Table of Contents
This summary is the last one between the full assessments of 1994 and 1998 on environmental effects of ozone depletion. The aim is to keep the Parties to the Montreal Protocol informed about new scientific developments.
Ozone and UV Changes
Monitoring of ultraviolet (UV) radiation has increased sharply in the last few years, and measurements from locations in all continents have recently been published. Particularly noteworthy is the increased UV monitoring in some developing countries where data had previously been sparse or non-existent. Such measurements are important contributions to the development of a global (land-based) UV climatology. Their continuation (together with the appropriate quality assurance) is essential towards an accurate, statistically representative characterisation of average and peak UV levels at the ground.
Detailed spectral measurements, although available at only relatively few locations, have been used extensively to enhance our understanding of the factors affecting atmospheric UV transmission, including not only stratospheric ozone (O3), but also clouds, stratospheric and tropospheric particulates (aerosols), urban pollutants (especially ozone), surface elevation and reflectivity. When atmospheric conditions are well-known and reasonably uniform, the difference between theoretical model predictions and direct measurements is often at the 10% level or better, and approaches that which is found among different instruments. Such results increase our confidence in the usefulness of both models and measurements.
There is no scientific doubt that when atmospheric O3 decreases, UV will increase, all other conditions being equal. However, the statistical significance of past trends in surface UV radiation associated with stratospheric ozone reductions has still not been established unequivocally by direct UV measurements. This is unlikely to change in the next few years due to (1) the lack of reliable historical (pre-ozone-depletion) UV measurements, (2) the continuing difficulty of maintaining instrument calibrations to a sufficient accuracy, and (3) the high natural variability in meteorological conditions that control UV transmissions (e.g., year-to-year fluctuations in cloud cover, aerosols, and pollutants). Re-analysis of historical Robertson-Berger (RB) meter network data has now been published, and shows that the previously reported decreasing UV trends (1974-1985) in the USA were probably an artifact from instrument calibration shifts rather than actual UV radiation change. The authors of that study concluded that the data from the original RB network were unsuitable for trend detection.
The effects on UV of shorter term variations in ozone have been clearly observed in recent years. Record-low ozone values have been observed at high and middle latitudes in the Northern Hemisphere during late winter and early spring in the last few years. At least one study has already been published, showing accompanying record-high UV radiation values (for that season) in central Europe.
Estimates of long-term UV radiation trends still rely on measured ozone decreases in combination with calculations of the propagation of solar radiation through the atmosphere (radiative transfer models). Although less direct than trend detection by surface UV measurements, the credibility of such model-based calculations has been greatly reinforced by the good agreement with direct UV measurements over shorter time scales, as described above. Furthermore, when based on satellite-derived ozone measurements, this approach allows estimation of surface UV with essentially global coverage. The earliest efforts (reported in previous years) were based on zonally averaged ozone data and cloud-free (or constant cloud) assumptions. Several new studies include both latitudinal and longitudinal variations, and have examined the effects of clouds and aerosols (as measured from satellite platforms) as well as estimates of tropospheric ozone change from atmospheric chemical transport models. The results show that cloud changes have been small on average and have not altered the ozone-induced UV trends since the beginning of the satellite ozone measurements (1979-1992, TOMS version 7). However, UV-absorbing aerosols associated with biomass burning plumes were estimated to reduce UV levels substantially in some regions. New estimates of tropospheric ozone increases also confirm earlier studies of its importance over broad regions surrounding industrialized areas, especially in the Northern Hemisphere. As already discussed in the earlier assessments, tropospheric ozone and aerosols are largely associated with increased human activity since pre-industrial days, so their effect on UV trends is not directly comparable to the more rapid and global-scale UV increases associated with the recent stratospheric ozone reductions. Nevertheless, these pollutants are clearly of some importance to surface UV levels in densely populated regions. Further study is required to fully characterize their effect on the surface irradiance, and on the angular distribution of the sky radiation (e.g., via scattering by the aerosols), which may be of special importance to some health effects (e.g., ocular) and to tropospheric photochemistry.
Recent information continues to expand our knowledge of both the adverse and the beneficial effects of UV as well as raise concerns about the use of chlorofluorocarbon (CFC) substitutes. Still of greatest concern are the impacts of UV on immune responses and their possible consequences for infectious diseases, the induction of skin cancers, particularly basal and squamous cell carcinomas and melanoma, and eye diseases, especially cataract.
Evidence continues to support the conclusion that UV-B radiation can suppress immune responses, both in laboratory animals and in humans of all skin types. New studies show that immune reactions elicited in UV-B irradiated human skin are diminished compared to responses occurring in unexposed skin. Studies on mechanisms of immune suppression by UV-B radiation reveal increasing complexity of the interactions between the immune system and UV. New evidence indicates that there are multiple pathways by which UV can perturb the immune system and multiple parts of the immune system that can be affected. For example, new information from studies in experimental animals indicates that production of certain types of antibodies can also be impaired by UV-B irradiation, in addition to the well-documented impairment of lymphocyte-mediated responses. Studies in experimental animals of infectious diseases of importance to humans show that UV-B irradiation not only can decrease immune responses against infectious organisms, but can increase the severity and duration of a broad spectrum of infectious diseases. It is still not possible to predict the impact of increasing UV-B exposure on infectious diseases in human populations. The accumulating new knowledge, however, increases the concern that UV exposure may increase the severity of certain infectious diseases and decrease the effectiveness of vaccinations.
The causal relationship between UV and skin cancer has already been established in animal experiments and epidemiological studies. Using this information, a recent risk assessment has estimated that, in the absence of the Montreal Protocol, elevations in UV due to ozone depletion would lead to skin cancer incidences as high as four times the current levels by the end of the next century. Under the agreements of the original Montreal Protocol, the incidence was estimated to double. With the Copenhagen Amendments, the relative increase in incidence would reach a maximum of almost 10 % in the year 2060, and then gradually return to baseline (i.e., levels without ozone depletion). Such estimates have a considerable uncertainty, as they are based on a number of simplifying assumptions, including that there will be full compliance with the agreements. These estimates nevertheless suggest that failure in the implementation of the Montreal Protocol will significantly increase the risks of skin cancer in susceptible populations.
Continued work analyzing the genetic alterations in skin cancers confirms earlier findings that mutations in the p53 gene represent early events in the development of these cancers and that these mutations appear to be specifically caused by UV-B. This has now been found for both basal cell and squamous cell carcinomas. Such information can contribute to refining future risk assessments either by helping to identify sensitive subpopulations, or by contributing to the development of more precise risk assessment models. Similar information on melanoma is not yet available but is an area of intensive research.
Although much fundamental research has been carried out on UV-induced damage to the lens of the eye, most of it does not contribute to our understanding of the connection between ambient UV radiation exposure and cataract in humans. One recent experiment, however, confirms that for rats whose eyes were exposed to UV the lens opacities (precursors to cataract) induced by that treatment tend to start at the periphery of the lens, just as is the case for the human cataracts that are associated with sun exposure.
A recent report in the medical literature indicates that workers accidentally exposed to the CFC substitutes HCFC-123 and HCFC-124, from a leak in an air conditioning system, developed acute liver disease. The authors concluded that strict containment measures are needed for these compounds and also recommended the rapid development of safer alternatives.
Effects on Terrestrial Ecosystems
In the past year, considerable progress has been reported at several levels of study ranging from molecular to ecosystem-level research. Ambient solar UV-B radiation was shown to induce plant DNA lesions in a few studies that also indicate competent repair of this damage. In other work, enhanced induction of antioxidants by UV-B radiation supports earlier research suggesting that oxidative damage may be caused by UV-B radiation. In several plant species, induction and accumulation of specific major flavonoid compounds occur. These compounds not only act as internal UV shields, but also have properties that may contribute to ameliorating oxidative damage. Still, in recent outdoor experiments, morphological changes, including shorter stems and greater branching, were the most common manifestations of UV-B exposure. Several studies have shown some potentially adverse effects of UV-B on various facets of plant reproductive biology including flowering, pollen development, seed production, and seed size. Varieties of rice differ considerably in response to UV-B in screening trials, but field studies have not generally shown pronounced effects of solar UV-B enhancement. UV-B radiation has been shown to affect the timing of plant development in UV-B attenuation studies.
Elevated UV-B can influence the effects of microorganisms and insects on higher plants. Some studies indicate that disease incidence may decrease under ambient solar UV-B, e.g., in tea plants. However, other studies have indicated increased severity of plant disease under UV-B radiation. Sizeable influences of solar UV-B on plant response to insect attack have been reported in at least one field study; other studies are in progress. Plant disease, microorganisms and insects may be directly influenced by solar UV-B, but many of the UV-B radiation effects on plant disease and plant-insect interactions are apparently the indirect result of changes in plant secondary chemistry.
Pronounced water stress increased susceptibility to elevated UV-B in some Mediterranean species; yet the plants exposed to elevated UV-B were better able to tolerate summer water stress due to less water loss from foliage. This reduced loss of water was attributed to thicker wax layers on leaf surfaces that developed under elevated UV-B. Further work on the interaction between elevated CO2 and UV-B has also been conducted. Both factors had some influence on plant growth, including a general stimulation under elevated CO2 and mild decreases under elevated UV-B. However, pronounced interactions between the two factors have been less apparent in the recent work. In a similar vein, a study of plant nitrogen nutrition and elevated UV-B showed that nitrogen-deficient plants were more UV-B sensitive.
Investigations at the ecosystem level are particularly important, since it is clear that extrapolation from studies with isolated plants to the ecosystem level is not reliable. Field ecosystem-level studies continue, including those at high latitudes (closer to the poles) where ozone reduction is most pronounced. Effects on plants are not always immediately apparent in the first season, but can appear in subsequent years. Thus, it is quite important that field ecosystem research of several years duration be conducted. Apart from direct effects on vegetation, effects on litter decomposition, insect consumption of vegetation and plant-plant competition are important secondary consequences of solar UV-B exposures that are being reported. Recent studies of peat-forming mosses, which are very important in global carbon storage, are not indicating reduced biomass accumulation under increased UV-B radiation.
Effects on Aquatic Ecosystems
Aquatic ecosystems provide a significant share of the world's animal protein for human consumption. Phytoplankton form the foundation of aquatic food webs. In addition, the oceans play a key role with respect to global warming, because marine phytoplankton is a major sink for atmospheric CO2. Recent studies continue to expand our knowledge of how increased exposure to solar UV-B radiation affects the structure and function of aquatic ecosystems and the consequent impact on global biogeochemical cycles.
Microbial mats, frequently the first colonizers of barren habitats, cover large areas in both marine and freshwater ecosystems. The members of the communities optimize their position in the mat by light-controlled vertical migration. Solar UV-B radiation has a negative effect on the migration, and consequently affects colonization, photosynthetic yield and biomass production. Cyanobacteria play a much larger role in biomass production than previously thought. They are capable of fixing atmospheric nitrogen and making it available for other members of aquatic and terrestrial ecosystems. UV-B radiation impairs nitrogen uptake and fixation, and thus has consequences for the availability of nitrogen compounds for natural and crop communities. These effects of solar UV-B radiation are augmented by the effects on cyanobacteria of toxic substances such as heavy metal pollutants.
Macroalgae and seagrasses are significant biomass producers in aquatic ecosystems and are of major economic importance. Numerous recent studies have indicated that many organisms (but not all) are under pronounced UV-B stress, even at current levels, and are likely to be further influenced by increased UV-B levels. As in higher plants, macroalgae and phytoplankton produce screening substances that protect the organisms against solar UV-B radiation. These substances are also passed on to the primary and secondary consumers through the food web. Some of these screening substances have been characterized and identified in algae and cyanobacteria, while others have been observed but not yet chemically identified. The efficiency of protection from thymine dimer production is being studied in microorganisms during active and passive movement within the mixing layer of the water column.
Both laboratory and field experiments suggest a direct impact of solar UV-B radiation on primary and secondary consumers ranging from zooplankton to fish. Correlations have been suggested between UV-B sensitivity and declines in both amphibians and corals. It has been speculated that UV-B may be one of several environmental factors influencing these declines.
Particulate and dissolved organic matter are breakdown products from primary and secondary biomass producers. The breakdown of these substances is augmented by solar UV-B radiation which increases the rate of uptake by bacteria. On the other hand, UV-B radiation has a negative impact on bacteria thus retarding the rate of degradation of organic material. These conflicting effects are currently under study.
Monitoring programs for the assessment of enhanced UV-B are increasing in sophistication and areal coverage. This includes the first underwater monitoring program aimed at providing continuous assessment of UV-B penetration and in-water optical properties across a latitudinal gradient in Europe. In-water constituents and the penetration of UV-B are intimately linked. Dissolved organic material (DOM) as well as particulate organic material (POM) significantly influence the penetration of UV-B in natural waters with absorption strongly increasing at shorter wavelengths. In turn, this absorption may lead to photobleaching of these constituents, altering the optical properties and leading to increased UV-B penetration relative to longer wavelengths. Complex feedback mechanisms exist between UV-B penetration, in-water optical properties that depend upon the concentration of DOM and POM, and subsequent UV impact on community structure of aquatic ecosystems. Acid deposition in boreal lakes, as well as re-oxidation of sediment sulfur resulting from drought-induced acidification, can decrease the concentrations of DOM within the lakes sufficiently to increase markedly the exposure of organisms to UV-B radiation. Continued research is necessary to elucidate these linkages.
Potential consequences of enhanced levels of exposure to UV-B radiation include loss of biomass such as food sources for humans, changes in species composition, decrease in availability of nitrogen compounds, and reduced uptake capacity for atmospheric carbon dioxide, augmenting global warming. Although there is significant evidence that increased UV-B exposure is harmful to aquatic organisms, damage to ecosystems is still uncertain.
Effects on Biogeochemical Cycles
Recent observational and theoretical evidence confirms the critical role of UV-B radiation in influencing biogeochemical cycles through alterations in photobiological and photochemical processes in the environment. These effects are manifested by changes in UV penetration into aquatic environments, nutrient cycles, carbon capture and storage in the aquatic and terrestrial environment, and the biosphere-atmosphere exchange of greenhouse and chemically-active gases such as CO2 and carbon monoxide (CO).
During the past year, significant new evidence from outdoor studies has indicated that UV penetration into freshwater and marine environments is primarily controlled by UV-absorbing dissolved organic substances, referred to as chromophoric dissolved organic matter (CDOM). UV absorption by the CDOM results in its photodegradation and loss of absorbance or bleaching that can result in an increase in the penetration of UV-B in lakes and oceans. A study of the algal-derived pigments preserved in Canadian lake sediments has revealed a possible link between climate change and UV exposure. Sedimentary profiles of the occurrence of UV-induced pigments indicate that the greatest UV penetration correlated with periods of drought, when levels of UV-absorbing CDOM in the lakes were minimal.
Other studies in lakes have confirmed that CDOM is photodegraded to CO2 as well as other organic and inorganic photoproducts and oxygen is consumed. Among the identifiable photoproducts are a large number of non-volatile, low-molecular-weight organic acids and other carbonyl compounds that are readily assimilated by microorganisms. Laboratory experiments using bacterial bioassay approaches have shown that the photochemical breakdown of CDOM can stimulate biomass production or activity in batch cultures by several fold. A recent report indicated that UV radiation photodegrades CDOM in rivers and coastal environments to volatile organic compounds (VOC). Action spectra for VOC photoproduction depend strongly on the VOC molecular structure, indicating that photodegradation of several components of the CDOM may be involved. The production of isoprene, an important precursor of tropospheric ozone, although resulting from the action of solar radiation on marine phytoplankton, is unaffected by UV-B. An extensive new set of action spectra for CO photoproduction in upper open ocean water from the Pacific Ocean was similar in the UV region to previously reported action spectra for coastal and freshwater environments. These data confirm that CO photoproduction is sensitive to changes in solar UV-B radiation in both freshwater and marine environments. CO is an important trace gas that strongly influences biogeochemical cycles through its effects on chemical reactions in the atmosphere. New results confirm that the photoproduction of carbonyl sulfide, the most-concentrated sulfur gas in the troposphere, correlates positively with UV irradiance and UV absorbance by CDOM in the upper ocean.
Studies of UV effects on terrestrial biogeochemical cycles include recent outdoor experiments on interactions between elevated UV-B radiation and other co-occurring environmental change variables. Research has continued on UV-induced changes in plant litter composition and their resulting effects on the microbial decomposability of litter. Newly published studies have shown that enhanced UV-B radiation increases tannin, lignin and other secondary compounds, thereby reducing the microbial digestibility of the litter from mid-latitude dune grassland species and high latitude shrubs. New evidence has appeared that UV-B photodegrades the lignaceous component of surface litter and dissolved organic matter in the sea. Studies of the effects of enhanced UV-B and elevated CO2 have indicated no combined effects on CO2 respired from sub-arctic shrub litter. Recent climate changes are enhancing fire frequency and extent in the boreal forest. Increased boreal fires may be having important regional effects on the UV-induced release of CO from the forest to the atmosphere. Although the floors of mature boreal forests generally take up CO via microbial oxidation processes, the charred surface residues following fire produce high levels of CO on exposure to solar UV radiation. The production is sufficiently large that the burned forest becomes a net source of CO to the atmosphere.
Trifluoroacetate (TFA), a persistent substance derived from the oxidation of certain CFC replacements (HCFC-123, HCFC-124, HCFC-134a), was found to be retained in vegetation and soil of a temperate North American forest, especially in the case of wetlands with organic soils. The potential buildup of TFA in the atmosphere and in localized water reservoirs such as vernal ponds requires multi-year monitoring.
Effects on Air Quality
Reductions in stratospheric ozone lead to increased penetration of UV-B radiation to the lower atmosphere, and therefore to a general increase in the photochemical reactivity of the troposphere. These changes are believed to affect concentrations of key tropospheric gases such as ozone (the major constituent of urban photochemical smog), peroxides (important contributors to the acidification of rain) and the hydroxyl radical (OH), which is the major oxidant responsible for the atmospheric residence time of species such as carbon monoxide, methane, VOC, nitrogen and sulfur oxides, and other constituents including many substitutes for ozone-depleting substances. However, the magnitude and even the sign of the tropospheric composition responses to increased UV-B levels depend on the chemical environment, especially the local amounts of nitrogen oxides, VOC, and water vapour.
Recent modelling studies confirm and extend earlier work showing that remote regions should experience lower tropospheric O3 levels due to enhanced UV-B radiation, and higher OH concentrations leading to shortened lifetimes for many atmospheric constituents as well as to higher levels of peroxides. However, the newest study also suggests that such effects will be rather minor in the upper troposphere due to the low levels of water vapour present at those altitudes. Another recently published study illustrates the effect of stratospheric perturbations on tropospheric chemistry by showing increases in tropical tropospheric methane and carbon monoxide for several months following the Mt. Pinatubo eruption, which injected large amounts of UV-absorbing sulfur dioxide into the stratosphere and thus temporarily reduced tropospheric OH radical production.
An increase in solar UV-B may result in shortening of useful lifetimes of plastics and increase the cost of using plastics, particularly in building applications.
Studies reported during the past year have added significantly to the understanding of spectral sensitivity of several widely used plastics. These included polyethylenes, acrylic polymers, nylon and textile materials. The findings are consistent with those for other polymers already investigated and quantify the spectral dependence of photodegradation. These data are useful for future cost estimates on the impacts of increased UV radiation in relation to materials damage. A report on the effect of nine types of common brominated flame retardant additives on the spectral sensitivity of polyolefins and polystyrene indicated a shift in the action spectrum to shorter wavelengths and increase in the UV-B induced degradation of the polymers. Plastics generally include flame-retardants and other additives to ensure processibility and performance, and their effect on the photodegradability of the polymer is of practical interest.
Data on the UV-susceptibility of two biopolymers, chitosan and collagen have also been recently reported, clarifying the chemistry of UV-induced oxidative processes and the spectral sensitivity of the degradation process. However, these findings should be considered preliminary because the sample preparation process may have changed the chemistry of biomaterials used in these studies.
Early data from controlled-temperature outdoor exposure experiments on polyethylene films illustrated the role of temperature in enhancing and supplementing the sunlight-induced degradation. At least in the case of polyethylene, the increased damage by an incremental increase in UV-B in sunlight is likely to be largely influenced by ambient temperature of the exposure location. Effects of stratospheric ozone depletion, and the subsequent increased UV-B radiation, on the useful lifetimes of plastics will be more severe in locations that experience high ambient temperatures. Understanding the role of temperature, together with UV-B irradiation, is scientifically and economically important.
ENVIRONMENTAL EFFECTS PANEL MEMBERS,