (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 CFCl 3 and CF 2 Cl 2. For the projected trace gas increase, we compute the stratospheric O 3 change by employing a photochemical model coupled to the radiative‐convective model. The O 3 change cools the stratosphere and the magnitude of the cooling is as large as that due to the projected CO 2 increase. Because of the O 3‐induced stratospheric cooling and the surface warming due to the greenhouse effect, the trace gas effects on climate are virtually indistinguishable from those of CO 2.