Item 3 of the provisional agenda: Presentations By The Assessment Panels Environmental Effects Of Ozone Depletion
5 September 2000
TWELFTH MEETING OF THE PARTIES TO
THE MONTREAL PROTOCOL ON
SUBSTANCES THAT DEPLETE
THE OZONE LAYER
Ouagadougou, 11-14 December 2000
Item 3 of the provisional agenda
for the High-level Segment*
SUMMARY PRESENTATIONS BY THE ASSESSMENT PANELS
ENVIRONMENTAL EFFECTS OF OZONE DEPLETION
Note by the secretariat
The latest full assessment report on the environmental effects of ozone depletion is that of November 1998. An update on recent findings is annexed to this document. Special attention has been paid to interactions between ozone depletion and climate change. Potential interactions are indicated within the various sections.
ENVIRONMENTAL EFFECTS OF OZONE DEPLETION
Update on recent findings
Ozone and changes in ultraviolet radiation
- Globally integrated ozone loss seems to have levelled off, but total column ozone amounts remain low, especially at high latitudes during spring. Owing to uncertainties in instrument calibrations and natural year-to-year variability, we cannot yet be sure whether global ozone has reached a minimum. In the Antarctic, the deficit in ozone mass in the spring of 1999 was slightly smaller than the previous year, in which ozone loss was the largest on record. In the spring of 2000 the ozone hole developed extremely rapidly, and by late August had already reached an area comparable with the all-time maximum previously recorded. In the Arctic during the spring of 2000, stratospheric temperatures were particularly low and substantial ozone losses occurred. At some altitudes, the rates of ozone destruction were larger than had been seen previously. However, the vertical extent of depletion in the Arctic was slightly less than in the record-setting years of 1993, 1995 and 1996; and the peak integrated column ozone loss in 2000 was smaller than in those years.
- The prospects for ozone recovery remain uncertain. Atmospheric chlorine concentrations are beginning to decrease. However, with halogen loadings in the atmosphere currently still high, the ozone layer remains vulnerable to further depletion from events such as volcanic eruptions that inject material into the stratosphere. Interactions between global warming and ozone depletion could delay ozone recovery by more than a decade, and this topic remains an area of intense research interest. One numerical climate model predicted that there will be longitudinal differences in the patterns of recovery, with larger ozone depletions being expected in the Northern European sector. Even with the expected decreases in atmospheric chlorine, it will be decades before the ozone recovery can be unambiguously identified at individual locations. Locations with largest ozone depletion are not necessarily those where recovery will be seen earliest. On the basis of statistical considerations of ozone variability, southern mid-latitudes are the first location at which ozone recovery may be detectable.
- The quality and availability of ground-based ultraviolet (UV) measurements continue to improve, and recent studies have contributed to delineating regional and temporal differences due to aerosols, clouds and ozone. In Mexico City, poor air quality has been shown to be responsible for reductions of about 20% in erythemal (skin-reddening) UV in the city centre compared with the suburbs. In Moscow, changes in atmospheric opacity from clouds and/or aerosols were also probably responsible for a reported gradual decrease in UV from the 1960s to the mid-1980s, followed by an increase back to 1960s levels by the late 1990s. Measurements in Poland have revealed increases in erythemal UV of about 6% per decade over the period 1976 to 1997 due mainly to changes in ozone. Results from more recently established UV monitoring networks (e.g. in Europe and South America) are contributing to the characterization of geographical differences.
- There has been increased use of satellite data to estimate geographical variability and trends in UV. Satellite-derived estimates of surface UV radiation have been evaluated by means of comparisons with ground-based UV measurements. While basic geographical and seasonal patterns are generally in agreement, systematic differences of 10-20% are noted for mid-latitudes, with larger discrepancies at higher latitudes. In one study, satellite-derived estimates of erythemal UV in Australia showed larger increases in the tropics than at mid-latitudes over the period 1979-1992, owing to the combined influence of changes in ozone and clouds. Another study of satellite-derived erythemal UV trends in the northern hemisphere (1979-1991) showed marked regional differences: at latitudes 30-40 ºN, trends were larger over oceans, while at 40-60 ºN they were larger over continental areas; the largest trends were seen over north-east Asia, where they exceeded 10% per decade for May-August. Progress has been made in inferring historical levels of UV radiation using measurements of ozone from satellites in conjunction with measurements of total solar radiation obtained from extensive meteorological networks.
- The list of infectious diseases influenced by UV-B exposures continues to grow. UV-B irradiation of rodents has been shown to increase the lethality of both malaria and influenza virus infections as well as the severity of neurological symptoms following infection with herpes simplex virus.
- Ozone depletion and climate change may interact to enhance the spread and/or severity of a number of diseases. In principle, changes in climate due to global warming could lead to alterations in the distribution of insects and other vectors that carry human and animal pathogens. Such alterations, combined with increased UV-B-induced immunosuppression from ozone depletion, could increase the incidence and/or severity of the diseases these pathogens induce. However, epidemiological or experimental evidence to demonstrate that such interactions do influence these diseases in human populations is currently lacking.
- It has long been known that exposure to UV can suppress the effectiveness of immunization in humans; recent work now shows that, even after successful immunization, UV can still suppress these immune responses. In earlier studies, suppression was demonstrated only in individuals who were first exposed to UV and then immunized. Now it has been reported that UV irradiation (exposure to simulated sunlight) of already immunized individuals can also suppress the immune response. As a consequence, such individuals could become susceptible to reinfection.
- Additional genetic information directly links UV-B exposures to the development of basal cell carcinoma in humans. UV-B-specific changes have been found in genes recently identified as essential to the development of basal cell carcinoma. Although new genes important to the development of cutaneous melanoma have also recently been identified, evidence of UV-B-induced changes in these genes is lacking.
- Experimental results in newborn as well as adult opossums (South American marsupials) indicate that UV-B irradiation induces the formation of cutaneous malignant melanomas, whereas UV-A does not. These experiments are in contrast to earlier work in fish that showed UV-A was almost as effective as UV-B in inducing these tumours.
- Regular use of a broad-spectrum sunscreen results in a reduced incidence of squamous cell carcinoma. In the first study of healthy volunteers, followed forward in time, adult Australians who regularly applied sunscreens over a 2-3 year period developed fewer squamous cell carcinomas than control subjects who applied a placebo cream. Most of the previous studies have asked individuals retrospectively about their sunscreen use and thus are subject to "recall bias", i.e., errors in memory, on the part of the participants. In this new study, the incidence of basal cell carcinoma was not reduced, perhaps because the application of sunscreen was limited to a short period during adult life.
- There is additional epidemiological evidence of a relationship between specific disorders of the eye and sun exposure. These disorders, called pterygia and pingueculae, are degenerative disorders of the cornea that can affect vision. In a population in Asia, dose-response relationships between sunlight exposure and the incidence of these diseases were established. It remains to be determined to what extent UV-B is the active component of sunlight causing these disorders in humans.
- A recently completed comprehensive analysis of over 100 field studies indicates that supplemental UV-B on average causes a small, but statistically significant, decrease in the productivity of higher plants. Furthermore, the effect on plant productivity was more pronounced when relatively high supplemental UV-B was used. This is the first comprehensive analysis of the broad literature on plant UV-B responses in the field, and involved both agricultural and non-agricultural plant species.
- Apart from direct responses to UV-B of higher plants, many other UV-B effects are being shown in terrestrial ecosystems. Studies have reported sizeable influences on plant-insect interactions, including increases or decreases in the degree to which plants are consumed by insects. Also, supplemental UV-B has been shown to cause changes in flowers that influence the behaviour of insect pollinators. Compared to sunlight with UV-B reduced by filters, ambient solar UV-B increased populations of some invertebrates and decreased the abundance of fungi living in peat bogs. It also resulted in a lower abundance of some fungi on leaf surfaces and fungi colonizing dead plant debris. Grasses with fungi living in their internal tissues showed growth greatly reduced by supplemental UV-B, while those without these internal fungi were little influenced by the additional UV-B.
- Experiments with combinations of elevated UV-B radiation and other environmental factors are showing unanticipated responses in terrestrial plants. Plants subjected to water stress or nitrogen deficiency appeared to be less influenced by elevated UV-B than well-watered or nitrogen-fertilized plants. It was also reported that elevated UV-B allowed plants to cope better with water stress. In other studies, various interactions in plant responses to combinations of elevated UV-B and other environmental factors such as increased temperatures, elevated CO2 and certain pollutants (e.g., cadmium) have been described. Research on Arctic plant species shows that the frost tolerance of some species was substantially decreased when the plants were exposed to elevated UV-B radiation.
- Recent studies of the effects of solar UV radiation on aquatic primary producers (phytoplankton and macroalgae) confirm the deleterious consequences for growth and survival of some species. Reduced productivity as a consequence of enhanced levels of solar UV-B results in reduced uptake capacity for atmospheric carbon dioxide, which may augment global warming. However, changes in community structure and ecosystem integrity may be the most important consequences of enhanced solar UV-B.
- The combination of UV with other stress factors has been studied in bacteria, cyanobacteria and other primary producers. Excessive visible radiation, non-optimal temperatures, pollutants such as toxic heavy metal ions and changes in salinity can synergistically enhance the inhibitory effects of solar UV, including growth, reproduction, ecosystem structure and food web dynamics.
- Research has continued to progress toward understanding the impact of UV on complete ecosystems (both freshwater and marine) rather than individual organisms and responses. In addition to mixing depth and other factors, penetration of solar UV into natural waters and the resulting UV gradient in the water column has been found to influence the vertical distribution of macroalgae within the tidal zone as well as the vertical migrations of individuals, e.g. in phytoplankton communities and microbial mats.
- Increasingly detailed models for the assessment of UV-induced damage to organisms in the aquatic environment show reasonable agreement with measurements. The effects on DNA damage and repair have been considered in models that include ozone reduction, vertical mixing and dissolved and suspended material in the water column. In studies to date, ozone column thickness has shown the largest influence in determining DNA damage within the mixed layer.
- The protective function of UV screening substances has been verified in both producers and consumers. A number of new compounds absorbing UV-A and UV-B have been identified in cyanobacteria, phytoplankton and macroalgae. Consumers (such as coral reef organisms, sea urchins and fish) can acquire protection by taking up these substances with their food. The synthesis of these substances is induced by UV radiation, and their role as photoprotectants during evolution is recognized.
- Additional research confirms that UV-B radiation affects freshwater and marine consumers. UV-B may play a role in changes in predator-prey interactions. It has also been shown to impair the palatability of small algae as food for copepods and other primary consumers, and it is an important factor affecting the success of early life history stages of some fish species (e.g. cod). The depth at which certain fish species breed appears to be related to the penetration of UV-B radiation into the water column. Also, UV-B has been shown to compromise the immune system of fish. It is one factor that may act in conjunction with other stresses to affect amphibian populations adversely.
- An important component of the terrestrial nitrogen cycle has been shown to be sensitive to enhanced UV-B. In the high Arctic, where the low availablility of nitrogen severely limits plant growth, nitrogen fixation by free-living blue-green algae was retarded by UV-B. Nitrogen fixation by symbiotic algae in a subarctic lichen was also reduced in the long term (8 years), while it increased slightly in the short term (3 months).
- The exchange of trace gases between terrestrial systems and the atmosphere has been shown to be influenced by changes in UV-B. Additional research on UV-induced carbon monoxide production from dead plant matter in terrestrial ecosystems indicates that the global annual carbon monoxide input from this source to the atmosphere is significant. Solar UV induced nitrogen oxide production in snowpacks located at diverse sites in Greenland, Antarctica, Canada and the northern United States. UV-driven emissions of carbon monoxide and nitrogen oxides may change local atmospheric chemistry.
- Several important sources of natural ozone-depleting halogenated substances have been identified in the terrestrial biosphere. Calculations of global atmospheric budgets of methyl bromide and methyl chloride indicate large missing sources. Recent experimental data indicate that natural emissions of these gases from terrestrial ecosystems, particularly salt marshes, contribute significantly to the global budgets.
- Additional research has shown that enhanced UV-B can increase or decrease the decomposition of dead plant matter in terrestrial ecosystems. An international study from the south to north in Europe showed that enhanced UV-B, on average, slightly retards decomposition of dead birch leaves, possibly owing to inhibition of fungal decomposers. In contrast, there is evidence that exposure of living leaves to enhanced UV-B can alter leaf chemistry, leading to more rapid decomposition. Recent work confirms that UV-B effects on dead leaf decomposition are species-dependent and highly variable; overall, there is almost a balance between cases in which decomposition is accelerated and in those where it is retarded.
- Field research shows that past climate change has affected UV-B penetration into surface freshwaters. Sedimentary records of fossil diatoms in Canadian lakes have provided new evidence of large shifts in underwater UV-B radiation associated with past changes in climate and related changes in the inputs of UV-absorbing organic matter.
- Continuing research on fresh and oceanic waters has further demonstrated that UV-B transforms dead organic matter to dissolved inorganic carbon, including carbon dioxide, and to organic substances that are either more or less easily available to micro-organisms. These UV-induced transformations vary regionally, with highest efficiencies observed in open ocean water. The extent of these transformations is correlated with loss of UV absorbance by the organic matter.
- Interactions between UV-B radiation and climate change affect sulphur emissions that influence the balance between incoming and outgoing radiation in the marine atmosphere. Enhancements of dimethylsulphide emissions from the ocean to the atmosphere were linked to an interaction between vertical mixing in the ocean and UV-B inhibition of bacterial growth.
- Models have been developed to assess the role of UV-B radiation in aquatic biogeochemical cycling. Significant advances have been made in conceptual, local and global models of trace gas production and destruction in the ocean as a function of changes in UV-B, ocean biology, climate and related environmental parameters.
- The influence of changing UV-B radiation levels on tropospheric chemistry, while recognized previously, has been examined recently in more detail by several modelling studies. Factors causing changes in UV-B included increases from stratospheric O3 depletion, as well as both increases and decreases due to tropospheric cloud cover, scattering and absorbing aerosols, and pollutant gases. These radiation changes lead to comparable changes in the concentrations of important tropospheric chemicals, including O3 (a common pollutant associated with urban and regional smog), and OH (a highly reactive chemical that controls the atmospheric residence time of many compounds, including methane and HCFCs).
- The effect of UV-B changes on tropospheric O3 production is very sensitive to local atmospheric conditions. Modelling studies for various regions predict responses that are determined by the concentrations of nitrogen oxides, hydrocarbons, O3 and water vapour. Large differences are found, especially as a function of the vertical distribution of NOx concentrations. These modelling studies are supported by recently reported observations of enhanced tropospheric O3 production at two sites in the Swiss Alps in February 1993, where normally low winter sunlight was enhanced by higher UV-B associated with unusually low total O3 column values.
- High ambient temperatures in the range of 25 to 45 ºC increased the rate of UV-B-induced degradation in plastics. To date, available data show that the damaging effects of high temperatures with high UV levels are significant and synergistic. This finding, previously reported for polyethylene plastic films used for agricultural and packaging applications, has now been extended to polycarbonate plastics commonly used in glazing applications.
- A new group of catalysts that produce better UV-resistant plastics is being developed. Polyethylene plastics with better mechanical properties can be produced at a relatively lower cost using the new metallocene catalysts. These catalysts are based on complex organic compounds of zirconium and titanium. Polyethylenes made using these catalysts have superior resistance to damage from exposure to UV-B radiation, compared to those produced using conventional catalysts.
- Several new additives that stabilize common plastics against UV-induced damage have been developed. In laboratory studies these additives were found to be significantly more effective than those available commercially. They can be used with most plastics in construction and other applications. However, the commercial viability of these new additives depends on economic factors and their effectiveness in a wide range of field environments.
- Sensitivity to UV-induced changes in the naturally occurring protein collagen has been determined. The wavelength sensitivity of collagen (a polymer used in medical applications), in both stabilized and unstabilized forms, has been reported. The findings were qualitatively similar to those for other polymers. A higher rate of damage was obtained upon exposure to UV wavelengths compared to visible radiation.