Figure L4. High concentrations of CIO radical!» and the simultaneous rapid ozone destruction occur in winter when the temperature becomes very cold. Measurements by J. G. Anderson and coworkers (1989) of Harvard University.
April, when the destruction trend approached 1% per year. Ozone destruction took place in other seasons as well but at the lower rate of around 0.4% per year (Stolarski et al.T 1991). The declining trend during winter and spring has increased during the present decade over the middle- to high-latitude zone of the Northern Hemisphere. In particular, during the winters 1995/1996 and 1996/1997 similar ozone depletions to those observed over the Antarctic about 15 years earlier were seen (e.g., Müller et aL, 1997), It is therefore especially gratifying to note that since early 1986 international agreements have been in effect that forbid the production of CFCs and several other industrial CI- and Hr-containing gases in the developed world, with a decade's respite for the developing world. Hopefully this means that the damage to the ozone layer may not grow much worse in the future. However, even in the best of circumstances, full recovery of the ozone layer will be a slow process. It will take up to half a century before the ozone hole will disappear. The slowness of the repair process is due to the long average atmospheric decay times of the CFC gases, on the order of 50 years for CFC-11 and 110 years for CFC-12.
Nevertheless, as a result of these measures the worst effects on the biosphere have probably been prevented. Thus, estimates by Slaper et al. (1996) would have predicted a fourfold increase in the incidence of skin cancer during the next century if no regulatory measures had been taken. The effects on other parts of the biosphere are
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harder to quantify, but they could have been important. It would be very surprising if light-skinned people would be the only species to be so strongly damaged by increased UV-B radiation.
Although the ozone layer may thus be recovering, a recent study by Shindell et al. (1998) indicates that as a result of the cooling of the stratosphere due to increasing CO2 levels, it may be possible that for another two decades stratospheric ozone in both hemispheres may further decrease, an effect that could be especially pronounced over the Arctic, maybe leading there to ozone columns similarly low as over Antarctica. In fact, Waibel et al. (1999) point to the possibility that, because of cooling of the stratosphere by increasing CO>, even by the third quarter of 2000 severe ozone depletion may still occur.
Humankind is having a considerable influence on the condition in the atmosphere, even in areas that are very far removed from the pollution sources. Most surprisingly and unexpectedly, over Antarctica during September-October, enormous damage is done to the ozone layer due to a remarkable combination of feedbacks: radiative cooling, giving very low winter and springtime temperatures, and the presence of chlorine gases in the stratosphere at concentrations about six times greater than that of the natural background provided by CH3CI. The cold temperatures promote the formation of solid or supercooled liquid polar stratospheric cloud particles consisting of a mixture of H2SO4, UNO;, and HiO, on whose surfaces, or within w hich, reactions take place that convert HC1 and CIONO? (which do not react with ozone) to highly reactive radicals CI and CIO. The latter rapidly remove ozone from the lower stratosphere by catalytic reactions. Nobody predicted this course of events. In fact, until the discovery of the ozone hole, it was generally believed that ozone at high latitudes could not be significantly affected at al! by chemical processes and was only subjected to transport. How w rong we all w ere. Exactly in the part of the stratosphere the farthest aw ay from the industrialized world, and exactly in that altitude region at which until about 1980 maximum concentrations of ozone had always been found, mainly during the month of September, all ozone is going to be destroyed for many more years to come, despite the international agreements that are now in place. This is due to a number of positive feedbacks. This ozone loss should be a warning. It can be difficult or impossible to predict precisely where the weak points in the environment are located. It is therefore important to w atch even for seemingly unlikely chains of positive feedbacks leading to major environmental impacts. Examples of potential instabilities were discussed at this workshop, including abrupt climate changes and a weakening of the Atlantic deep-water formation.
1.4 Epilog: And Things Could Have Been Much Worse
Gradually, if the studies by Shindell et al. (1998) and Waibel et al. (1999) turn out not to be valid, over a period of a century or so, stratospheric ozone should largely
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recover to its natural state. However, it was a close call. Had Farman and his colleagues not persevered in making their measurements in the harsh Antarctic environment for all those years since the International Geophysical Year 1958/1959, the discovery of the ozone hole might have been substantially delayed, and there might have been far less urgency to reach international agreement on the phasing out of CFC production. There might thus have been a substantial risk that an ozone hole could also have developed in the higher latitudes of the Northern Hemisphere.
Furthermore, whereas the establishment of an instability in the Oj/GOx system requires chlorine activation by heterogeneous reactions on solid or in supercooled liquid particles, this is not required for inorganic bromine, which, bccausc of gas-phase photochemical reactions, is normally largely present in its activated forms Hr and BrO. This makes bromine almost 100 times more dangerous for ozone than chlorine on an atom-to-atom basis. This brings up the nightmarish thought that if the chemical industry had developed organohromine compounds instead of the CFCs - or alternatively, if chlorine chemistry had behaved more like that of bromine - then without any preparedness, we would have been faced with a catastrophic ozone hole everywhere and in all seasons during the 1970s, probably before atmospheric chemists had developed the necessary knowledge to identify the problem and the appropriate techniques for the necessary critical measurements. Noting that nobody had worried about the atmospheric consequences of the release of CI or Hr before 1974,1 can only conclude that we have been extremely lucky. This shows that we should always be on our guard for the potential consequences of the release of new products into the environment. Continued surveillance of the composition of the stratosphere, therefore, remains a matter of high priority for many years to come,
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