Environmental effects of ozone depletion and its interactions with climate change: Progress report, 2007

Abstract

This year the Montreal Protocol celebrates its 20th Anniversary. In September 1987, 24 countries signed the ‘Montreal Protocol on Substances that Deplete the Ozone Layer’. Today 191 countries have signed and have met strict commitments on phasing out of ozone depleting substances with the result that a 95% reduction of these substances has been achieved. The Montreal Protocol has also contributed to slowing the rate of global climate change, since most of the ozone depleting substances are also effective greenhouse gases. Even though much has been achieved, the future of the stratospheric ozone layer relies on full compliance of the Montreal Protocol by all countries for the remaining substances, including methyl bromide, as well as strict monitoring of potential risks from the production of substitute chemicals. Also the ozone depleting substances existing in banks and equipment need special attention to prevent their release to the stratosphere. Since many of the ozone depleting substances already in the atmosphere are long-lived, recovery cannot be immediate and present projections estimate a return to pre-1980 levels by 2050 to 2075. It has also been predicted that the interactions of the effects of the ozone layer and that of other climate change factors will become increasingly important.

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Introduction

The Parties to the Montreal Protocol are informed by three panels of experts. One of these is the Environmental Effects Assessment Panel (EEAP), which deals with effects of ozone depletion and its interactions with climate change. The EEAP produces an extensive assessment report for the Parties to the Montreal Protocol every four years, and in the intermediate years a brief Progress Report is prepared. These assessments aim at readability for non-specialists, but are based on scientific information published in the scientific literature. The latest full report was published in Photochem. Photobiol. Sci., 2007, 6, 201–332. The present Progress Report gives an update since the last full report.

Ozone and changes in biologically active UV radiation reaching the Earth's surface

• Although the Montreal Protocol has succeeded in controlling most of the ozone depleting substances, the ozone layer remains under threat. Thus future surface UV-B radiation is still a matter of concern

Ozone depletion from anthropogenically produced nitrogen oxides is likely to be more important in the future as the concentrations of atmospheric chlorine decline.¹ Larger amounts of chlorofluorocarbons (CFCs) than previously estimated are contained in existing products, and a large proportion of these CFCs may eventually be released to the atmosphere, where they will continue to destroy ozone.² A new analysis of global ozone observations suggests that, although the slowdown of ozone depletion is statistically significant at northern mid-latitudes, this slowdown is in part due to changes in atmospheric circulation. In the Southern Hemisphere, where these changes in atmospheric circulation have been smaller, the levelling off is less conclusive. Thus the attribution of the levelling off of ozone columns due to reductions of chlorine and bromine has not yet been verified.³Ozone recovery should be detectable earlier at southern middle- and high-subpolar latitudes where changes are larger, and the natural ozone variability is smaller.⁴

• There are strong interactions between ozone depletion and climate change, and implementation of the Montreal Protocol has had a substantial effect in mitigating global warming

The regulation of ozone depleting substances under the Montreal Protocol has had a far greater impact on reducing the severity of global warming than the calculated benefits from the reduction targets of greenhouse gases for the first commitment period of the Kyoto Protocol.⁵ In recent decades, increases in Antarctic temperatures may have been suppressed by changes in stratospheric ozone affecting wind patterns, so melting of the west Antarctic ice sheet may proceed faster in future decades as ozone recovers.⁶ The effects of changes in stratospheric chemistry and circulation associated with ozone recovery have not been included in the models used in previous assessments of climate change.⁷ Improved predictions of climate change should be achieved if the upper boundary of these models is extended to include the stratosphere.

• Further studies have confirmed the increases in atmospheric transparency to total solar radiation in the 1990s, but this tendency has been less pronounced since 2000

Solar radiation measurements under all sky conditions in the USA showed an increase from 1997 to 2000 followed by a decrease from 2001 to 2004.⁸ Analysis of satellite data over Europe also shows an increase in transmittance between 1987 and 2002.⁹ UV radiation changes due to changes in aerosols observed in Greece over the period 1997–2005, have been broadly consistent with these findings.¹⁰ The periods above are very short, but if the current trends in atmospheric transmittance continue, that would imply that the peaks of UV radiation may already have occurred in these regions.

• In the early period of the Earth's history, there may have been times when UV-B irradiances were much higher than at present. However, these occurred long before the emergence of mankind

A recent modelling study showed that in the period ∼4000 million years ago, UV-B radiation may have been several orders of magnitude higher than at present.¹¹ Another modelling study suggested that ∼250 million years ago the UV-B levels were also elevated, mainly at higher latitudes.¹² These elevated levels of UV-B were primarily due to differences in the composition of the Earth's atmosphere, including oxygen and ozone. Past changes in UV-B, their causes, and methods to probe them have recently been reviewed.¹³

• Balancing the risks and benefits of solar UV radiation is a challenge for policymakers and health advisors

Because of the success of the Montreal Protocol, increases in UV-B radiation due to ozone depletion have been modest in populated regions of the world (i.e., outside the region of the Antarctic ozone hole). The most important determinant of UV radiation at the surface is the path length of the radiation through the atmosphere. Seasonal changes in sun angle are therefore responsible for huge latitudinal and seasonal variations in UV. For example, at mid latitudes, the UV index (UVI) at noon in winter is typically 1, which is only ca. 10% of that in the summer. At higher latitudes, the UVI at noon is lower, and the summer/winter contrast (the ratio of UVI between summer and winter) also increases rapidly. The daily doses of UV radiation show even more marked seasonal variation due to the shorter length of day in winter.

The main beneficial effect of UV radiation on human health is the synthesis of vitamin D in the skin. The action spectrum for vitamin D production¹⁴ is confined mainly to the UV-B region. Therefore, vitamin D-weighted UV radiation shows even larger summer/winter contrasts than the UVI. Despite the increases in UV-B that have occurred due to ozone depletion at mid to high latitudes in both hemispheres, any wintertime deficiencies in vitamin D-weighted UV radiation are unlikely to have been ameliorated (see under Health, below). Monthly climatological maps of the mean vitamin D-weighted UV radiation, and various other biological weightings, are now available.¹⁵ However, it should be noted that information about the action spectrum for vitamin D production is still incomplete.

Health

• Solar UV-B, which is increased due to ozone depletion, represents a major risk factor for skin cancer: the incidences of the non-melanoma skin cancers, basal cell carcinoma and squamous cell carcinoma, and cutaneous melanoma continue to rise

The most recent data from several countries indicate that the incidence of cutaneous melanoma (CM) is rising at the highest rate for any form of reported cancer. In addition, although CM is uncommon in individuals under the age of 20, its incidence in teenagers in the USA has increased by 2.9% per year between 1973 and 2003, and is higher in areas of the country with greater sun exposure.¹⁶ This observation is consistent with the results of a study that monitored trends in sunburn prevalence amongst adults living in the USA and found that it increased from about 32% in 1999 to about 34% in 2004.¹⁷ Similarly to CM, in many countries there are also significant annual increases in the incidences of the non-melanoma skin cancers (NMSC).¹⁸ As an example of the seriousness of the health issues involved, the incidence of NMSC in Australia in 2002 was five times greater than the incidence of all other cancers combined and the treatment and diagnosis costs were also the largest. In 2002, almost 2% of the fair-skinned Australian population was treated for NMSC. Trends between 1985 and 2002 revealed increases in overall rate but analysis by age cohort found that rates had stabilized for subjects under the age of 60 for basal cell carcinoma (BCC) and under the age of 50 for squamous cell carcinoma (SCC). This stabilisation may be due to the introduction of public health educational programmes, begun in Australia around 1970, that emphasised moderate sun exposure.¹⁹ In Europe, there is a need to record and analyse comparative epidemiological data relating to NMSC if an effective public health policy is to be developed.²⁰ The sensitivity of an individual to sunburn and the number of lifetime sunburns have been linked with beta-papillomavirus infection and the subsequent development of SCC. These associations merit further investigation.^21,22

• A recent study in new cutaneous melanoma patients has linked residence in locations with high ambient solar UV-B during the first two decades of life to an increased risk of mutations in an oncogene (BRAF) commonly found in cutaneous melanomas

This finding further supports the need to protect children, particularly those with fair skins, from excessive exposure to solar UV radiation.²³

• The association between ocular melanoma and solar UV exposure is still poorly understood

A recent investigation has shown that the risk of external (eyelid and conjunctival) ocular melanoma was increased at lower latitudes (20°–22°).²⁴ This finding is similar to that observed for skin melanoma. On the other hand, the incidence of internal (uveal) ocular melanoma increased at higher latitudes (47°–48°), possibly via the mechanism outlined below for various other internal tumours.^25,26 An alternative hypothesis is that the increased incidence of internal ocular melanoma at higher latitudes supports a direct carcinogenic effect,²⁷ based on greater direct and indirect ocular exposure at these latitudes.^28,29 Another population-based study compared the association between ocular melanomas and 32 other types of primary cancers and found that the risk of ocular melanoma was significantly increased only after prostate cancer.³⁰

• Subjects with skin cancer, the occurrence of which is strongly linked with solar UV-B radiation, have a lowered risk of developing many other internal cancers and a lower risk of osteoporotic fractures

A study by Tuohimaa et al.³¹ showed that the risk of subjects developing an internal solid cancer following the diagnosis of skin cancer was decreased by 35% for BCC and by 42% for SCC in sunny countries, such as Australia, compared with less sunny countries, such as Finland, as assessed by the average annual erythemal UV doses. Grant³² undertook a meta-analysis of second cancers that had developed following a diagnosis of NMSC and found that the relative risks of several internal cancers, e.g., cervical, oesophageal, and gastric, were reduced. Similarly, de Vries et al.²⁶ demonstrated that skin cancer patients in the south-east region of the Netherlands had an 11% decreased risk of developing prostate cancer compared to the general population. In addition, the risk of advanced prostate cancer was reduced by 27%. Finally a retrospective cohort study in people over the age of 50 living in southern Tasmania found that the incidence of prior NMSC in those with a fracture was 31% lower than in the general population.³³ As solar UV-B exposure is the major environmental risk factor for skin cancer, these results have been interpreted to indicate that UV radiation can lower the chances of developing several internal cancers, and can protect against osteoporotic fractures in later life. In all the above studies, it was suggested that the protective factor induced by the solar UV radiation was vitamin D (see the following bullet).

• Exposure of the skin to solar UV-B radiation is associated with reduced risk of certain internal cancers and autoimmune diseases

Vitamin D, synthesised in the skin in response to the UV-B radiation, has been proposed as the protective agent in these effects. For optimisation of vitamin D levels, there are two options available: additional UV-B exposures or added vitamin D, via diet and/or supplementation. Both options have advantages and disadvantages which require careful consideration by individuals and public health agencies. It is recognised that a significant proportion of the general population, particularly those living at mid to high latitudes, do not have optimal vitamin D levels.³⁴Vitamin D is not only essential for good bone health but also helps to protect against several internal cancers and autoimmune diseases. Recent reports have revealed that there is an inverse correlation between sun exposure and the incidence, severity or mortality from several internal cancers (for example, see^25,35,36) and the autoimmune disease, multiple sclerosis (for example, see^37,38). As the protection associated with sun exposure could be mediated by vitamin D, recent discussion has centred on the best method of attaining optimal levels of this hormone. In most people, giving additional UV-B exposures results in increased synthesis of vitamin D in the skin. Such additional exposures may be required in situations where the solar UV-B irradiance is low, for example in the winter months at mid to high latitudes, in dark-skinned people where much of the UV-B is absorbed by the melanin pigment, in the elderly living indoors and in veiled women. The drawback of increasing UV-B exposures lies in increasing the risk of skin cancer development. Advocates of this option limit this risk by advising that the additional exposures take place only when the UV-B irradiance is low such as in the winter months or when the UV Index is less than 3 (www.cancer.org.au/positionVitD; www.cancer.ca). Advocates of administering vitamin D in the diet or by supplementation argue that consistent optimal levels can be reached with safety and at all latitudes throughout the year.^39,40 Each public health agency dealing with this matter will require to make its own recommendations but will need to consider individual subject groups, as well as the whole population. Their task should become easier as more data become available.

• Regular sunscreen use protects against skin cancer. However, it may also contribute to inadequate vitamin D status in those for whom sun exposure is the principal source of this nutrient

A study in Australia earlier reported that a sunscreen group (subjects who regularly applied sunscreen to the head, neck, hands and forearms over a 4.5 year period) had a lower incidence of first SCCs but not BCCs when compared to a control group (subjects who did not regularly apply sunscreen).⁴¹ This benefit has now been shown to persist even 8 years after the end of the study since first SCCs continued to occur at a lower rate in the sunscreen group than in the control group.⁴² BCC and melanoma rates also tended to decrease in the group formerly using the sunscreen. Earlier information indicated that sunscreen usage in practice does not significantly lower the individual's vitamin D level. A new report on theoretical computations confirms the obviously expected reduction in UV-driven synthesis of previtamin D by adequate application of a sunscreen (sun protection factor, SPF 15),⁴³ but also claims that the sunscreen blocks previtamin D formation much more effectively than it does sunburn. However, it should be noted that these calculations are hampered by a lack of data on previtamin D formation at wavelengths longer than 315 nm, and possible back reactions and wavelength interactions at high UV doses. Moreover, sunscreens allow the transmission of a fraction of incident UV photons: for example for SPF 15, it would be 7%.⁴⁰ In addition the common usage of sunscreens is considerably below the recommended amount, and there are also unprotected or poorly protected body sites that may accidentally receive sun exposure. Very recently, it has been discovered that compounds that activate the tanning pathway can also reduce inflammation and promote DNA repair. Skin protection products based on these observations are under development.^44–46

• Further studies implicate chronic sunlight exposure as the main risk factor for the development of pterygium. Agents with the potential to protect against pterygium are beginning to be identified

Pterygium is a wing-shaped inflammatory, proliferative and invasive growth on the conjunctiva and cornea of the human eye that can impair vision.⁴⁷ Several reports confirm the association between repeated exposure to solar UV-B and primary and recurrent pterygium in different populations.^48–52 Two topical anti-inflammatory agents that could reduce the progression of the disease and the necessity for surgical intervention have been found.⁵³ Conversely, it is possible that photorefractive keratectomy (PRK), a laser surgical procedure on the cornea to correct refractive error, may induce the rapid growth of an existing pterygium.⁵⁴ Fluorescence photography might reveal areas of cellular activity within a pterygium that would be useful to the further understanding of pterygium growth and pathogenesis.⁵⁵

• There is considerable research evaluating whether changes in vector-borne diseases are associated with global climate change. In many of these studies the possibility that such diseases may be exacerbated by the impairment of immune responses from increased solar UV-B has not been considered

Several groups have examined the relationship of changes in climate to changes in the natural history of vector-borne diseases,^56–58 but have not considered the possible contribution of the concomitant changes in solar UV-B flux. Thus, for example, a recent evaluation of the role of climate change in the increased incidence of tick-borne encephalitis in the Baltic countries concluded that changes in climate were insufficient to explain the observed effects.⁵⁷ However, alterations in solar UV-B exposure during the time period were not explored, despite the fact that the resistance to ticks and tick-borne diseases can be modified by this environmental factor.^59,60

Terrestrial ecosystems

• Plant growth can be significantly reduced by enhanced UV-B radiation, but this is dependent on the species and can be influenced by other environmental variables common to climate change

For example, recent research with trees has shown no significant UV-B response in some species^61,62 but significant reductions in growth of others.^63–67 Sensitivity of plants to UV-B radiation can be modified by other environmental variables that might be affected by climate change including elevated CO₂,⁶⁸ drought,^64,65 and nutrient supply. Increases in UV-B resulting from ozone depletion are unlikely to have large-scale effects on carbon capture and storage by terrestrial vegetation even though some plant species or communities may have reduced photosynthesis and growth.

• Water deprivation stress can alter response to enhanced UV-B radiation

One study,⁶⁴ conducted in a UV-transparent greenhouse supplementing ambient UV-B radiation with a filtered lamp system, examined the response of an upland and lowland species of poplar tree (Populus) to this combination of stresses. For both the lowland and upland species, leaf thickness exhibited an additive response to the combined effects of water stress and UV-B radiation. For leaf area and plant height there were synergistic effects of the combined stresses in the lowland species, while there were no interactive effects of these stresses for the upland species. Another study conducted in a greenhouse⁶² reported a significant interaction between water stress and UV-B radiation. The UV-B radiation increased the balance between root and shoot mass (root : shoot ratio) of drought stressed plants to a greater extent that water stress by itself. The UV-B radiation had no effect on root : shoot ratios of well watered plants.

• Abundant nitrogen supply to plants can increase their sensitivity to UV-B radiation

Several recent studies show that plant sensitivity to UV-B radiation generally increases when plants are provided with supplemental nitrogen, which in some cases has been linked to a decreased synthesis of protective UV-absorbing compounds⁶⁹ or to a lower antioxidant capacity in plants that had received supplemental nitrogen.⁶⁷ Excess nitrogen supply to plants occurs at large regional scales on the Earth due to atmospheric nitrogen deposition and is a component of global change. Nitrogen as fertiliser is also often applied to agricultural fields.

• In some regions, available nitrogen is in short supply and is dependent on biological nitrogen fixation, which can be reduced by enhanced UV-B radiation

In a high-latitude, low-nitrogen Arctic ecosystem, much of the nitrogen input comes from biological nitrogen fixation by cyanobacteria. A review of multi-year field studies reported that in this Arctic vegetation, nitrogen fixation by cyanobacteria associated with one species of moss was significantly reduced by exposure to enhanced UV-B radiation. This was not found in cyanobacteria associated with another sub-Arctic moss species nor for those associated with several species of lichens.⁷⁰

• Solar UV-B radiation can influence nitrogen oxide emissions from vegetation

Nitrogen oxides (NO and NO₂) are a common group of trace gases that play a critical role in atmospheric chemistry. Earlier work has shown that plants may release nitrogen oxides to the atmosphere when exposed to solar UV-B radiation.⁷¹ The nitrogen compounds released from plants are generally thought to originate from inside the leaf. Plant leaves with a higher content of nitrogen show greater nitrogen dioxide emission rates, whereas plants with high leaf concentrations of an antioxidant, ascorbate, tend to take up nitrogen dioxide instead.⁷² Since solar UV-B radiation can increase the content of ascorbate in leaves⁷³ variations in solar UV-B may indirectly affect the flux of nitrogen oxides (especially NO₂) to the atmosphere. Other recent work⁷⁴ indicates that solar UV-B can directly cause the release of nitrogen oxides through the degradation of nitrogen-containing compounds (such as nitrate or nitric acid) deposited on foliage surfaces from air pollution.

• Ambient UV-B radiation can alter the composition of leaf-surface bacteria and this, in turn, can increase the incidence of blister blight, a fungal disease on tea plants

The dynamics of leaf-surface microorganisms can be greatly influenced by solar UV-B radiation. Recent field studies⁷⁵ showed that following rains when leaves of the tea plant were still moist, removal of ambient solar UV-B radiation increased the prominence of one leaf surface bacteria species. This also led to a decrease in the blister blight disease. However, earlier studies had shown that the fungus causing blister blight was sensitive to UV-B radiation. Thus, this research suggests that ambient solar UV-B radiation increases the prominence of one bacterial species on the leaf surface leading to a change in competition among leaf microorganisms that was more important in reducing the blight disease than direct solar UV-B damage to the blight-causing fungus. Other recent studies have shown that solar UV also damages bacteria-consuming viruses present on the leaf surface.⁷⁶ Thus, the effects of ambient solar UV-B radiation on plant diseases may be due to several biological interactions on the leaf surface. Similarly, plant-pest interactions may be affected by UV effects on micro-organisms that attack pest insects.⁷⁷

• Solar UV-B radiation affects insect activity and this is mediated by changes in plant tissue chemistry

Many previous studies have found that UV-B radiation reduced herbivory by insect larvae. A recent study⁷⁸ has shown that adult diamondback moths laid fewer eggs on plants grown under natural levels of solar UV-B radiation than on plants grown under attenuated levels of UV-B radiation. Decreased egg laying was not detectable on plants that were genetically impaired in their ability to respond to jasmonic acid, indicating that this defence-related plant hormone plays a role mediating the responses to solar UV-B under field conditions. Another recent study⁷⁹ reported that some, but not all, of the UV-B induced changes in plant tissue chemistry are similar to those induced by insect herbivory.

• Understanding the diversity of UV-B-induced responses in plants is being further strengthened by combining physiological approaches with molecular and genetic tools

Some plant responses to UV-B are similar to those induced by several other environmental stresses, as indicated by several examples of responses involving convergent gene expression.^69,80 However, some responses appear to be largely UV-B-specific. Both general-stress and UV-B-specific responses appear to occur under natural conditions. For example, solar UV-B radiation induces accumulation of UV-screening pigments (phenolic acids and flavonoids) in wild tobacco,⁷⁹ whereas, insect herbivory led to the induction of phenolic acids, but did not result in the induction of flavonoids.

Aquatic ecosystems

• Human-induced global climate change, ozone depletion and the links between them have important implications for marine coastal ecosystems and the economic and social systems that depend on them

Marine habitats from the intertidal zone through the continental shelf are estimated to provide over US$14 trillion worth of ecosystem goods and services per year, such as food, raw material, and nutrient cycling. These estimates include costs involved in restoring ecosystem integrity from potential damage. If human-induced changes continue at their present rate, we risk serious degradation of marine ecosystems, with far-reaching consequences for human health and welfare.⁸¹

• A comprehensive new database has been established for major UV-absorbing compounds in primary producers and consumers in aquatic ecosystems

The database, which is continuously updated, is available online covering the occurrence, photophysical properties and biochemical and structural data of UV-absorbing substances in fungi, cyanobacteria, macroalgae, phytoplankton and many taxonomic groups of animals.⁸² The molecular spatial structure of one key substance (porphyra-334) has been identified.⁸³ These UV-absorbing substances are also considered as natural substitutes for synthetic UV-protective sunscreens for humans.⁸⁴

• The degree of protection against enhanced solar ultraviolet radiation in phytoplankton and macroalgae depends on the availability and concentration of nutrients in the water column

UV-absorbing substances such as mycosporine-like amino acids (MAAs) and scytonemin protect exposed aquatic organisms from damage.^85–87 Several studies indicate that sufficient supply of nitrogen is a prerequisite for the production of UV-protective MAAs, which are also considered as a form of nitrogen storage.^88,89 Likewise, non-limiting levels of phosphorus and sulfur also enhance the synthesis of UV-absorbing compounds. Since MAAs provide efficient protection against enhanced solar UV radiation, the organisms show a higher photosynthetic yield, lower photoinhibition and better growth rate when nutrients are available. Other environmental variables which regulate MAA accumulation are UV irradiation, salinity and temperature.⁹⁰ Global climate change-related temperature increases may reduce UV-B-induced stress by increasing the production of UV-screening pigments.

• Microcosm experiments demonstrate the complexity of determining impacts of multiple stressors including enhanced UV-B on aquatic communities

The combination of the water-soluble fraction of crude oil and enhanced UV-B radiation exposure resulted in a synergistic effect of both stressors. Results are dependent on the time and space variability of many environmental factors including nutritional state, species composition, as well as light conditions.⁹¹ Experiments with surface containers simulating natural ecosystems at three different latitudes (northern Canada, Brazil and southern Argentina) demonstrate that communities that differ in terms of light-adaptation characteristics respond differently to enhanced solar UV-B. Enhanced UV-B significantly increased photoinhibition in these surface containers, but no effects were observed in the stirred mesocosms at the 3 sites, indicating that mixing moderated the UV-B effects.⁹² In alpine lakes UV had a negative effect on larval stages and egg-bearing females at low temperatures and at low, ambient concentrations of dissolved organic matter (DOM), but had no effect at higher temperatures or when DOM was added.⁹³ Copepods fed on non-irradiated diatoms produced three times more eggs, healthier offspring with fewer lethal larvae deformities than those fed on UV-B-exposed, live diatoms.⁹⁴

• In nature, thin layers of microalgae (biofilms) typically show high resistance to UV radiation as compared to related aquatic strains

There are relatively few studies on green algae which colonize architectural surfaces but UV-exposure experiments indicated that all investigated species were capable of synthesizing and accumulating UV-absorbing compounds (MAAs), explaining the conspicuous ecological success of these green algae in biofilms on facades of buildings.⁹⁵

• Recent observations suggest that penetration of solar UV-B into Crater Lake (Oregon) has a significant ecological impact

The density and structure of the phytoplankton community is inversely correlated with the level of UV-B due to natural variability in column ozone. The ecological adaptation of the community includes the appearance of UV-resistant organisms and the low diversity of organisms in the surface waters of the lake. The absence of organisms higher in the food chain near the surface of Crater Lake is also likely influenced directly or indirectly by UV radiation although experimental evidence to exclude other factors (grazing, predation, high levels of photosynthetic available radiation (PAR)) is lacking.⁹⁶

• Temporal changes in UV transparency, particularly in deep water layers, are mainly controlled by the growth of phytoplankton. Dissolved organic carbon (DOC) concentration is not an accurate predictor of coloured dissolved organic material (CDOM) in alpine lakes

While earlier models used DOC as a predictor for estimating the penetration of UV-B, recent results indicated that more than 50% of the DOM did not have significant UV absorption.⁹⁷

Biogeochemical cycles

• Acidification of the surface ocean related to climate change is causing increased stress on ocean ecosystems that will increase the vulnerability and sensitivity of ocean biogeochemistry to variations in UV-B fluxes

Recent evidence indicates that the surface ocean pH is decreasing in response to increases in the atmospheric concentration of CO₂.⁹⁸ This acidification-related stress potentially can interact with UV impacts on photosynthetic organisms,^99,100 resulting in altered CO₂ uptake by the ocean. Ocean acidification can also alter sources and sinks of halocarbons, sulfur compounds, carbon monoxide, and hydrocarbons and change the biological availability of metals. The interactions between acidification and ozone depletion are intensified at mid to high latitudes where ozone depletion is most pronounced.

• Climate change interacts with UV-related biogeochemical carbon cycling by changing the transport of coloured dissolved organic matter (CDOM) from terrestrial to aquatic systems and by altering CDOM transport within aquatic systems

Aquatic organisms that play important roles in carbon uptake and storage, such as phytoplankton and coral reefs, are damaged by UV radiation. CDOM absorbs UV so changes in the quality and quantity of CDOM can increase UV penetration into aquatic environments, damaging biological processes (also see Aquatic ecosystems, above).^101,102 The role of terrestrial ecosystems as significant sources for CDOM in aquatic systems has been confirmed,^103–106 and the transport of CDOM from terrestrial to aquatic systems is affected by factors such as rainfall and run-off that are expected to change as a result of climate change. Melting of land and sea ice alters the quality and quantity of CDOM and other substances delivered to aquatic systems. Within aquatic systems, climate change is affecting currents, vertical mixing, residence time,^107,108 and stratification^102,107,109 which influence CDOM variability.

• Solar UV-B can significantly influence the decomposition of dead plant material (litter) in terrestrial ecosystems

When litter from tree species from a semi-arid region was exposed to UV in microcosms, the rate of mass loss during decomposition was not altered compared to non-irradiated litter.¹¹⁰ Earlier reports have indicated that UV exposure can accelerate mass loss from litter of some other species.¹¹¹ Changes did occur in the quantity and chemical composition of the dissolved organic carbon (DOC) that could be extracted from the decomposing litter.¹¹⁰ We concluded in previous reports that such changes in decomposition due to changes in UV-B may have wide-ranging effects on ecosystem processes, even though effects on total carbon released from terrestrial ecosystems are small. For example, changes in DOC released from litter and, perhaps, changes in the phenolic chemical component of leaves (which are a well-defined response to natural and experimental variation in UV-B)^112–115 may influence soil processes, and the release of DOC from terrestrial to aquatic systems (see above).

• The chemical composition of organic matter in aquatic systems is linked to climate-induced changes in its sources and water chemistry, with consequences for UV exposure and the biological availability of the organic matter

DOM from terrestrial sources has a greater capacity to absorb UV radiation than DOM from within aquatic systems.¹⁰³ Algal-derived DOM has been reported to be biologically more labile than terrestrial-derived DOM.^116,117 UV can transform bioavailable into non-bioavailable compounds and vice versa.¹¹⁸ Upon exposure to UV radiation, terrestrial-derived DOM becomes more like that from aquatic sources.^119–121 Contraction of sea ice cover could accelerate photomineralization of terrestrial-derived DOM in Arctic surface waters,¹²² and the effects of climate change on the chemical composition of aquatic environments (salinity, pH, iron content) also influences rates of UV-induced CDOM transformations, i.e. CDOM photobleaching.¹²⁰ As a consequence, interactive effects of solar UV radiation and climate-related changes in continental hydrology could cause a net loss of organic carbon from terrestrial ecosystems and a UV-mediated positive feedback of CO₂ accumulation in the atmosphere.¹¹¹

• Changes in UV may affect phytoplankton emissions of sulfur compounds and hydrocarbons that form aerosols that affect clouds over the ocean

Dimethylsulfide (DMS) is the major source of volatile sulfur to the marine atmosphere. DMS concentrations in the subpolar and subtropical North Pacific have been shown to exhibit a linear increasing trend between 1970 and 2000 with a concomitant increase of the DMS flux from sea to air.¹²³ Melting sea ice can release substantial quantities of DMS, leading to elevated seawater DMS concentrations,¹²⁴ and this input would be expected to increase due to climate change. The effects of changing UV radiation on DMS are likely to be complex. Both UV radiation and nitrogen limitation have been shown to enhance the algal metabolism that produces DMS.^125,126 On the other hand, UV exposure can reduce nitrogen limitation in surface waters,^127,128 and this process may decrease algal DMS production. Furthermore, photolysis of DMS is an important sink of DMS in the upper ocean.^111,129 In addition to the key role of DMS, a new study has proposed that the link between clouds and a phytoplankton bloom in the Southern Ocean was related to organic aerosols produced by oxidation of phytoplankton-derived isoprene.¹³⁰

• Changes in agricultural and natural sources of methyl halides are significant in atmospheric budgets of ozone-depleting substances

Coastal vegetation provides important sinks and sources for atmospheric trace gases. Annual budgets of methyl halides suggest that coastal vegetation may be net sources,^131–133 or net sinks.^134,135 Even within individual ecosystems, methyl halide fluxes vary due, for example, to weather, the extent of flooding, and the removal of plant biomass.^131–135 Recent data suggest that plant biomass production in salt marsh plants may be decreased by increased UV-B radiation¹³⁶ and this decrease likely reduces net methyl halide fluxes from coastal ecosystems. A laboratory study demonstrated that mangrove trees can be net emitters of methyl halides.¹³⁷ Soil fungi have been confirmed as potential sources of methyl halides.¹³⁸ Agriculture and horticulture remain significant sources from the use of methyl bromide, but recent research has demonstrated the efficacy of a number of alternative technologies that may ultimately replace methyl bromide.^139–143

• UV-B-mediated degradation of DOM may enhance the toxicity of copper

The complexation of copper (Cu) by DOM regulates Cu toxicity by decreasing the concentration of the bioavailable form of copper, which is Cu²⁺.¹⁴⁴ UV-mediated degradation of DOM compounds that form strong complexes with copper has been shown to increase the concentration of the bioavailable and hence toxic form of Cu.^108,144,145 This phenomenon may be especially critical in freshwater aquatic ecosystems that receive sewage discharges with high concentrations of Cu.

Air quality

• The direct links between stratospheric ozone, climate change, and air quality will continue to be important for human health, the environment, and agricultural productivity for several decades

It is well recognized that reductions in air quality, which cause adverse effects such as acid rain and respiratory problems, play a significant role in both environmental and human health.^146–148 Increased solar UV-B radiation (280–315 nm) provides the energy for many of the chemical transformations that affect concentrations of pollutants (i.e., sulfur dioxide, formaldehyde, and ozone) in the troposphere. These processes are all affected by climate change and will continue to be important in the near future. In this regard, future increases in tropospheric ozone concentrations resulting from the effects of climate change combined with stratospheric ozone recovery may have more serious effects in humans than expected. Results from studies in human volunteers indicate that greater concentrations of ozone elicit a larger effect on physiological response (such as forced expiratory volume in 1 s), than would be expected.¹⁴⁹ This implies a nonlinear dose-response relationship with greater than expected effects at larger exposures. These findings may require reassessment of current exposure guidelines for ozone.

• Present predictions of stratospheric ozone recovery do not consider the interactions with tropospheric ozone chemistry

According to recent studies,^150–153 as our understanding of the changes in stratospheric ozone improves, the high latitude middle atmosphere consequences of stratospheric ozone recovery are likely to become more certain. However, the effects of direct (stratospheric troposphere exchange) and indirect (changes in UV) processes will need to be considered together with longer model integrations for the predictions of ozone recovery.¹⁵¹

• Analysis of long term changes in tropospheric ozone have revealed as yet unexplained inconsistencies related to where the measurements were carried out

Over Europe, trends in tropospheric ozone are inconsistent between different measurement sites, due at least partially to changes in methods of measurement over time.¹⁵⁴ The significant correlation observed in Canadian data between tropospheric and stratospheric ozone¹⁵⁵ was not observed in the European data. For the Southern Hemisphere, significant increases in tropospheric ozone have been observed at “clean air” sites in September since 1973.¹⁵⁶ The cause is unclear; the data do not show a long-term change in the upper troposphere that could be indicative of enhanced fluxes from the stratosphere. Interactions with atmospheric pollutants from burning of biomass also do not explain this observation.¹⁵⁶

• A number of recent studies have clarified how ozone is generated in the troposphere, which improves the ability for evaluation of global and regional atmospheric chemical models

A simulation with the climate-chemistry model showed a good representation of the changes in the stratospheric ozone layer, including the ozone hole. These models can improve the simulation of natural variability of tropospheric ozone.¹⁵⁷ The model also explains changes in the stratospheric ozone and its flux into the troposphere, which tends to reduce the simulated positive trend in tropospheric ozone due to emissions from industry and traffic during the late 1980s and early 1990s. While differing in detail, similar conclusions have been reported using a different atmospheric model.¹⁵⁸ This provides useful and necessary information on the impact of large-scale processes and inter-annual/decadal variations on regional air quality and indicates that ozone trends can be weakened or strengthened by stratospheric processes, climate-chemistry interactions and circulation patterns, such as El Niño. In addition, the results of studies by several groups who studied local atmospheric chemistry^159–161 and satellite-based estimates of ozone precursors^162,163 have substantially improved the ability for evaluation of global and regional atmospheric chemical models. There are still significant differences between models of tropospheric ozone chemistry, so that predictions of future tropospheric ozone trends may change significantly with refinement of these complex atmospheric chemical models.¹⁶⁴

Materials

• Outdoor lifetimes of plastic and wood materials are determined primarily by the dose of solar UV-B radiation received by them

Materials exposed to solar UV radiation undergo dose-dependent degradation leading to loss in their useful properties over a period of time. This damage is exacerbated by higher ambient temperatures, higher humidity levels and atmospheric pollutants. Therefore an increase in ambient UV levels and average temperatures due to climate change can significantly shorten the outdoor useful lifetimes of natural and synthetic materials. While the effects can be countered by using light-stabilizers in these materials, higher solar UV-B levels will require correspondingly higher levels of stabilizers and therefore result in higher material costs.

• Improvement in solar UV-B resistance of polycarbonate glazing materials was recently demonstrated

Polycarbonates used in glazing applications undergo yellowing and loss of mechanical strength upon exposure to solar UV wavelengths.¹⁶⁵ Novel inorganic surface coatings (with zinc oxides and alumino-zinc oxides),¹⁶⁶ or acrylic film laminates,¹⁶⁷ were shown to effectively stabilize polycarbonate materials against such degradation. Novel co-polymers of polycarbonate that generate a protective UV-B absorbing surface coating on initial exposure to sunlight were also reported.¹⁶⁸ These approaches may allow low-cost alternative light-stabilization to be achieved in commercial polycarbonate glazing materials.

• Nanoparticle fillers are being identified as better UV-stabilizers compared to conventional fillers in plastics formulations

Some types of inorganic fillers used in plastics compositions act as UV stabilizers. Nanoparticle fillers of UV-absorbing materials (such as the rutile form of titania) yield better UV-resistant plastics formulations compared to those with conventional fillers.^169,170 This is the result of high specific surface area per unit mass and consequent efficient light screening ability, of ultra-small nanofiller particles. This effect was demonstrated for several common plastics such as polyethylenes,^171,172 polypropylene,¹⁷⁰ and polyurethane,¹⁷³ used in outdoor applications and may lead to efficient UV stabilizers based on nanofillers.

• Better performing novel UV-stabilizer systems for plastics including synergistic mixtures of stabilizers have been identified

Further evidence of synergistic UV stabilization by mixtures of hindered amine type light stabilizers (HALS) and light absorbers has been reported in polyester clearcoat formulations.¹⁷⁴ Novel polymeric UV absorbers that performed well in acrylic plastics and in polycarbonates were also reported.¹⁷⁵ A new class of bifunctional UV stabilizers that combines the chemical structure of HALS with that of light absorbers in a single molecule was synthesized.¹⁷⁶ Exploiting these developments can result in longer outdoor lifetimes for plastics as well as wood materials.

• Improved approaches for stabilizing wood-plastic composites have been explored

Synergy was observed in hardwood-polyethylene composite (WPC) materials when UV absorbers and HALS were used together as a mixed stabilizer.¹⁷⁷ Lamination of the surface of wood already stabilized with HALS with thin light-barrier plastic films also resulted in significant increased outdoor durability. However, it is the manufacturing conditions that have a large and direct effect on their durability.¹⁷⁸ Fabricating WPCs with a higher plastic content at the surface relative to the bulk resulted in their better outdoor durability.

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Footnote

† List of contributing authors in alphabetical order: Anthony Andrady, Pieter J. Aucamp, Alkiviadis F. Bais, Carlos L. Ballaré, Lars Olof Björn, Janet F. Bornman (Co-Chair), Martyn Caldwell, Anthony P. Cullen, David J. Erickson, Frank R. de Gruijl, Donat-P. Häder, Mohammad Ilyas, G. Kulandaivelu, H. D. Kumar, Janice Longstreth, Richard L. McKenzie, Mary Norval, Nigel Paul, Halim Hamid Redhwi, Raymond C. Smith, Keith R. Solomon (Secretary), Barbara Sulzberger, Yukio Takizawa, Xiaoyan Tang (Co-Chair), Alan H. Teramura, Ayako Torikai, Jan C. van der Leun (Co-Chair), Stephen R. Wilson, Robert C. Worrest and Richard G. Zepp.

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