The 1976 Reports of the National Academy of Sciences on the Chlorofluorocarbon-Ozone Problem 770020

Two NAS reports, both released on September 13,1976, will be discussed. They are entitled “Halocarbons: Effects on Stratospheric Ozone” issued by the Panel on Atmospheric Chemistry of the Committee on Impacts of Stratospheric Change (CISC); and “Halocarbons: Environmental Effects of Chlorofluoromethane Release” issued by the parent CISC.

The Panel report deals mainly with the chemistry, transport, modeling, an measurement of the chlorofluoromethanes (CFM's) released into the atmosphere, particularly F-1l (CFCl₃) and F-12 (CF₂Cl₂) their photolysis products and subsequent reactions with stratospheric constituents which are thought to lead to ozone depletion. It presents in detail what was known up to the time of its publication about the world-wide production and release of various halocarbons, their possible removal processes in the troposphere (no appreciable ones found), transport to the stratosphere, photolysis to liberate chlorine atoms, and their catalytic destruction of ozone by chemical reactions which are investigated in laboratory experiments.

All of this is combined in computer modeling calculations which predict that, at constant, continuing 1973 release rates a steady state would be slowly approached in which our ozone shield would be reduced by about 7% with an overall uncertainty of about a factor of three upward or downward. Half of this reduction would be reached in roughly 50 years. Both magnitude and time history of the effect were found to be similar to those predicted earlier by Rowland and Molina. A substantial amount of supporting evidence was found and cited, but several further laboratory and field measurements are under way which will clarify the problem and reduce its uncertainties.

The parent CISC report builds on the results of the Panel report, considers the wider implications of ozone reduction and of the direct infrared “greenhouse” effect due to CFM's on climate, non-human biology, and medicine (skin cancer). It weighs the available knowledge on both the ozone perturbation and its consequences, as well as the cost of delaying a decision to regulate in terms of increased ozone reduction which it probably incurs (about 0.1% further reduction for each additional year). It strongly argues the case for selective regulation of uses and releases, treating different uses differently, if and when such regulation is required. The case for such regulation seems strong now, but, in view of certain inadequacies in the bases of the calculations and of current research programs which will remove some of these inadequacies, CISC recommends against immediate regulation, but allows no more than two years for this further evaluation process. It also calls for informative labeling (not as a substitute for regulation) and for readying legislative authority to deal with all facets of such global problems.

The two summary chapters of the NAS reports are reproduced below in their entirety: The Executive Summary of the parent CISC report and Chapter 5, Expected Changes in Ozone-- Findings, of that same report which is also Chapter 1, Findings, of the Panel report. Although these chapters are taken from the preliminary copies released on September 13,1976, the final versions, to be published in January 1977, will differ from the earlier ones only in minor, editorial detail.