CIESIN Reproduced, with permission, from: Tolba, M. K., O. A. El-Kholy, E. El-Hinnawi, M. W. Holdgate, D. F. McMichael, and R. E. Munn, eds. 1992. Ozone depletion. Chapter 2 in The world environment 1972-1992. New York: Chapman and Hall.

Scientific milestones: 1970 to present

1970-74 A number of papers published hypothesizing that nitrogen oxides from supersonic aircraft, and chlorine from solid rocket fuels, could catalytically reduce the level of stratospheric ozone.

1974 First postulate that CFCs would result in the catalytic destruction of stratospheric ozone.

1975 Observations of the vertical distribution of CFCs demonstrate that CFCs are removed in the stratosphere.

1977 Observations of ClO in the stratosphere demonstrate the presence of the key chlorine species responsible for the catalytic removal of ozone.

1983 Multi-year observations of CFCs 11 and 12 demonstrate that they are long-lived compounds with no significant tropospheric removal processes.

1984 First observations of the vertical extent of depletion of ozone above Antarctica based on ozone-sonde data from Syowa indicating that ozone is being depleted in the lower stratosphere between 13km and 20km altitude.

1985 Two-dimensional models are developed and used for assessment purposes to predict that CFC-induced depletion of ozone will be greatest at high altitudes (consistent with the predictions from one-dimensional models) and high latitudes.

1985 First report of the Antarctic ozone hole based on observations of total-column ozone using a ground-based Dobson spectrophotometer at Halley Bay.

1986 Spatial and temporal extent of Antarctic ozone hole established using satellite observations.

1986 Ground-based Antarctic ozone expedition (NOZE I) demonstrates that the chemical composition of the Antarctic stratosphere is highly perturbed, lending credence to the hypothesis that the Antarctic ozone hole is caused by CFCs.

1987 Aircraft campaign establishes that the Antarctic ozone hole is caused by CFCs.

1988 The International Ozone Trends Panel re-analysis of ground-based total-column ozone data shows a statistically significant decrease in ozone between 30deg.N and 60deg.N from 1969 to 1986 that is greatest in winter and that cannot be accounted for by any known natural phenomena.

1989 Aircraft campaign demonstrates that the chemical composition of the Arctic stratosphere is highly perturbed during late winter and is primed for ozone depletion.

1991 Satellite data demonstrate that ozone is being depleted at all latitudes throughout the year between 30deg.S and the South Pole, and from November to May between 30deg.N and the North Pole.

1991 Satellite (SAGE I and II), Umkehr, and balloonsonde observations of the vertical distribution of ozone have shown that ozone has decreased between 15km and 20km altitude during the last decade or so.

1992 Satellite and aircraft observations show elevated levels of stratospheric concentrations of ClO. particularly north of 50deg.N. At the same time, ozone depletions of 50% are found at a height of 21km in the tropics from 10deg.S to 20deg.N (due to volcanic aerosols from Mount Pinatubo).