A change in the composition of the stratosphere becomes relevant to society only if it has noticeable effects. This places the assessment of effects in a pivotal role in the problem of ozone depletion.
Decreases in the quantity of total-column ozone, as now observed in many places, tend to cause increased penetration of solar UV-B radiation (290-315 nm) to the Earth's surface. UV-B radiation is the most energetic component of sunlight reaching the surface. It has profound effects on human health, animals, plants, microorganisms, materials and on air quality. Thus any perturbation which leads to an increase in UV-B radiation demands careful consideration of the possible consequences. This is the topic of the present assessment made by the Panel on Environmental Effects of Ozone Depletion.
The assessment is given in seven chapters, summarized as follows:
The quality and quantity of UV measurements has increased greatly in the last few years. Variations among measurements from different instruments are diminishing toward the 5% level. Long-term trend detection is still a problem, with little historical data available for baseline estimation.
Enhanced UV levels are clearly associated with the Antarctic springtime ozone reductions. Measurements show that maximum UV levels at the South Pole are reached well before the summer solstice, and DNA-damaging radiation at Palmer Station, Antarctica (64°S) during the springtime ozone depletion can exceed maximum summer values at San Diego, USA (32°N). UV increases at mid-latitudes are smaller. However, increases associated with the record low ozone column of 1992/93 in the northern hemisphere are evident when examined on a wavelength-specific basis.
Measurements in Argentina, Chile, New Zealand, and Australia show relatively high UV levels compared to corresponding northern hemispheric latitudes, with differences in both stratospheric ozone and tropospheric pollutants likely to be playing a role. Tropospheric ozone and aerosols can reduce global UV-B irradiances appreciably. At some locations, tropospheric pollution may have increased since pre-industrial times, leading to decreases in surface UV radiation. However, recent trends in tropospheric pollution probably had only minor effects on UV trends relative to the effect of stratospheric ozone reductions.
Global ozone measurements from satellites over 1979/93 imply significant UV-B increases at high and mid-latitudes of both hemispheres, but only small changes in the tropics. Such estimates however assume that cloud cover and tropospheric pollution have remained constant over these years. Under the current CFC phase-out schedules, global UV levels are predicted to peak around the turn of the century in association with peak loading of chlorine in the stratosphere and the concomitant ozone reductions. The recovery to pre-ozone depletion levels is expected to take place gradually over the next 50 years.
The increase in UV-B radiation associated with stratospheric ozone depletion is likely to have a substantial impact on human health. Potential risks include increases in the incidence of and morbidity from eye diseases, skin cancer, and infectious diseases. Quantitative estimates of risk are available for some effects. (e.g., skin cancer), but other (e.g., infectious diseases) are associated with considerable uncertainty at the present time.
UV radiation has been shown in experimental systems to damage the cornea and lens of the eye. Chronic exposure to UV-B (resulting in a high, cumulative, lifetime dose) is one of several factors clearly associated with the risk of cataract of the cortical and posterior subcapsular forms. The 1989 Report noted that a 1% increase in stratospheric ozone depletion has been predicted to be associated with a 0.6 to 0.8% increase in cataract; this estimate, although crude, has not been improved upon in the intervening years.
Some components of the immune system are present in the skin, which makes the immune system accessible to UV radiation. Experiments in animals show that UV exposure decreases the immune response to skin cancers, infectious agents, and other antigens and can lead to unresponsiveness upon repeated challenges. Studies in human subjects also indicate that exposure to UV-B radiation can suppress the induction of some immune responses. The importance of these immune effects for infectious diseases in humans is unknown. However, in areas of the world where infectious diseases already pose a significant challenge to human health and in persons with impaired immune function, the added insult of UV-B induced immune suppression could be significant.
In susceptible (light-skinned) populations, UV-B radiation is the key risk factor for development of non-melanoma skin cancer (NMSC). Using information derived from animal experiments and human epidemiology, it is estimated that a sustained 1% decrease in stratospheric ozone will result in an increase in NMSC incidence of approximately 2%. The relationship between UV-B exposure and melanoma skin cancer is less well understood and appears to differ fundamentally from that of NMSC. Epidemiologic data indicate that the risk of melanoma increases with sunlight exposure, especially during childhood. There is, however, uncertainty about the relative importance of UV-B radiation, which directly determines the magnitude of the increase in melanoma that would result from ozone depletion.
Physiological and developmental processes of plants are affected by UV-B radiation, even by the amount of UV-B in present-day sunlight. Plants also have several mechanisms to ameliorate or repair these effects and may acclimate to a certain extent to increased levels of UV-B. Nevertheless, plant growth can be directly affected by UV-B radiation.
Response to UV-B also varies considerably among species and also cultivars of the same species. In agriculture, this will necessitate using more UV-B-tolerant cultivars and breeding new ones. In forests and grasslands, this will likely result in changes in species composition; therefore there are implications for the biodiversity in different ecosystems.
Indirect changes caused by UV-B (such as changes in plant form, biomass allocation to parts of the plant, timing of developmental phases and secondary metabolism) may be equally, or sometimes more, important than damaging effects of UV-B. These changes can have important implications for plant competitive balance, herbivory, plant pathogens, and biogeochemical cycles. These ecosystem-level effects can be anticipated, but not easily predicted or evaluated. Research at the ecosystem level for solar UV-B is barely beginning. Other factors, including those involved in climate change such as increasing CO2 also interact with UV-B. Such reactions are not easily predicted, but are of obvious importance in both agriculture and in nonagricultural ecosystems.
More than 30% of the world's animal protein for human consumption comes from the sea, and in many countries, particularly the developing countries, this percentage is significantly higher. As a result, it is important to know how increased levels of exposure to solar UV-B radiation might affect the productivity of aquatic systems.
In addition, the oceans play a key role with respect to global warming. Marine phytoplankton are a major sink for atmospheric carbon dioxide, and they have a decisive role in the development of future trends of carbon dioxide concentrations in the atmosphere.
Phytoplankton form the foundation of aquatic food webs. Marine phytoplankton are not uniformly distributed throughout the oceans of the world. The highest concentrations are found at high latitudes while, with the exception of upwelling areas on the continental shelves, the tropics and subtropics have 10 to 100 times lower concentrations. In addition to nutrients, temperature, salinity and light availability, the high levels of exposure to solar UV-B radiation that normally occur within the tropics and subtropics may play a role in phytoplankton distributions.
Phytoplankton productivity is limited to the euphotic zone, the upper layer of the water column in which there is sufficient sunlight to support net productivity. The position of the organisms in the euphotic zone is influenced by the action of wind and waves. In addition, many phytoplankton are capable of active movements that enhance their productivity and, therefore, their survival. Exposure to solar UV-B radiation has been shown to affect both orientation mechanisms and motility in phytoplankton, resulting in reduced survival rates for these organisms.
Researchers have measured the increase in, and penetration of, UV-B radiation in Antarctic waters, and have provided conclusive evidence of direct ozone-related effects within natural phytoplankton communities. Making use of the space and time variability of the UV-B front associated with the Antarctic ozone hole, researchers assessed phytoplankton productivity within the hole compared to that outside the hole. The results show a direct reduction in phytoplankton production due to ozone-related increases in UV-B. One study has indicated a 6-12% reduction in the marginal ice zone.
Solar UV-B radiation has been found to cause damage to early developmental stages of fish, shrimp, crab, amphibians and other animals. The most severe effects are decreased reproductive capacity and impaired larval development. Even at current levels, solar UV-B radiation is a limiting factor, and small increases in UV-B exposure could result in significant reduction in the size of the population of consumer organisms.
Although there is overwhelming evidence that increased UV-B exposure is harmful to aquatic ecosystems, the potential damage can only be roughly estimated at the present time.
Increases in solar UV radiation could affect terrestrial and aquatic biogeochemical cycles thus altering both sources and sinks of greenhouse and chemically-important trace gases e.g., carbon dioxide (CO2), carbon monoxide (CO), carbonyl sulfide (COS) and possibly other gases, including ozone. These potential changes would contribute to biosphere-atmosphere feedbacks that attenuate or reinforce the atmospheric buildup of these gases.
In terrestrial ecosystems increased UV-B could modify both the production and decomposition of plant matter with concomitant changes in the uptake and release of atmospherically-important trace gases. Decomposition processes can be accelerated when UV-B photodegrades surface litter, or retarded when the dominant effect is on the chemical composition of living tissues resulting in reduced biodegradabilty of buried litter. Primary production can be reduced by enhanced UV-B, but the effect is variable between species and even cultivars of some crops. Likewise, photoproduction of CO from plant matter is species dependent and occurs more efficiently from dead than living matter.
In aquatic ecosystems solar UV-B radiation also might have significant impacts. Studies in several locations have shown that reductions in current levels of solar UV-B result in enhanced primary production, and Antarctic experiments under the ozone hole demonstrated that primary production is inhibited by enhanced UV-B. In addition to its effects on primary production, solar UV radiation can reduce bacterioplankton growth in the upper ocean with potentially important effects on marine biogeochemical cycles. Solar UV radiation stimulates the degradation of aquatic dissolved organic matter (DOM) resulting in loss of UV absorption and formation of disolved inorganic carbon (DIC), CO, and organic substrates that are readily mineralized or taken up by aquatic microorganisms. Aquatic nitrogen cycling can be affected by enhanced UV-B through inhibition of nitrifying bacteria and photodecomposition of simple inorganic species such as nitrate. The marine sulfur cycle may be affected by UV-B radiation resulting in possible changes in the sea-to-air emissions of COS and dimethylsulfide (DMS), two gases that are degraded to sulfate aerosols in the stratosphere and troposphere, respectively.
New research on the environmental fate and impact of the hydrofluorocarbon (HFC) and hydrochlorofluorocarbon (HCFC) substitutes for CFCs has focused on trifluoroacetate (TFA), a tropospheric oxidation product of certain HFCs and HCFCs. TFA is mildly toxic to most marine and freshwater phytoplankton. The results indicate that TFA, although it may become globally distributed with increased usage of alternative fluorocarbons, is not likely to accumulate in soils and organisms. Although resistant to chemical degradation, very recent evidence indicates that TFA can be broken down by microorganisms.
Reductions of stratospheric ozone and the concomitant increases of UV-B radiation penetrating to the lower atmosphere result in higher photodissociation rates of key trace gases that control the chemical reactivity of the troposphere. This can increase both production and destruction of ozone (O3) and related oxidants such as hydrogen peroxide (H2O2), which are known to have adverse effects on human health, terrestrial plants, and outdoor materials. Changes in the atmospheric concentrations of the hydroxyl radical (OH) may change the atmospheric lifetimes of climatically important gases such as methane (CH4) and the CFC substitutes.
Trends in the photodissociation rate coefficient of tropospheric O3, of about +0.36+/-0.04% per year in the northern hemisphere and +0.40+/-0.05% per year in the southern hemisphere, have been estimated from satellite measurements of the ozone column between 1979 and 1992. The corresponding model-calculated changes in tropospheric chemical composition are non-linear and sensitive to the prevailing levels of nitrogen oxides (NOx). In polluted regions (high NOx), tropospheric O3 is expected to increase, reaching potentially harmfulä concentrations earlier in the day, and leading to more frequent exceedance of oxidant standards for air quality in urban areas where O3 levels are routinely near such air quality thresholds. In more pristine regions (lower NOx), O3 increases can be lower or even negative. Other oxidants, such as H2O2 and OH, are projected to increase for both polluted and pristine regions. Changes to H2O2 concentrations may have some impact on the geographical distribution of acid precipitation. Rural regions may become more urban-like and the percentage of areas with remote tropospheric conditions may decline.
Increases in OH concentrations cause a nearly proportionate decrease in the steady state tropospheric concentrations of CH4 and CFC substitutes such as the HCFCs and HFCs. Thus, the measured reductions in the ozone column (TOMS, 1979-92) are likely to have moderated CH4 increases over the past decade, and may account for about 1/3 of the slowing of the global CH4 trends.
Increased tropospheric reactivity could also lead to increased production of particulates such as cloud condensation nuclei, from the oxidation and subsequent nucleation of sulfur of both antropogenic and natural origin (e.g. carbonyl sulfide and dimethylsulfide). While these processes are still not fully understood, they exemplify the possibility of complex feedbacks between stratospheric ozone reductions, tropospheric, chemistry, and climate change.
Synthetic polymers, naturally occurring biopolymers, as well as some other materials of commercial interest are adversely affected by solar UV radiation. Application of these materials, particularly plastics, in situations which demand routine exposure to sunlight is only possible through the use of light-stabilizers and/or surface treatments to protect them from sunlight. Any increase in solar UV-B content due to partial ozone depletion will therefore accelerate the photogradation rates of these materials, limiting their service lifetimes outdoors.
The nature and the extent of such damage due to increased UV-B radiation in sunlight is quantified in action spectra. In spite of the several polymer action spectra available in the research literature the information is often inadequate to make reliable estimates of the increased damage. However, it is clear from the available data that the shorter UV-B wavelengths processes are mainly responsible for photodamage ranging from discoloration to loss of mechanical integrity. The molecular level interpretation of these changes remain unclear in many instances.
The use of higher levels of conventional light-stabilizers in polymer formulations will undoubtedly be attempted as a means of mitigating the effects of increased UV levels in sunlight. However, such an approach assumes that a) these stabilizers continue to be effective under spectrally-altered sunlight conditions; b) they are themselves photostable on exposure to UV-rich sunlight; and c) they can be sufficiently effective at low enough concentrations to serve the purpose. Experimental data bearing on these issues is sparse. Ongoing research, particularly that relating to extreme-environment exposure of polymers, is expected to shed more light on these questions. Substitution of the affected materials by more photostable plastics and other materials also remains an attractive possibility. Both these approaches will add to the cost of plastic products in target applications.
The increases in UV-B radiation already observed and expected in the future will have consequences of significant magnitude in several respects. This applies to the UV-B increases predicted on the basis of the most favorable scenario of ozone depletion; it applies even more if ozone depletion would be greater, for instance, due to incomplete compliance to the phaseout agreed for ozone depleting chemicals. This strongly supports continued determination to protect the ozone layer.
In many areas the uncertainties about effects are still so great that quantitative predictions are not possible. Incompleteness of knowledge does not diminish the concerns for the consequences, e.g., an increase in infectious diseases or disturbance of natural ecosystems. Some of these consequences of ozone depletion may well be more serious than the effects now quantifiable. Further investigations are necessary in order to achieve more certainty about such effects and, if necessary, about possibilities for protection or mitigation.