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The latest full assessment report
on Environmental Effects of Ozone Depletion is that of November,
1998. This Interim Summary is an update on recent findings. Special
attention has been paid to interactions between ozone depletion
and climate change. Potential interactions are indicated within
the various sections.
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UNITED
NATIONS
ENVIRONMENT PROGRAMME
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Pursuant
to Article 6 of the Montreal Protocol on Substances that Deplete the Ozone
Layer under the Auspices of the United Nations Environment Programme (UNEP)
Table of Contents
- Ozone and
UV Changes
- Health
Effects
- Effects
on Terrestrial Ecosystems
- Effects
on Aquatic Ecosystems
- Biogeochemical
Cycles
- Effects
on Air Quality
- Materials
Damage
- Panel
Members and UNEP Representatives
Ozone and UV Changes
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Globally
integrated ozone loss seems to have levelled off, but total column
ozone amounts remain low, especially at high latitudes during spring.
Due 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 less 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 less than in those
years.
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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. Based on statistical considerations of ozone
variability, southern mid-latitudes are the first location at which
ozone recovery may be detectable.
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The
quality and availability of ground-based 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 ca 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 ca 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 characterisation
of geographic differences.
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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 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 incident
in Australia showed larger increases in the tropics than at mid-latitudes
over the period 1979-1992, due to the combined influence of changes
in ozone and clouds. Another study of satellite-derived erythemal
UV trends in the Northern Hemisphere (1979-91) 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 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.
Health Effects
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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 is currently lacking to demonstrate that such
interactions do influence these diseases in human populations.
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It has
long been known that exposure to UV can suppress the effectiveness
of immunisation in humans; recent work now shows that, even after
successful immunisation, UV can still suppress these immune responses.
In earlier studies, suppression was only demonstrated in individuals
who were first exposed to UV and then immunised. Now it has been reported
that UV irradiation (exposure to simulated sunlight) of already immunised
individuals can also suppress the immune response. As a consequence,
such individuals could become susceptible to re-infection.
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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.
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Experimental
results in newborn as well as adult opossums (a South American marsupial)
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.
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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 retrospectively asked individuals 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.
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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.
Effects on Terrestrial Ecosystems
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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 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.
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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 colonising dead plant debris. Grasses with
fungi living in their internal tissues showed greatly reduced
growth by supplemental UV-B, while those without these internal fungi
were little influenced by the additional UV-B.
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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-fertilised
plants. It was also reported that elevated UV-B allowed plants to
better cope 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.
Effects on Aquatic Ecosystems
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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.
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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.
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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.
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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.
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The
protective function of UV screening substances has been verified in
both producers and consumers. A number of new compounds absorbing
in the UV-A and UV-B has been identified in cyanobacteria, phytoplankton
and macroalgae. Consumers (such as coral reef organisms, sea urchins,
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 recognised.
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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 was also 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 was 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.
Effects on Biogeochemical
Cycles
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An important
component of the terrestrial nitrogen cycle has been shown to be sensitive
to enhanced UV-B. In the high Arctic, where unavailable 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).
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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 the nitrogen oxide production in snowpacks located
at diverse sites in Greenland, Antarctica, Canada and the northern
United States. The UV-driven emissions of carbon monoxide and nitrogen
oxides may change local atmospheric chemistry.
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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.
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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 due 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
is 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.
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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.
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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.
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Interactions
between UV-B radiation and climate change affect sulfur emissions
that influence the balance between incoming and outgoing radiation
in the marine atmosphere. Enhancements of dimethylsulfide 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.
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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.
Effects on Air Quality
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The
influence of changing UV-B radiation levels on tropospheric chemistry,
while recognised 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).
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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.
Materials Damage
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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.
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A new
group of catalysts that produce better UV-resistant plastics is being
developed. Polyethylene (PE) 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.
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Several
new additives that stabilise 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 building 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.
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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) both in stabilised and unstabilised
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.
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