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.


Ozone depletion

The issue

Stratospheric ozone depletion has been a major environmental issue of the last two decades--first as an interesting hypothesis following publication of the seminal paper by Molina and Rowland (1974), and then as a matter of urgency and intergovernmental action following the discovery of the ozone 'hole' in the Antarctic stratosphere in 1984 (Farman et al., 1985). The primary concern regarding ozone depletion is that a decrease in the total column content of ozone leads to an increase in the amount of UV-B radiation reaching the Earth's surface, with adverse effects on human health and ecosystems (Box 1) (UNEP, 1989). Ozone depletion may also contribute to changes in the Earth's climate (see Chapter 3).

While the largest ozone depletion is occurring at high latitudes in both hemispheres, it is happening everywhere except the tropics, and enhanced levels of UV-B radiation will have adverse effects on people of all nations, independent of their geographical position or economic status. Peoples with lightly pigmented skins are most susceptible to melanoma and non-melanoma skin cancer, but all peoples are at risk of contracting eye disorders and suppression of the immune response system. Societies in developing countries with inadequate health services are at greatest risk. Because of the impact of UV-B radiation on some plants, and hence on ecosystem functioning, ozone depletion will also reduce agricultural and fisheries productivity in the long term, and again the people most likely to be affected are those living where shortages of food already exist.

The considerable advancement in scientific knowledge since 1970 (Box 2) has been accompanied by similar advances in national and international regulatory action (Box 3). Within these past 70 years, three separate time intervals can be identified: (i) 1974 to mid-1987, including the periods during which the Vienna Convention for the Protection of the Ozone Layer (1985) and the Montreal Protocol on Substances that Deplete the Ozone Layer (1987) were negotiated; (ii) mid-1987 to mid-1990. when the London amendments to the Montreal Protocol were negotiated; and (iii) mid-1990 to the present. (Current research and monitoring studies are establishing the scientific basis for further amendments to the Montreal Protocol to be considered at the fourth meeting of the Contracting Parties in November 1992.)

Chemical processes controlling stratospheric ozone

About 90 per cent of the Earth's protective ozone layer resides in the stratosphere between 15km and 50km altitude (Figure 1). Molecular oxygen is broken down in the stratosphere by solar radiation to yield atomic oxygen, which then combines with molecular oxygen to produce ozone. Ozone is destroyed naturally through a series of catalytic cycles involving oxygen, nitrogen, hydrogen and to a lesser extent chlorine and bromine species. The abundance of stratospheric ozone is therefore chemically controlled by the stratospheric abundances of compounds containing hydrogen, nitrogen, chlorine and bromine. Increases in the abundances of methane and nitrous oxide (sources of hydrogen and nitrogen oxides respectively) thus affect the abundance and distribution of stratospheric ozone. Stratospheric ozone is also affected by the abundance of carbon dioxide (CO2), because the rates of the chemical reactions that control the abundance of ozone are temperature-dependent, and the abundance of CO2 plays a key role in determining the temperature structure of the stratosphere.

During the 1970s and early 1980s, theoretical model calculations focused on predicting the response of stratospheric ozone to changes in chlorine, assuming that the atmospheric abundances of other trace gases remained constant. However, since the early 1980s, with advances in understanding of trace gas trends and model formulation, model calculations have been used to predict the response of stratospheric ozone to simultaneous increases in chlorine (from chlorofluorocarbons, hydrochlorofluorocarbons, carbon tetrachloride, methylchloroform and methyl chloride) and bromine (from halons and methyl bromide), as well as methane, nitrous oxide and carbon dioxide.

Trends in ozone depletion and scientific understanding

Trends in chlorofluorocarbons (CFCs)

Table 1 shows the current atmospheric concentrations and trends in a number of important halocarbons, including CFCs 11, 12 and 113.

Figure 2 shows that the estimated global production of chlorofluorocarbons (primarily CFCs 11 and 12) increased during the 1960s and early 1970s, stabilized during the mid-1970s and early 1980s due to national regulations on the use of these gases as aerosol propellants and a world-wide economic recession. and increased during the mid to late 1980s as economies improved. Figure 2 also shows how the uses of CFCs have changed considerably from 1974, when aerosol propellants accounted for almost 70 per cent of the market, to 1988 when refrigerants and foam-blowing agents accounted for about 60 per cent of the market. Figure 3 shows how the atmospheric abundance of CFC11 (CC13F) has continued to increase over the last twenty years at about 4-5 per cent per year, even though emission rates have not continued to increase. Figure 4 shows the estimated consumption of CFCs in 1986 by geographic region.

Model calculations 1970-87

Photochemical models are used to predict the extent of ozone depletion for a variety of assumptions concerning future emission rates of halocarbons, methane, nitrous oxide and in some cases carbon dioxide. All models predict that total-column ozone depletion would be greatest at high latitudes, and that these changes would be greatest in the upper stratosphere at altitudes of around 40km. Models predict that the abundance of ozone: (i) decreases with increasing concentrations of chlorine, bromine and nitrous oxide, and (ii) increases with increasing atmospheric abundances of methane and carbon dioxide. Figure 5 shows a typical model calculation of the predicted change in the vertical distribution of ozone between 1985 and 2040 assuming constant emissions of CFCs (1985 emission rates), but growth in the emission rates of methane (1 per cent per year) and nitrous oxide (0.2 per cent per year). Scenarios that assume growth in the emissions of CFCs predict greater depletions of ozone.

Ozone observations 1970-87

Statistical analyses of ground-based total-column ozone measurements show that between 1970 and 1980 there was no statistically significant decrease in the annual global average content of ozone (i.e. the decrease was less than 1 per cent). Model calculations, taking into account increases in carbon dioxide, methane, nitrous oxide and halocarbons, predicted that total-column ozone should have decreased by less than 0.5 per cent at all latitudes in the summer and between 0.5 and 1 per cent in the winter, with the largest depletions predicted at high latitudes. Thus the model results are not inconsistent with the observations.

Estimates of trends in the vertical distribution of ozone, using ground-based data, suggest that ozone decreased in the middle and upper stratosphere by 23 per cent in the period 1970 to 1980. While this data base is limited, both in number of stations and data quality, the trends are consistent with model predictions.

In 1985 the Antarctic ozone hole was first reported (Farman et al., 1985). Observations of total-column ozone, using a ground-based spectrophotometer at Halley Bay, showed that the October level of ozone decreased from a value of typically 300 Dobson Units (DU) during the 1960s and early 1970s to about 200 DU in the early 1980s (Figure 6). This decrease of about 30-40 per cent in a decade was totally unexpected. Satellite data (Plates I and II. between pages 52 and 53) showed the spatial extent of the depletion. and balloonsonde data (Figure 7) showed that it occurred in the lower stratosphere. By 1986 three mechanisms (two involving changes in natural processes (1 and 2) and one involving human activities (3)) had been proposed to explain the observations of ozone loss:

  1. solar cycle mechanism: periodic increases in the abundance of nitrogen oxides in the lower Antarctic stratosphere due to variations in solar output:

  2. dynamical mechanism: a change in the circulation pattern from downwelling of ozone-rich air from the upper stratosphere to upwelling of ozone-poor air from the troposphere; and

  3. halogen mechanisms: several different theories invoking the catalytic destruction of ozone due to CFCs and halons.

A ground-based study based at McMurdo, Antarctica, in 1986 showed that:

  1. the abundances of nitrogen oxides were exceptionally low, which largely disproved the solar cycle mechanism:

  2. the levels of long-lived tracers were such that a dynamical mechanism was unlikely: and

  3. the elevated levels of active chlorine species suggested that chlorine catalysis was the most probable mechanism.

At the time that the Montreal Protocol was signed in September 1987, the cause of the Antarctic ozone hole was not well established. However, the scientific synthesis upon which the Protocol was based provided convincing evidence that the stratosphere had become perturbed:

Advances in scientific understanding 1987-92

There have been tremendous advances in scientific understanding of the ozone layer in the last five years. These advances, primarily based on laboratory studies, special field campaigns, theoretical studies and a re-assessment of trends in stratospheric ozone, were reviewed during the 1989 and 1991 international scientific assessments, which were required by the Montreal Protocol. The key conclusions of these two international assessments are summarized in Boxes 4 and 5. Scientific understanding of the environmental impacts of ozone layer depletion were also reviewed at that time (Box 1).

Recent special field campaigns

A scientific campaign mounted from Punta Arenas, Chile, during August and September of 1987 utilizing aircraft flights over Antarctica provided good scientific evidence that anthropogenic chlorine, and to a lesser extent bromine, were the primary cause of the observed Antarctic ozone hole. Figure 8 shows that the abundance of the ClO radical, an active chlorine species. increases significantly from about 50 pptv outside the seasonal meteorological vortex, which is located between about 65deg.S and 68deg.S, to between approximately 0.75 and 1.0 ppbv within the vortex. Figure 8 also shows how a strong anticorrelation developed between ClO and ozone from 23 August to 16 September 1987. These data, combined with the observations of: (i) the BrO radical: (ii) low abundances of water vapour, nitrogen oxides and long-lived tracers such as CFCs and nitrous oxide; and (iii) polar stratospheric clouds (PSCs), demonstrated that the ozone loss over Antarctica is initiated by chemical reactions that occur on PSCs and that convert the long-lived chlorine into chemically more-reactive forms, which in the presence of sunlight, leads to a catalytic destruction of ozone of up to 1-2 per cent per day in total column content.

A second Airborne Arctic Stratospheric Expedition (AASE-II) was launched in early October 1991. In the course of this six-month field experiment, the highest levels of chlorine monoxide (ClO) ever found in the polar stratosphere were recorded over eastern Canada and northern New England in mid to late January 1992 (1.5 ppbv) (NASA, 1992). During this period, the NASA Upper Atmosphere Research Satellite (UARS) also detected elevated levels of ClO over large parts of Europe and Asia north of 50deg.N. The observations of high concentrations of ClO and also of bromine monoxide imply human-induced ozone destruction rates of 1-2 per cent per day,

At the same time, satellite measurements of ozone levels at a height of about 21km in the tropics (from 10deg.S to 20deg.N) were about 50 per cent less than usual due to the eruption of Mount Pinatubo The presence of enhanced stratospheric concentrations of volcanic aerosols is likely to cause ozone depletion in the tropics for several years (NASA, 1992).

A similar campaign using aircraft based at Stavanger, Norway, between January and March 1989, provided compelling scientific evidence that the chemical composition of the Arctic stratosphere was highly perturbed. The chemical perturbations were similar to those found in Antarctica, namely an increase in the abundance of the ozone-depleting forms of chlorine in association with PSCs. Reactive chlorine abundances were enhanced by a factor of 50 to 100.

Recent assessments of trends in stratospheric ozone

In 1986 the International Ozone Trends Panel (IOTP) was established to analyse ground-based and satellite ozone data in terms of both total column content and vertical distribution. The most important aspect of the studies was that the ozone record was analysed for seasonal and latitudinal trends rather than for just global annual averaged trends. Some of the conclusions of the IOTP, and the more recent findings of Bishop and Bojkov (1992) and Bojkov et al. (1990) are summarized in Boxes 4 and 5. The key points included in the synthesis report of the assessment panels (November 1991) are:


Policy responses

Box 3 summarizes the main international policy responses in the 1970s and 1980s to the ozone layer issue. Established by UNEP in the 1970s, the Coordinating Committee on the ozone Layer (CCOL) has played a major role in establishing scientific research priorities, and in describing the implications of the latest scientific findings in terms understandable to policy-makers. In 1980, for example, the CCOL reported that continued releases of CFCs would eventually deplete the ozone layer, and that this could have serious impacts on the health of people and the biosphere (Holdgate et al., 1982). Subsequently, the CCOL synthesized the knowledge base to provide the scientific basis for the Montreal Protocol. The policy implications of the scientific findings of the last decade have been profound (see Box 6), leading, for example, to a significant revision of the control measures of the Montreal Protocol.

The most recent scientific findings also have substantial policy implications (Box 7) and could lead to further significant revisions in the control measures of the Montreal Protocol at the Fourth meeting of the Contracting Parties, which will be held in November 1992.

Using simulation models to explore policy options

Nowadays, simulation models are used more and more to explore policy options for resolving the ozone depletion issue. A particular example, performed in 1989, was a sensitivity study of atmospheric chlorine-loading scenarios for a range of global emissions of compounds currently regulated under the Montreal Protocol, and for possible new HCFC substitutes. The results are summarized in Figure 9, which shows that:

The study concluded that in order to minimize future ozone layer depletion and to facilitate the most rapid elimination of the Antarctic ozone hole (i.e. by about 2075):

Concluding remarks

Depletion of the stratospheric ozone layer by halogenated chemicals is a global problem. While the issue was primarily a matter of scientific curiosity during the 1970s and early 1980s, it has now become an urgent policy question for governments in both developed and developing countries. The weight of scientific evidence strongly indicates that chlorinated and brominated chemicals (largely man-made) are primarily responsible for the substantial decreases in stratospheric ozone (greater than 50 per cent in the total column) that occur over Antarctica every Southern Hemisphere springtime. In addition, satellite data supported by ground-based observations have demonstrated that ozone has been decreasing since the late 1960s at middle and high latitudes in both the northern and southern hemispheres, and that this decrease cannot be explained by known natural processes. Indeed, the conclusion of the 1991 International Scientific Assessment was that 'the weight of evidence suggests that the observed middle- and high-latitude ozone losses are largely due to chlorine and bromine'.

The current and historic use of CFCs and other such chemicals in developed countries is the primary cause of the problem. However, adequate protection of the ozone layer will require a full partnership between the developed countries, whose industries and consumers have caused the problem, and the developing countries, whose people wish to use these chemicals in refrigeration and other ways.

As in the case of atmospheric concentrations of lead, which are now declining, even in Greenland, as a result of tighter emission controls (Chapter 1), the stratosphere would gradually recover if emissions of CFCs and similar chemicals were to cease. Thus, the most urgent task is to strengthen the Montreal Protocol and to develop an international method of monitoring national programmes to control CFCs and related gases. However scientific research and monitoring should continue into the next century to provide a basis for better policies. The findings of the December 1991 Technology and Economic Assessment Panel (Box 8) indicate that effective action for an early phase-out is both technically and economically feasible.


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