The Effects of Chlorofluoromethanes on Climate

Abstract

The chlorofluoromethanes, CFCl₃, CF₂Cl₂ are chemically quite inert. Their atmospheric destruction is slow and takes place essentially in the stratosphere. With present release rates the tropospheric CFM concentration is expected to increase until a steady state of a 2.1 ppb is reached. Their climatic impact proceeds through a variety of pathways which makes an evaluation more complex and more uncertain than for other trace gases. The CFM have strong infrared absorption bands between 9 to 12 urn, wavelengths at which the atmosphere is relatively transparent. Thus their direct contribution to the greenhouse effect will be relatively large. One-dimensional radiative convective models predict an eventual increase in the surface temperature of about 0.4° C.

Other climatic influences are caused by the CFM induced depletion of the stratospheric ozone layer. Calculations with a one dimensional chemical model predict that stratospheric O₃ concentrations will be depleted by an average of 15% – 20%, if the present rates of release of CFM are continued. The depletion of stratospheric O₃ is expected to have the following consequences for the radiation budget and the surface temperature of the earth: (1) A smaller fraction of the solar visible and UV radiation is absorbed in the stratosphere and the stratosphere cools. (2) More solar short wave radiation reaches the earth’s surface; the surface temperature increases. (3) Because of the decrease in stratospheric O₃ concentration and temperature, less infrared radiation is emitted to the troposphere; the surface temperature decreases. Effects (2) and (3) nearly cancel leaving a small net cooling of the earth’s surface, (4) The change in stratospheric temperature changes the stratospheric density structure and thus influences the stratospheric circulation.

The overall effect, direct and O₃ mediated, on the surface temperature is a small warming of 0.15 K with an uncertainty rage of 0-0.6 K.