There is an oft quoted statement that the Antarctic ozone hole was discovered in 1956 and therefore it can't be caused by CFCs. This remark originates from a paper by Professor G M B Dobson, the scientist who designed the ozone spectrophotometer which has been the standard for ozone measurements since the 1930s. The big advantage in standardising on one make of instrument is that we can be certain that changes in ozone amount that are measured, are changes in the atmosphere rather than changes due to observational technique. The following is taken from Dobson's paper in Applied Optics, March 1968, Vol 7, No3.
'One of the more interesting results on atmospheric ozone which came out of the IGY (International Geophysical Year) was the discovery of the peculiar annual variation of ozone at Halley Bay (76 south, 26 west). The annual variation of ozone at Spitzbergen was fairly well known at that time, so, assuming a six months difference, we knew what to expect. However, when the monthly telegrams from Halley Bay began to arrive and were plotted alongside the Spitzbergen curve, the values for September and October 1956 were about 150 units lower than was expected. We naturally thought that Evans has made some large mistake or that, in spite of checking just before leaving England, the instrument had developed some fault. In November the ozone values suddenly jumped up to those expected from the Spitzbergen results. It was not until a year later, when the same type of annual variation was repeated, that we realized that the early results were indeed correct and that Halley Bay showed a most interesting difference from other parts of the world. It was clear that the winter vortex over the South Pole was maintained late into the spring and that this kept the ozone values low. When it suddenly broke up in November both the ozone values and the stratosphere temperatures suddenly rose.'
This table shows the difference between what Dobson expected from Spitzbergen, the normal values observed at Halley between 1956 and 1975 and the values presently observed. Mean October ozone values have fallen by around 3% per year since 1976, while the amount of chlorine has risen by 3% per year.
Spitz Feb Mar Apr May Jun Jul Aug Sep Oct 1956 440 470 450 400 350 320 300 280 280 Halley Aug Sep Oct Nov Dec Jan Feb Mar Apr 1956 300 300 300 330 350 320 300 280 280 1996 172 155 149 181 260 278 265 245 242
The Antarctic ozone hole is the depletion in the spring over and above that caused by the different atmospheric circulations in the two hemispheres. Signs can be seen in data from 1976 when you know what to look for, but suspicion didn't really arise until the end of the decade and the paper announcing the discovery of ozone loss in the Antarctic was not published until 1985. When American satellite data was reanalysed it became apparent that it was a phenomena that covered the whole of the Antarctic and it was given the name ozone hole. The latest data also show ozone depletion during the summer and autumn months, in addition to the spring- time 'hole'.
There are suggestions that observations made in 1958 at the French Antarctic station of Dumont d'Urville show significant ozone depletion. These measurements were made using a photographic spectrometer and are subject to large errors, in the region of 20 - 50%. The observations disagree with measurements made by Dobson spectrophotometer at the same time. There was also no correlation between the photographic measurements and stratospheric conditions and all they show is random scatter.