S. Fred Singer, Ph.D.

The evidence from ground stations and satellites is not yet compelling to identify a long-term trend of anthropogenic ozone depletion. UN reports [1] do not squarely address problems of data contamination and instrument calibration [2]. A separate issue, inadequately discussed, is the shortness of the global record, making the removal of the natural variations a daunting task [2]. Another fact is the absence of any credible evidence for a corresponding long-term upward trend of solar UV radiation at the earth's surface. The widely publicized claim for such a trend [3] is acknowledged to be spurious and based on a faulty statistical analysis [4].

Yet, ozone depletion is entirely plausible and to be expected. But chlorine (from CFCs) may not be a major destroyer of ozone molecules in the lower stratosphere where most of the ozone resides. Important clues are provided by the existence of the Antarctic ozone hole and the pronounced ozone depletion caused by the Pinatubo eruption; they indicate that the presence of particles is essential [5]--something not anticipated nor predicted by the CFC-ozone theory of Rowland-Molina, which applies more appropriately to the upper stratosphere. Another clue comes from the absence of ozone destruction in the middle stratosphere [6]; it suggests that global depletion of ozone is related to the increasing levels of lower-stratospheric sulfate aerosols [7]. In addition, there is evidence, both from theory [8] and from observations [9], that radicals derived from water vapor are the most effective ozone destroyers in the lower stratosphere; their concentration may be rate-limiting. But stratospheric WV is known to be rising [10], likely because of the increasing production of methane by human activities [11].

Could manmade sources of methane, like cattle raising and rice growing, be more damaging to the ozone layer than CFCs? Has the growth of commercial air traffic become an important factor in ozone depletion? Is there any observational evidence for an upward trend in stratospheric bromine compounds? These are questions that need to be debated before putting into effect far-reaching policies to ban all halocarbon chemicals.

1. Scientific Assessment of Ozone Depletion:1994, WMO Report No.37, p. 1.14 and p.1.19
2. S.F. Singer "Ozone Depletion" Chem Engr News 71, 2 (12 July 1993); -----"Ozone Depletion Theory" Science 261, 1101-2, 1993
3. J.B. Kerr and C.T. McElroy, "Evidence for Large Upward Trends of Ultraviolet-B Radiation Linked to Ozone Depletion," Science 262, 1032-4, 1993
4. P.J. Michaels, S.F. Singer, P.C. Knappenberger, "Analyzing UV-B radiation: Is there a trend?" Science 264, 1341-2, 1994
5. Scientific Assessment [1], pp. 3.10-3.27
6. Scientific Assessment [1], p. 1.37
7. D.J. Hofmann, "Increase in the Stratospheric Background Sulfuric Acid Aerosol Mass in the Past 10 Years" Science 248, 996-1000, 1990
8. Scientific Assessment [1], pp. 4.16-4.17
9. P.O. Wennberg et al, "Removal of Stratospheric O3 by Radicals: In Situ Measurements of OH, HO2, NO, NO2, ClO, and BrO" Science 266, 398-404, 1994
10. S.J. Oltmans and P.J. Hofmann, "Increase in Stratospheric Water Vapor at a Midlatitude Northern Hemisphere Site from 1981-1994" Nature 374, 146-9, 1995
11. S.F. Singer, "Stratospheric Water Vapour Increase Due to Human Activities" Nature 223, 543-7, 1971; also Scientific Assessment [1], p. xxii
Keywords: ozone depletion, stratosphere, CFC, methane, water vapor, bromine