Decreased quantities of total-column ozone are now observed
over large parts of the globe, permitting increased penetration of solar
UV-B radiation (280-315 nm) to the Earth´s surface. The present assessment
deals with the possible consequences. The Atmospheric Science Panel predicts
that the ozone layer will be in its most vulnerable state during the coming
two decades. Some of the effects are expected to occur during most of the
next century. Recent studies show that the effects of ozone depletion would
have been dramatically worse without the protective measures taken under
the Montreal Protocol.
The assessment is given in seven chapters, summarised
Changes in Ultraviolet Radiation
Stratospheric ozone levels are near their lowest point
since measurements began, so current UV-B radiation levels are thought
to be close to their maximum. Total stratospheric content of ozone-depleting
substances is expected to reach a maximum before the year 2000. All other
things being equal, the current ozone losses and related UV-B increases
should be close to their maximum. Increases in surface erythemal (sun-burning)
UV radiation relative to the values in the 1970s are estimated to be:
- about 7% at Northern Hemisphere mid-latitudes in winter/spring;
- about 4% at Northern Hemisphere mid-latitudes in summer/fall;
- about 6% at Southern Hemisphere mid-latitudes on a
- about 130% in the Antarctic in the spring; and
- about 22% in the Arctic in the spring.
The correlation between increases in surface UV-B radiation
and decreases in overhead ozone has been further demonstrated and quantified
by ground-based instruments under a wide range of conditions. Improved
measurements of UV-B radiation are now providing better geographical and
temporal coverage. Surface UV-B radiation levels are highly variable because
of sun angle, cloud cover, and also because of local effects including
pollutants and surface reflections. With a few exceptions, the direct detection
of UV-B trends at low and mid-latitudes remains problematic due to this
high natural variability, the relatively small ozone changes, and the practical
difficulties of maintaining long-term stability in networks of UV-measuring
instruments. Few reliable UV-B radiation measurements are available from
pre-ozone depletion days.
Satellite-based observations of atmospheric ozone and
clouds are being used, together with models of atmospheric transmission,
to provide global coverage and long-term estimates of surface UV-B radiation.
of long term (1979-1992) trends in zonally-averaged UV-irradiances that
include cloud effects are nearly identical to those for clear-sky estimates,
providing evidence that clouds have not influenced the UV-B trends. However,
the limitations of satellite-derived UV estimates should be recognized.
To assess uncertainties inherent in this approach, additional validations
involving comparisons with ground-based observations are required.
Direct comparisons of ground-based UV-B radiation measurements
between a few mid-latitude sites in the Northern and Southern Hemispheres
have shown larger differences than those estimated using satellite data.
measurements show that summertime erythemal UV irradiances in the Southern
Hemisphere exceed those at comparable latitudes of the Northern Hemisphere
by up to 40%, whereas corresponding satellite-based estimates yield only
10 to 15% differences. Atmospheric pollution may be a factor in this discrepancy
between ground-based measurements and satellite-derived estimates. UV-B
measurements at more sites are required to determine whether the larger
observed differences are globally representative.
High levels of UV-B radiation continue to be observed
in Antarctica during the recurrent spring-time ozone hole. For example,
during ozone hole episodes, measured biologically-damaging radiation at
Palmer Station, Antarctica (64° S) has been
found to approach and occasionally even exceed maximum summer values at
San Diego, USA (32° N).
Long term predictions of future UV-B levels are difficult
and uncertain. Nevertheless, current best estimates suggest that a slow
recovery to pre-ozone depletion levels may be expected during the next
half-century. Although the maximum ozone depletion, and hence maximum
UV-B increase, is likely to occur in the current decade, the ozone layer
will continue to be in its most vulnerable state into the next century.
The peak depletion and the recovery phase could be delayed by decades because
of interactions with other long-term atmospheric changes, e.g. increasing
concentrations of greenhouse gases. Other factors that could influence
the recovery include non-ratification and/or non-compliance with the Montreal
Protocol and its Amendments and Adjustments, and future volcanic eruptions.
The recovery phase for surface UV-B irradiances will probably not be detectable
until many years after the ozone minimum.
Effects on Human and Animal Health
Recent estimates suggest that the increase in the risk
of cataract and skin cancer due to ozone depletion would not have been
adequately controlled by implementation of the Montreal Protocol (1987)
alone but can be achieved through implementation of its later provisions.
Risk assessments for the US and the Northwestern Europe indicate large
increases in cataracts and skin cancers under either the ‘no Protocol’
or the early Montreal Protocol scenarios. Under scenarios based on the
later amendments, Copenhagen (1992) and Montreal (1997), increases in cataracts
and skin cancer attributable to ozone depletion return almost to zero by
the end of the next century.
The increases in UV-B radiation associated with ozone
depletion are likely to lead to increases in the incidence and/or severity
of a variety of short-term and long-term health effects, if current exposure
practices are not modified by changes in behavior.
- Adverse effects on the eye will affect all populations
irrespective of skin color. Adverse impacts could include: more cases
of acute reactions such as ‘snowblindness’; increases in cataract incidence
and/or severity (and thus the incidence of cataract-associated blindness);
and increases in the incidence (and mortality) from ocular melanoma and
squamous cell carcinoma of the eye.
- Effects on the immune system will also affect
all populations but may be both adverse and beneficial. Adverse effects
include depressed resistance to certain tumors and infectious diseases,
potential impairment of vaccination responses, and possibly increased severity
of some autoimmune and allergic responses. Beneficial effects could include
decreases in the severity of certain immunologic diseases/conditions such
as psoriasis and nickel allergy.
- Effects on the skin could include increases in photoaging,
and skin cancer with risk increasing with fairness of skin. Increases
in UV-B are likely to accelerate the rate of photoaging, as well as increase
the incidence (and associated mortality) of melanoma and the non-melanoma
skin cancer, basal cell and squamous cell carcinoma.
Research is generating much new information that is being
used to help reduce the uncertainties associated with the current risk
estimates. Evaluation of the impact of susceptibility genes is helping
to identify highly susceptible populations so that their special risk can
be assessed. Examination of the impacts of behavior changes such as consuming
diets that are high in antioxidants, avoiding sun exposure during the four
hours around solar noon, wearing covering apparel, e.g., hats, sunglasses,
is beginning to identify important exposure patterns as well as possible
Quantitative risk assessments for a variety of other effects,
such as UV-B induced immunosuppression of infectious diseases, are not
yet possible. New information continues to confirm the reasonableness
of these concerns, but data adequate for quantitative risk assessment are
not yet available.
Effects on Terrestrial Ecosystems
Increased UV-B can be damaging for terrestrial organisms
including plants and microbes, but these organisms also have protective
and repair processes. The balance between damage and protection varies
among species and even varieties of crop species; many species and varieties
can accommodate increased UV-B. Tolerance of elevated UV-B by some species
and crop varieties provides opportunities for genetic engineering and breeding
to deal with potential crop yield reductions due to elevated UV-B in agricultural
Research in the past few years indicates that increased
UV-B exerts effects more often through altered patterns of gene activity
rather than damage. These UV-B effects on regulation manifest themselves
in many ways including changes in life cycle timing, changes in plant form
and production of plant chemicals not directly involved in primary metabolism.
These plant chemicals play a role in protecting plants from pathogens and
insect attack, and affect food quality for humans and grazing animals.
Terrestrial ecosystem responses to increased UV-B are
evident primarily in interactions among species, rather than in the performance
of individual species. Much of the recent experimentation indicates
that increased UV-B affects the balance of competition among higher plants,
the degree to which higher plants are consumed by insects and susceptibility
of plants to pathogens. These effects can be mediated in large part by
changes in plant form and chemistry, but effects of UV-B on insects and
microbes are also possible. The direction of these UV-B-mediated interactions
among species is often difficult to predict based only on single-organism
responses to increased UV-B.
Effects of increased UV-B radiation may accumulate from
year to year in long-lived perennial plants and from generation to generation
in annual plants. This effect has been shown in a few recent studies,
but the generality of this accumulation among species is not presently
known. If this phenomenon is widespread, this would amplify otherwise subtle
responses to UV-B seen in a single growing season, for example in forest
Effects of increased UV-B must be taken into account together
with other environmental factors including those associated with global
change. Responses of plants and other organisms to increased UV-B are
modified by other environmental factors such as CO2,
water stress, mineral nutrient availability, heavy metals and temperature.
Many of these factors also are changing as the global climate is altered.
Effects on Aquatic Ecosystems
Recent studies continue to demonstrate that solar UV-B
and UV-A have adverse effects on the growth, photosynthesis, protein and
pigment content, and reproduction of phytoplankton, thus affecting the
food web. These studies have determined biological weighting functions
and exposure-response curves for phytoplankton, and have developed new
models for the estimation of UV-related photoinhibition. In spite of this
increased understanding and enhanced ability to model aquatic impacts,
considerable uncertainty remains with respect to quantifying effects of
ozone-related UV-B increases at the ecosystem level.
Macroalgae and seagrasses show a pronounced sensitivity
to solar UV-B. They are important biomass producers in aquatic ecosystems.
Most of these organisms are attached and so cannot avoid being exposed
to solar radiation at their growth site. Effects have been found throughout
the top 10-15 m of the water column.
Zooplankton communities as well as other aquatic organisms
including sea urchins, corals and amphibians are sensitive to UV-B.
There is evidence that for some of these populations even current levels
of solar UV-B radiation, acting in conjunction with other environmental
stresses, may be a limiting factor but quantitative evaluation of possible
effects remains uncertain.
UV-B radiation is absorbed by and breaks down dissolved
organic carbon (DOC) and particulate organic carbon (POC) and makes the
products available for bacterial degradation and remineralization.
The degradation products are of importance in the cycling of carbon in
aquatic ecosystems. Because UV-B breaks down DOC as it is absorbed, increases
in UV-B can increase the penetration of both UV-B and UV-A radiation into
the water column. As a consequence, the quantity of UV-B penetrating to
a given depth both influences and is influenced by DOC. Warming and acidification
result in faster degradation of these substances and thus enhance the penetration
of UV radiation into the water column.
Polar marine ecosystems, where ozone-related UV-B increases
are the greatest, are expected to be the oceanic ecosystems most influenced
by ozone depletion. Oceanic ecosystems are characterized by large spatial
and temporal variabilities that make it difficult to select out UV-B specific
effects on single species or whole phytoplankton communities. While estimates
of reduction in both Arctic and Antarctic productivity are based upon measurable
short-term effects, there remain considerable uncertainties in estimating
long-term consequences, including possible shifts in community structure.
Reduced productivity of fish and other marine crops could have an economic
impact as well as affect natural predators; however quantitative estimation
of the possible effects of reduced production remain controversial.
Potential consequences of enhanced levels of exposure
of aquatic ecosystems to UV-B radiation include reduced uptake capacity
for atmospheric carbon dioxide, resulting in the potential augmentation
of global warming. The oceans play a key role with respect to the budget
of greenhouse gases. 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. The relative
importance of the net uptake of carbon dioxide by the biological pump and
the possible role of increased UV-B in the ocean are still controversial.
Effects on Biogeochemical Cycles
Effects of increased UV-B on emissions of carbon dioxide
and carbon monoxide (CO) and on mineral nutrient cycling in the terrestrial
biosphere have been confirmed by recent studies of a range of species and
ecosystems. The effects, both in magnitude and direction, of UV-B on
trace gas emissions and mineral nutrient cycling are species-specific and
operate on a number of processes. These processes include changes in the
chemical composition in living plant tissue, photodegradation (breakdown
by light) of dead plant matter, including litter, release of carbon monoxide
from vegetation previously charred by fire, changes in the communities
of microbial decomposers and effects on nitrogen-fixing micro-organisms
and plants. Long-term experiments are in place to examine UV-B effects
on carbon capture and storage in biomass within natural terrestrial ecosystems.
Studies in natural aquatic ecosystems have indicated that
organic matter is the primary regulator of UV-B penetration. Enhanced
UV-B can affect the balance between the biological processes that produce
the organic matter and the chemical and microbial processes that degrade
it. Changes in the balance have broad impacts on the effects of
enhanced UV-B on biogeochemical cycles. These changes, which are reinforced
by changes in climate and acidification, result from clarification of the
water and changes in light quality.
Increased UV-B has positive and negative impacts on microbial
activity in aquatic ecosystems that can affect carbon and mineral nutrient
cycling as well as the uptake and release of greenhouse and chemically-reactive
gases. Photoinhibition of surface aquatic micro-organisms by UV-B can
be partially offset by photodegradation of dissolved organic matter to
produce substrates, such as organic acids and ammonium, that stimulate
Modeling and experimental approaches are being developed
to predict and measure the interactions and feedbacks between climate change
and UV-B induced changes in marine and terrestrial biogeochemical cycles.
These interactions include alterations in the oxidative environment in
the upper ocean and in the marine boundary layer and oceanic production
and release of CO, volatile organic compounds (VOC), and reactive oxygen
species (such as hydrogen peroxide and hydroxyl radicals). Climate related
changes in temperature and water supply in terrestrial ecosystems interact
with UV-B radiation through biogeochemical processes operating on a wide
range of time scales.
Effects on Air Quality
Increased UV-B will increase the chemical activity in
the lower atmosphere (the troposphere). Tropospheric ozone levels are
sensitive to local concentrations of nitrogen oxides (NOx)
and hydrocarbons. Model studies suggest that additional UV-B radiation
reduces tropospheric ozone in clean environments (low NOx),
and increases tropospheric ozone in polluted areas (high NOx).
Assuming other factors remain constant, additional UV-B
will increase the rate at which primary pollutants are removed from the
troposphere. Increased UV-B is expected to increase the concentration
of hydroxyl radicals (OH) and result in faster removal of pollutants. Increased
concentrations of oxidants such as hydrogen peroxide and organic peroxides
are also expected. The effects of UV-B increases on tropospheric ozone,
OH, methane, carbon monoxide, and possibly other tropospheric constituents,
while not negligible, will be difficult to detect because the concentrations
of these species are also influenced by many other variable factors (e.g.,
No significant effects on humans or the environment have
been identified from TFA produced by atmospheric degradation of HCFCs and
HFCs. Numerous studies have shown that TFA has, at most, moderate short-term
toxicity. Insufficient information is available to assess potential chronic,
developmental, or reproductive effects. The atmospheric degradation mechanisms
of most substitutes for ozone depleting substances are well established.
HCFCs and HFCs are two important classes of substitutes.Atmospheric
degradation of HCFC-123 (CF3CHCl2),
and HFC-134a (CF3CH2F)
produces trifluoroacetic acid (TFA). Reported measurements of TFA in rain,
rivers, lakes, and oceans show it to be a ubiquitous component of the hydrosphere,
present at levels much higher than can be explained by currently reported
sources. The levels of TFA currently produced by the atmospheric degradation
of HFCs and HCFCs are estimated to be orders of magnitude below those of
concern and make only a minor contribution to the current environmental
burden of TFA.
Effects on Materials
Physical and mechanical properties of polymers are negatively
affected by increased UV-B in sunlight. Increased UV-B reduces the
useful lifetimes of synthetic polymer products used outdoors and of biopolymer
materials such as wood, paper, wool and cotton. The reduction in service
life of materials depends on the synergistic effect of increased UV-B and
other factors, especially the temperature of the material during exposure
to sunlight. Even under harsh UV exposure conditions the higher temperatures
largely determine the extent of increased UV-induced damage to photostabilized
polyethylenes. However, accurate assessment of such damage to various materials
is presently difficult to make due to limited availability of technical
data, especially on the relationship between the dose of UV-B radiation
and the resulting damage of the polymer or other material.
Conventional photostabilizers are likely to be able to
mitigate the effects of increased UV levels in sunlight. More effective
photostabilizers for plastics have been commercialized in recent years.
The use of these compounds allows plastic polymer products to be used in
a wide range of different UV environments found worldwide. It is reasonable
to expect existing photostabilizer technologies to be able to mitigate
these effects of an increased UV-B on polymer materials. This, however,
would increase the cost of the relevant polymer products, surface coatings,
and treated biopolymer materials. However, the efficiencies of even the
conventional photostabilizers under the unique exposure environments resulting
from an increase in solar UV-B have not been well studied.
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