Environmental effects of ozone depletion and its interactions with climate change: progress report, 2015

Abstract

The Environmental Effects Assessment Panel (EEAP) is one of three Panels that regularly informs the Parties (countries) to the Montreal Protocol on the effects of ozone depletion and the consequences of climate change interactions with respect to human health, animals, plants, biogeochemistry, air quality, and materials. The Panels provide a detailed assessment report every four years. The most recent 2014 Quadrennial Assessment by the EEAP was published as a special issue of seven papers in 2015 (Photochem. Photobiol. Sci., 2015, 14, 1–184). The next Quadrennial Assessment will be published in 2018/2019. In the interim, the EEAP generally produces an annual update or progress report of the relevant scientific findings. The present progress report for 2015 assesses some of the highlights and new insights with regard to the interactive nature of the effects of UV radiation, atmospheric processes, and climate change.

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We would like to dedicate this progress report to the memory of Dr David J. Erickson III (20 May, 1960–16 November, 2015), member of the Environmental Effects Assessment Panel since 1994 and contributor to the section on biogeochemical cycles. He was a scientist, colleague, and friend to all the members of the Panel. His expertise and knowledge of issues related to climate change and the effects of UV radiation on chemical processes in the environment will be sorely missed. We will remember Dave for his colourful, larger than life, and humorous character – full of spirit and courage.

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

1.1 By 2013, the implementation of the Montreal Protocol had already achieved significant benefits for the ozone layer and, consequently, for surface UV-B radiation

Model calculations have shown that, without the Montreal Protocol, a deep Arctic “ozone hole”, with total ozone values of below 120 DU, would have occurred in 2011 given the meteorological conditions in that year (Fig. 1). The decline of stratospheric ozone over the Northern Hemisphere mid-latitudes would have continued, more than doubling to about 15% by 2013 relative to the onset of ozone-depletion.

Fig. 1 (a) Without the Montreal Protocol (MP), there would have been a deep ozone ‘hole’ (with calculated ozone of less than 120 Dobson Units (DU)) in the Arctic in 2011, of comparable area to that in Antarctica in recent years. These Arctic ozone depletions would have recurred in future years. The area of the Antarctic ozone hole would also have continued to increase. The area of the ozone hole is defined as the area with a column ozone of 220 DU. (b) The observed area of the Antarctic ozone hole from 2007 to 2014 estimated from Ozone Monitoring Instrument (OMI) data (black symbols), and calculated from model runs including the MP (blue) and without the MP (red). (For the Arctic, no ozone hole has been observed during this period). (c) The percentage difference in the daily zonal mean ultraviolet index at local noon between model runs without and including the MP. (Adapted from Chipperfield, et al. Nat. Commun., 2015, 6).

In addition, the Antarctic ozone hole would have been 40% larger in 2013 relative to what was observed, with an enhanced loss of ozone also at sub-polar latitudes of the Southern Hemisphere.¹ These large reductions in ozone would have resulted in springtime increases in UV-B radiation at the Earth's surface of about 10% at mid-latitudes and over 20% at high latitudes of both hemispheres. For the implications of these changes for human health, see section 2 below.

1.2 Measurements at several sites over the last decade have shown decreases in surface UV-B radiation that are consistent with observed increases in total ozone

However, at some sites, changes in aerosols, clouds and, at high latitudes, sea ice, were the main drivers of changes in UV-B radiation. Between 1990 and 2011, changes in UV-B irradiance ranging between +0.4 and −8% per decade were found at four northern high-latitude locations (59°–71°N) in agreement with opposite trends in total ozone.² In contrast, UV irradiance at 325 nm, aerosol optical depth and cloud fraction did not have significant long-term trends. No significant trends in UV irradiance were found for the three southern high-latitude locations considered in this study.

Particularly in urban areas, the effects of aerosols can mask changes in UV-B radiation that arise from changes in ozone. Despite increasing total ozone over the 1991 to 2013 period, erythemally weighted irradiance at Uccle, Belgium, had increased by 7% per decade due to a combination of decreasing amounts of aerosol and cloud, which more than counteracted the effect of ozone.³ Statistical analysis showed that trends in UV-B irradiance and total ozone changed in the late 1990s, consistent with the recovery of stratospheric ozone starting at about the same time.

UV radiation for the period 1950–2011 was reconstructed from a variety of proxy data for nine locations in Spain. The erythemal irradiance increased between 1950 and 2011 by about 13%, of which half was due to decreases in ozone. Between 1985 and 2011, an increase of about 6% was calculated, mostly due to decreasing amounts of aerosols and clouds.⁴ The day-to-day variability of the noon-time clear-sky UV Index at Thessaloniki, Greece, was found to be influenced more by aerosols than total ozone, even for extreme total ozone events.⁵

Reflectivity data from several satellites between 1979 and 2012 suggest that decreases in sea-ice and small increases in the amount of clouds over the region of the Southern Ocean around the Antarctic Peninsula have resulted in decreases in UV-B radiation at the surface.^6,7

1.3 New measurements have confirmed high levels of UV radiation at low-latitude and high-altitude locations

Extremely large values of the UV Index (peaking at >20) have been measured at four sites of the Tibetan Plateau. These large values are due to the high elevation (3600–4500 m) and low latitude (29–31°N), resulting in small solar zenith angles, and low total ozone.⁸ In Kampala, Uganda (0.3°N, elevation 1200 m), UV Index values of more than 17 were measured on several occasions between 2005 and 2015,⁹ while in La Paz, Bolivia (6.5°S, elevation 3420 m), up to 70% of the measurements in 2008–2012 exceeded UV Index values of 10.¹⁰

1.4 New studies have improved understanding of the effects of clouds on UV radiation and the physical mechanisms of the processes involved

The weak wavelength-dependence of the cloud optical depth predicted through theoretical calculations was confirmed with measurements. Erythemal and total solar irradiance data recorded at Valencia, Spain, indicate that the difference in the cloud optical depth derived from these measurements is smaller than 2%.¹¹ The effective albedo caused by clouds below a mountain summit was quantified using measured and modeled UV irradiance data from the Izaña observatory (28°N, 2400 m above sea level).¹² The largest observed effective albedo value in the UV was 0.58 (i.e., 58% of the incident radiation is reflected upwards). More typical values range between 0.2 and 0.5 and are about 10 times larger than the local surface albedo (0.02–0.05). Measurements combined with radiative transfer modeling confirmed the theoretical predictions that clouds composed of small water droplets attenuate UV radiation more efficiently than clouds composed of large droplets, for the same liquid water content.¹³ The magnitude of this effect depends on the solar zenith angle and liquid water content.

1.5 Surface UV radiation may have been affected by changes in the amount and optical properties of anthropogenic aerosols, which have been observed at multiple sites worldwide since the year 2000

However, at most sites, no UV radiation data are available to assess these effects quantitatively. Aerosol data from the AERONET network revealed decreases in the amount and absorption efficiency of aerosols (quantified using the single scattering albedo) at most stations since 2000.^14–17 These changes were caused by decreases in air-pollution and should have led to increases in surface UV radiation. In contrast, aerosol amounts at many locations in China remain high (the aerosol optical depth at 440 nm ranged between 0.3 and 1.0 and have exceeded 5 during extreme events in some locations) with no significant trend since 2002.¹⁸ Measurements at six stations in the USA in 1995–2010 indicated no trend in direct irradiance but a positive trend in diffuse irradiance. This was attributed to an increase in the size of aerosols, possibly due to increased aircraft traffic.¹⁹

1.6 The increasing frequency and extent of wildfires due to climate change become important sources of aerosols with significant effects on surface UV radiation

Wildfires across the globe emit black carbon (BC) and organic carbon (OC) smoke particles that can persist in the atmosphere for days to weeks. The inclusion of emissions from fires in climate models increased the predicted global mean annual aerosol optical depth at visible wavelengths by 10%.²⁰ Modeling studies^21,22 projected that climate change will increase summertime concentrations of OC aerosol over the western USA by 40–70% and concentrations of BC aerosol by 20–27% in 2050, relative to present levels. Most of this increase (75% for OC and 95% for BC) is caused by larger emissions from wildfires with the remainder caused by changes in meteorological conditions. Such increases in carbonaceous aerosols would lead to significant reductions in UV radiation at the Earth's surface. A recent study has shown that wildfires in Russia in 2010 caused reductions of up to 50% in the daytime averaged photolysis rates of NO₂ and O₃ (which is driven by UV radiation) along the transport path of the aerosol plume.²³

1.7 Over the Arctic and northern high latitudes, surface UV-B radiation is projected to decrease during the 21^st century due to the recovery of total ozone, increases in cloud cover, and reduction of surface reflectivity caused by the shrinking of sea-ice and snow cover

Relative to the present, the greatest reductions in surface UV-B radiation (280–315 nm) over land are projected for April under all skies, locally reaching about 30% for the noon UV Index and about 50% for the noon effective dose for the production of vitamin D.^24,25 For mid latitudes, the UV Index is projected to show a decrease from 1975 to 2100 of about 5% in the Northern Hemisphere and of about 10% in the Southern Hemisphere, with about equal contributions from projected increases in stratospheric ozone, and cloud cover.²⁶

1.8 In the Arctic region, the processes that led to the sporadic large losses of stratospheric ozone observed in recent years are still unclear

A recent study²⁷ suggests that these losses have not been caused by increasing amounts of greenhouse gases. This was concluded from the analysis of long-term changes in the volume of polar stratospheric clouds derived from measurements in 1979–2011 and model simulations for the 21^st century. However, loss of Arctic sea ice may lead to changes in the circulation patterns over the polar region, which could ultimately lead to substantial reductions of polar ozone in March and April, delaying the ozone recovery by up to 12 years.²⁸

1.9 Depletion of Antarctic ozone has influenced the climate in the Southern Hemisphere but there is still no consensus that the ozone hole has caused the recently-observed increasing extent of Antarctic sea ice

Ozone depletion in the late 20^th century has likely driven the observed changes in circulation, temperature and salinity of the Southern Ocean.²⁹ Long-term records of data show that changes in circulation due to the ozone hole are contributing to a decrease in summer temperatures over south-east and south-central Australia, and inland areas of the southern tip of Africa.³⁰ In the decades since the appearance of the ozone hole, anomalously high and low total ozone in the spring have been significantly correlated with hotter and colder than normal summers, respectively, over large regions of the Southern Hemisphere, and in particular over Australia.

Increases in the extent of sea ice leads to increased UV-B radiation above the surface, but to decreased radiation penetrating the water under the ice. Potential influences of the Antarctic ozone depletion on sea ice are complex and not well understood. Two processes with different time scales have been proposed:³¹ in the short-term (a few years to a few decades depending on the model), changes in the ocean circulation cool the sea surface around Antarctica leading to seasonal expansion of the sea ice, but also to upwelling of warm waters in the region of the seasonal sea ice. This leads to an annual expansion and the retreat of sea ice. In the long-term (multiple decades), the effect resulting from upwelling of warm water will dominate, slowly leading to warming of the ocean and ultimately to an overall reduction of sea ice. Based on the results of an improved model it has been argued²⁹ that the cooling due to ozone depletion is weak, does not occur in the month of maximum sea ice extent, and is overwhelmed by the warming due to increases in greenhouse gases.

1.10 El Niño may indirectly influence UV-B radiation

Measurements and model simulations for China suggest that decreases in total ozone during El Niño events lead to increases in clear-sky erythemal irradiance.³² Increases due to El Niño events of up to 6% have been calculated for the middle and lower reaches of the Yangtze River in winter and up to 10% for the northwestern Tibetan Plateau during spring. Changes in cloud patterns associated with these events could also be important for the modulation of UV radiation.

1.11 High quality measurements are necessary to detect and quantify changes in surface UV radiation caused by ozone depletion and, eventually, by ozone recovery

Array spectrographs are increasingly being used for monitoring spectral UV irradiance; however, their accuracy is limited by their inherent instrumental characteristics. A recent study confirmed that measurement errors caused by stray light limit the useful spectral range of these instruments to wavelengths longer than about 305 nm, in particular for small solar elevations.³³ Bearing in mind these limitations and the increased uncertainty of data in the UV-B range due to stray light, the usefulness of these instruments is mainly in monitoring the spectrum of radiation in situations when the radiation field changes rapidly (e.g., in partly cloudy conditions or under tree canopies).

1.12 Instruments on satellites can provide reliable estimates of surface UV doses under low-aerosol and clear-sky conditions, but these estimates may be affected by large systematic biases over polluted areas, overcast skies, or snow-covered surfaces

UV Index data from the Ozone Monitoring Instrument (OMI) were validated with ground-based measurements with a NILU-UV multifilter radiometer near New York.³⁴ For clear-sky conditions, the data have good agreement (within 2%), while the OMI bias increases greatly with the optical depth of clouds, reaching −24% on average for overcast conditions.

OMI UV data were also compared with ground-based measurements at 13 stations located in the Arctic and Scandinavia from 60° to 83°N.³⁵ When the surface albedo is known, OMI data typically exceed ground-based measurements by 0–11%. Otherwise biases are much larger; ranging between +55%, when the albedo assumed by OMI is too high, to −59% when it is too low. These large negative biases are observed when reflections from snow and ice, which ultimately increase downwelling UV irradiance, are misinterpreted as reflections from clouds.

By combining satellite data on ozone from GOME-2 and clouds from AVHRR/3, an improved algorithm was developed to estimate surface UV radiation.³⁶ New global datasets of erythemal dose were constructed from OMI measurements for large-scale ecological studies. These data are provided at a 0.25° spatial resolution (about 25 km at the equator).⁶

1.13 Personal UV dosimeters are useful to estimate exposure to UV radiation and to quantify related effects, such as the status of vitamin D and the damage to skin from UV radiation

While electronic UV dosimeters are more expensive and can be more difficult to use than traditional dosimeters (e.g., spore-³⁷ or polysulfone-dosimeters³⁸) they have several advantages: firstly, electronic dosimeters can be tuned to more closely match the biological weighting of interest than is the case for polysulfone dosimeters, for which differences between their response function and action spectrum of interest are large.³⁹ Secondly, even if there are differences between the instrument response and the target weighting, the latter can still be derived from the measurements using radiative transfer models that include ozone and the solar zenith angle (SZA) as inputs. This is generally not possible for the older dosimeters because of their long integration periods that include a wide range of SZA.⁴⁰ Thirdly, the time series of data from the electronic dosimeters can assist in quality assurance and in verifying compliance of the users to the measurement protocols. For example, if the dosimeter is not actually worn, but left stationary, that will be obvious from the data record. Finally, because of their more linear response, calibration accuracy will generally be better in the reusable electronic devices, compared with polysulfone dosimeters, which have variable non-linear responses, e.g.ref. 39.

Further research has confirmed that UV irradiance on a horizontal surface (e.g., the UV Index) is not a good indicator of personal exposure. To more realistically model personal exposure, information on the directional distribution of radiation (radiance) would be preferable instead of the more commonly available irradiance data. However, such measurements are not widely available.⁴¹

UV dosimeters have recently been used to measure the exposure to UV radiation of members of an Antarctic expedition,⁴² high school students in Switzerland,⁴³ seafarers working on the decks of vessels,⁴⁴ skiers,⁴⁵ tennis players,⁴⁰ hikers,⁴⁰ and runners.^40,46 Amongst others, it was found that the Swiss students were exposed to 1.7% of the ambient UV irradiance during school days, while 85% of the cumulative UV dose was obtained on weekends and holidays. Skiers in Italy received 65% of the ambient dose on average, as measured by polysulphone detectors. The median daily UV exposure of hikers and tennis players typically exceeded 5 standard erythemal doses (SED), according to one study, but maximum exposures can be much higher.

2 Human health: the effects of solar UV radiation and interactions with climate change

2.1 Risky behaviour regarding sun exposure remains very common among fair-skinned populations, putting them at increased risk of skin cancer and other adverse effects of sun exposure

In the 2010 US National Health Interview Survey, 37% of adults reported sunburn on at least one occasion in the previous year.⁴⁷ Sunburn was particularly common in young adults (18–29 years, 52%), and in individuals with fair skin (44%). Amongst those reporting sunburn in the previous year, 12% had 4 or more sunburns. In a nationally representative sample of adolescents in the USA, self-reported use of sunscreen declined from 68% in 2001 to 56% in 2011.⁴⁸ In France, 78% of respondents in a telephone interview reported deliberate sun exposure and 38% did not use sufficient sun protection.⁴⁹ In one study, the eyes were the least likely part of the body to be protected from sun exposure.⁵⁰ East Asian immigrants to western countries typically have comparatively low sun exposure, but this increases as they become integrated into the culture of the host country.⁵¹ This change in behaviour is beneficial for vitamin D status, but may be associated with an increased risk of skin cancers. In a cross-sectional study of 386 university students in north China, over 90% were aware that exposure to UV radiation caused skin cancer and sunburn, while only 28% were aware of the effects on the risk of developing cataracts and 3% on risk of pterygium (a growth on the surface of the eye, usually on the side closest to the nose, that may cover the cornea and impair vision).⁵⁰

2.2 The incidence of cutaneous melanoma continues to increase globally, but, in many countries, mortality may have peaked. Exposure to solar UV radiation is the most important cause

A personal or family history of cutaneous melanoma (CM) does not reduce risky sun behaviour. Ongoing exposure of CMs to UV radiation may promote metastasis. The incidence of CM is highest in fair-skinned populations and has risen considerably in recent decades. For example, the age-standardised incidence rate of CM per 100 000 persons in the UK was 17.4 for 2009–2011, an increase of 57% in men and 39% in women, compared to 2000–2002.⁵² The age-standardised incidence rate increased from 26.7 in 1982 to 48.8 in 2010 in Australia⁵³ and doubled from 1982 to 2011 in the USA.⁵⁴ A further doubling in incidence by 2030 is predicted for the USA.⁵⁴ The incidence of CM increases with increasing age.^52,54 The incidence rates in children and young adults have decreased over the last ten years in countries such as Australia, which have had several decades of strong public health programs promoting sun protection, but rates continue to increase in other countries.^54–59 High-dose sun exposure at any time during life increases the risk of CM, but exposure occurring in childhood years, and associated with the development of nevi, may be particularly important.⁶⁰

Age-standardised mortality rates due to CM have stabilised in some countries, probably due to a combination of earlier detection, improvements in treatment and reductions in risky behaviour regarding the sun in younger cohorts who have grown up with strong sun protection programs. For example, in the USA the mortality rate was 2.7 per 100 000 persons in 2011, the same as in 1982.⁵⁴ In Australia, mortality has been stable at around 6.0 per 100 000 from 2004 to 2012.⁵³ In fair-skinned individuals globally, time trends in annual mortality rates for CM from 1955 to 2010 were closely related to the year of birth (age-cohort) with the change in overall rates reflecting a downturn in mortality in younger age groups.⁶¹ The peak years for CM mortality were for those born in 1936–1940 in Oceania, 1937–43 in North America, 1945–53 in the UK and Ireland and 1957 in Central Europe.

One study showed that, following a diagnosis of CM, patients in Denmark still expose themselves to high levels of solar UV radiation without using personal protection, particularly when on holiday.⁶² Similarly, children of people who have survived CM had higher levels of sun exposure and sunburn than average-risk populations in a study from California.⁶³ Contrary to a previous report,⁶⁴ higher levels of sun exposure prior to diagnosis of CM were not associated with a reduction in subsequent mortality⁶⁵ when examined with a rigorous study design focused on this specific question. Furthermore, in a mouse model, repetitive exposure of primary CMs to UV radiation promoted a spread to blood vessels and metastasis to the lung.⁶⁶

2.3 The incidence of non-melanoma skin cancers continues to increase, primarily in fair-skinned populations

Changes in patterns of sun exposure over time are resulting in a more rapid increase in the incidence of basal cell carcinoma (BCC) compared to that of squamous cell carcinoma (SCC). In the USA there was a 14% increase in the age-adjusted rate of procedures (a proxy for incidence) for non-melanoma skin cancers (NMSCs) between 2006 and 2012.⁶⁷ Extrapolating to the US population, the estimated total number of NMSCs was almost 5.5 million in 2012, with approximately 3.3 million people treated. Patterns of sun exposure have changed over time, with increased intermittent recreational exposure, most frequently occurring in locations with high ambient levels of UV radiation. As a consequence, the incidence of BCC is increasing at a faster rate than that of SCC in younger people.^68,69 Sun protection programs that have been shown to be effective in lowering the incidence of NMSC should have a particular focus on leisure-time sun exposure in young people.

A study in South Africa showed that the mean age-standardised annual incidences (per 100 000 people, figures are female, male incidence) of BCC, SCC and CM in 2000–2004 were highest in groups classified in the study as white (BCC: 113, 198; SCC: 32, 70; CM: 17, 21), followed by the coloured (BCC: 27, 59; SCC: 15, 26; CM: 4, 6), Asian (BCC: 5, 8; SCC: 3, 4; CM: 1, 1) and then black (BCC: 2, 3; SCC: 2, 3; CM: 1, 1) population groups.⁷⁰ SCC and BCC were most commonly found on the head.

A recent review of studies quantifying the direct costs of NMSC to health care systems found that these costs were highest for the USA (nearly 600 million € annually) followed by Australia (around 350 million € annually), although after adjustment for population, costs were greatest for Australia, followed by New Zealand, Sweden and Denmark.⁷¹ Skin cancer prevention initiatives were shown to be highly cost-effective and possibly cost-saving.

2.4 Both UV-A and UV-B radiation can lead to the formation of skin cancer

While UV-B irradiation is well known to cause DNA damage, UV-A (315–400 nm) irradiation also causes cellular damage through the generation of reactive oxygen species, as well as causing immunosuppression. There is increasing evidence to suggest that UV-A irradiation can also cause the production of thymine dimers in the DNA of basal keratinocytes and influence gene expression.⁷² UV-A radiation may be implicated in the development of both CM and NMSC through its role in the inhibition of nucleotide excision repair.⁷³

2.5 Several eye diseases, including cancers of the eyelid and the surface of the eye, cortical cataract and pterygium, are strongly related to exposure to UV radiation

The evidence is less clear for other diseases of the eye, such as nuclear and posterior subcapsular cataract, ocular melanoma and age-related macular degeneration. Exposure to UV radiation causes cancers of the eyelids, principally BCC.⁷⁴ The causative role of exposure to UV radiation in ocular squamous neoplasia may involve a combination of DNA damage, local and systemic immunosuppression, and the reactivation of latent viruses.⁷⁵ Specific genetic and epigenetic changes in genes involved in oxidative stress may increase susceptibility to UV radiation-induced development of cortical cataracts.^76,77 In China, there was an increase in disability-adjusted life years (DALYs) for cataract of 92 per 100 000 population for every 1000 J m⁻² increase in ambient erythemal UV radiation estimated from satellite data.⁷⁸ For the population aged ≥75 years this figure increased to 1342 DALYs per 100 000 people.

The risk of exfoliation syndrome (an age-related disease that is the most common identifiable cause of glaucoma) increased with greater time outdoors and a history of work over water or snow, and reduced with the use of sunglasses in a clinic-based case-control study in the USA and Israel.⁷⁹ Blue light, rather than UV radiation, may play a previously-unacknowledged role in the pathogenesis of uveal melanoma,⁸⁰ as well as age-related macular degeneration.⁸¹

2.6 Less time outdoors appears to be a major risk factor for the development of myopia, but whether this is an effect of UV radiation is not clear

In a birth cohort study, myopia (defined as a mean refractive error of both eyes ≤−0.5 diopters) in young Australian adults was significantly less common in participants with objective evidence of greater sun damage to the eye (measured as the area of conjunctival autofluorescence),⁸² and was significantly more common in individuals with vitamin D deficiency.⁸³ These findings support the hypothesis that outdoor light exposure may be protective against myopia, with the relevant wavelengths, or the role of UV radiation, not yet determined.

2.7 Exposure to solar UV radiation can alter the immune response to a variety of microorganisms in animal studies, and recent reports support a similar role in humans

The risk of recurrence of infection with herpes simplex virus in the eye was increased three-fold in people in North Carolina who had the highest outdoor exposure (8 hours or more per week outdoors when the UV Index was ≥4) compared with those having the least outdoor exposure (7 hours or fewer outdoors when the UV Index was <4).⁸⁴ In studies in Korea⁸⁵ and Taiwan,⁸⁶ the occurrence of shingles, caused by the reactivation of herpes zoster virus, was more common in the sunny months than in the winter, possibly due to the immunosuppression induced by exposure to higher levels of solar UV radiation.

In a systematic review of 24 randomised trials, the effectiveness of the BCG vaccine against tuberculosis (TB) decreased with closer proximity to the Equator and increasing levels of UV radiation, possibly due to UV-induced suppression of the immune response to the vaccination.⁸⁷ Furthermore, in a study over 28 years in Birmingham, UK, there was strong evidence for seasonality of TB, with notifications being 24% higher in the summer than in the winter.⁸⁸ The lesions of dermal leishmaniasis (a parasitic infection spread by sandflies) that can arise after an apparent cure from visceral leishmaniasis, appear on sun-exposed areas of the body.⁸⁹ Exposure to solar UV radiation is thought to play a key role, acting by suppressing immune responses in the skin at the exposed body sites.

A study in the USA found a significant positive correlation between the average annual levels of UV radiation for each State and the incidence of oral and pharyngeal cancer in people with fair skin, but not in those with deeply pigmented skin.⁹⁰ There was also a positive correlation between levels of UV radiation and the incidence of cervical cancer in white women. Human papilloma virus (HPV) is commonly found in each of these cancers; the variation in incidence in relation to levels of UV radiation suggests that exposure to solar UV radiation may induce the production of soluble mediators that promote HPV-associated tumorigenesis at non-exposed body sites.

2.8 Despite much research into a wide range of disease outcomes, the role of vitamin D in human health, beyond its importance for calcium homeostasis and the maintenance of bone, has not been confirmed⁹¹

Recent reviews^92,93 show that vitamin D deficiency (a concentration of 25-hydroxyvitamin D [25(OH)D] in serum or plasma of less than 50 nmol L⁻¹) is common worldwide, although the use of different assays for which accuracy is not assured makes it difficult to fully assess the magnitude of the problem. One study showed a high prevalence of vitamin D deficiency in young children in Iran that was the result of high levels of air pollution, leading to low levels of exposure to UV radiation.⁹⁴ New work is providing greater guidance on how much sun exposure is required for the avoidance of vitamin D deficiency, including in people with different skin types.⁹⁵ In Australia, a program of sun exposure led to increased 25(OH)D levels in older people in residential care but was only effective when started in spring or autumn, and some individuals did not achieve vitamin D sufficiency.⁹⁶

Many observational studies show that low serum 25(OH)D concentrations are associated with increased risk, progression, severity and mortality for many disorders, including cancers,⁹⁷ autoimmune diseases such as lupus,^98,99 inflammatory bowel disease¹⁰⁰ and multiple sclerosis,¹⁰¹ poor cognitive function,¹⁰² attention deficit hyperactivity disorder^103,104 and myopia.⁸³ Other studies show no effect of vitamin D. Intervention studies using supplementation with vitamin D show no benefit for most health outcomes tested.⁹¹ The major determinant of serum 25(OH)D concentration in many populations is recent sun exposure or time outdoors.¹⁰⁵ This means that serum 25(OH)D, as well as being the usual measure of vitamin D status, is a marker for recent sun exposure and time outdoors. Intervention studies using supplementation with vitamin D test a specific effect of vitamin D. However, associations with 25(OH)D may be due to the link with other factors related to recent sun exposure and time outdoors such as physical activity or exposure to UV radiation, rather than vitamin D status per se. The lack of a beneficial effect of supplementation with vitamin D suggests that there is, in fact, no causal association between higher vitamin D status and better health, but that other factors associated with the level of 25(OH)D may be important for health. Alternatively, supplementation studies may have had null results due to study designs where the supplementation dose was too low, for too short a time, in people without vitamin D deficiency at the beginning of the study, or the sample size of the study was too small. In a few supplement studies there has been evidence of improvement in disease symptoms, for example for lupus,^98,99 and autism¹⁰⁶ but the designs used in the studies do not allow firm conclusions to be drawn. The role of vitamin D in health and disease, the optimal level and the size of any effect, remain unclear.

Sun exposure and/or vitamin D during pregnancy may be particularly important for the health of offspring. Vitamin D status in pregnancy was inversely related to the risk of attention deficit hyperactivity disorder in the offspring¹⁰⁷ in one study, but was not in another, and low 25(OH)D level in umbilical cord blood has been correlated with an increased risk of transient wheezing and atopic dermatitis in early childhood.¹⁰⁸ Higher maternal vitamin D status was associated with an increased risk of depression in the offspring in another report.¹⁰⁹

A few studies show that both low and high levels of 25(OH)D are associated with increased risk of an adverse health outcome, such as all-cause mortality,¹¹⁰ or an adverse effect of higher 25(OH)D levels for prostate cancer,¹¹¹ and hypertension¹¹² suggesting that optimal levels are not necessarily high levels.

2.9 Other potential benefits and risks of exposure to UV radiation are being revealed. Higher sun exposure is associated with a reduced risk of some cancers and autoimmune diseases, but whether this is a causal association is not clear

In Finland, fishermen, but not their wives, had higher incidence of lip cancer but lower incidence of colon cancer than the general Finnish population.¹¹³ Whether these cancers are linked to the higher exposure to UV radiation, or the presumed higher consumption of fish, was not explored, although the discrepancy in risk between the men and their wives might lend weight to a UV-related explanation rather than a dietary cause. In the USA, the incidence of several sub-types of non-Hodgkin lymphoma was significantly lower in the highest compared to the lowest quintile of ambient UV radiation (average daily cloud-adjusted noon-time UV irradiance at 305 nm (UV-B) over 1982–92) for the residential location, with some effects being more pronounced in fair- compared to dark-skinned populations.¹¹⁴ Similarly, incidence rates for multiple myeloma, a cancer of the plasma cells which often spreads to the bone marrow, were greater at higher latitudes and there was an independent inverse association between UV-B irradiance and the incidence of multiple myeloma.¹¹⁵ In contrast, another study showed that higher residential erythemal UV radiation was associated with an increased risk of B-cell acute lymphoblastic leukemia in children aged less than five years; there was a threshold effect, with risk increasing at levels of ambient UV radiation above 100 J cm⁻².¹¹⁶

Several studies report an association between higher levels of sun exposure or ambient UV radiation and a reduction in inflammatory bowel disease: the risk of Crohn's disease,¹¹⁷ and hospitalisation and need for bowel surgery for both ulcerative colitis and Crohn's disease.¹¹⁸ The well-described seasonal variation in disease relapses in people with multiple sclerosis shows latitudinal variation, with a shorter gap between relapses in people living at higher latitude.¹¹⁹ There is a strong latitudinal gradient of increasing rates of hospital admission for anaphylaxis (a severe allergic reaction) in children in Chile; that is, with increasing latitude above 34°S and lower levels of solar UV radiation.¹²⁰

An inverse association between mortality from all causes and levels of UV radiation or personal sun exposure have been described for dialysis patients,¹²¹ female participants of a large cohort study in Sweden at the 20-year follow-up,¹²² and in premature babies (23–28 weeks gestation) in the first 28 days of life.¹²³

Many commonly used drugs can be phototoxic or cause photosensitivity. Photosensitivity reactions are reported with angiotensin II receptor blockers, used to treat hypertension.¹²⁴ In addition, long-term use of these drugs is associated with an increased risk of CM.¹²⁵ Long-term use of diuretics may increase the risk of SCC and BCC.¹²⁵ Additional advice on sun protection should be provided when these drugs are prescribed.

Higher levels of UV radiation have been associated with lower levels of serum folate,¹²⁶ including in women of childbearing age.¹²⁷ Whether this effect is sufficient to alter the risk of disorders such as neural tube defects in humans is not clear. The combination of high dietary folate intake and high levels of exposure to UV radiation may potentiate DNA damage¹²⁸ and the risk of BCC.¹²⁹

2.10 Infrequent use and poor understanding of labels and application of sunscreens are common, resulting in people overestimating their sun protection. There is mixed evidence on the long-term benefits of the use of sunscreens for the prevention of skin cancers

Sunscreens are not used commonly in some countries.^130–132 Even where their use is common, sunscreens are often applied at a thickness that is one-fifth to one-half of that required to achieve the rated sun protection factor (SPF).^133,134 Poor application may continue even after training,¹³⁵ but new approaches to training for application may improve effective protection.¹³⁶ The use of higher SPF sunscreens (e.g. SPF 50+) may compensate to some extent for the application thickness being less than optimal. New sunscreens provide better protection within the UV-A waveband, but, for a given SPF, increasing protection within the UV-A is accompanied by reduced protection within the UV-B waveband.¹³⁷

A randomised controlled trial in an older Australian cohort showed that regular daily use of sunscreen reduced the risk of actinic keratosis (a scaly growth on the skin that may be a pre-malignant stage of SCC), SCC, and CM.¹³⁸ In contrast, increasing use of sunscreens from 1997–2007 in a large cohort of Norwegian women was associated with an increased risk of sunburn,¹³⁹ and a meta-analysis of observational data from Europe, America and Oceania showed no significant increase in the risk or benefit of sunscreen use for CM.¹⁴⁰

A wide range of oral/nutritional photo-protective agents is being investigated.^141,142 There is considerable interest in the development of natural biocompatible sunscreens,¹⁴³ including plant extracts^144–146 and compounds ingested by marine animals (e.g. mycosporine-like amino acids) that probably function as sunscreens in tissues exposed to UV radiation.^147–150 Recent work also notes adverse health effects such as allergic and photo-allergic contact dermatitis from the use of synthetic sunscreens,^151,152 and that sunscreens may be endocrine disrupters¹⁵³ and have adverse effects on marine ecosystems (see section 4.5 below).

2.11 Sunglasses and contact lenses provide excellent protection from UV radiation, although in some countries labelling of sun protection factors may be inaccurate

A proposed new UV protection label for eyewear, the eye-sun protection factor (E-SPF), integrates the transmission of UV radiation through the lens and the reflection of UV radiation from the back of the lens (caused by coatings on both sides of the lens to reduce glare).¹⁵⁴ Validation of its biological relevance is now required.¹⁵⁵ A high proportion of sunglasses purchased from unauthorized dealers in developing countries do not comply with international standards for UV protection.¹⁵⁶ Testing of four commercially available soft contact lenses showed that they blocked 89–99% of UV-B radiation,¹⁵⁷ Use of UV-blocking contact lenses and spectacles was associated with reduced UV-induced damage as measured by autofluorescence in the conjunctiva near the nose but not in that on the outer side of the eye.¹⁵⁸

2.12 Research on the interactions between UV radiation and the effects of climate change on human health remains sparse

UV radiation may alter the physiology of some mosquitoes leading to an increased tolerance to some chemical insecticides and this effect may be accentuated by environmental pollution.¹⁵⁹ The importance of these interactions for future control of vectors of diseases is not clear.

In a study of people with white skin in the USA, the incidence of BCC increased in relation to increasing levels of lifetime residential ambient UV radiation and was also higher in the fourth (compared to the first) quintile of ambient temperature.¹⁶⁰ It is difficult to predict from this ecological study whether there might be interactions between exposure to UV radiation and warmer temperatures due to climate change that could affect the risk of skin cancer. In another study, temperature was an important determinant of time spent outdoors in cooler, but not hotter, climates.¹⁶¹ Thus, at locations where climate change leads to ambient temperatures in a “comfortable” range, people are likely to spend more time outdoors and have higher exposure to UV radiation. In contrast, if climate change leads to temperatures outside this range, people are more likely to spend more time indoors, and receive less UV radiation (also see a further discussion in section 4.4 below).

3 Terrestrial ecosystems: the effects of solar UV radiation and interactions with climate change

3.1 Agriculture and food security are likely to be more adversely affected as a result of the projected climate variability and changing levels of UV radiation, but some beneficial outcomes may also become evident

Projected increases in the intensity and frequency of unfavourable climate events such as heatwaves, flooding, and recurring dry seasons (see sections 1.4 above and 3.10 below) together with changing levels of UV radiation that are linked to climate change, can have adverse effects on the production of crops. On the other hand, in addition to an improved understanding of the way in which interactions between exposure to UV radiation and climate change can alter plant response, opportunities for exploiting some of the resultant modifications of plants are relevant with respect to food production, nutrition, and resilience of plants to stress in general. However, crop plants are usually cultivated in dense stands, where beneficial effects of solar UV-B radiation on e.g., the immunity of plants, could be lost.^162,163

Further work has been directed to productivity, quality traits of plants, and adaptive mechanisms^164,165 in order to address the effects of UV radiation and rising temperatures on plants. Across a range of plant species, increasing temperature during the early development of plants enhances growth, while exposure to realistic levels of UV-B radiation^165,166 counteracts the growth response. Other growth stages, such as the onset of flowering, are delayed by elevated levels of UV-B radiation, with resultant decreases in fruit and seed production (ref. 167 and references therein). These modifications during plant growth may become more common in areas that are receiving high levels of UV radiation, mostly because of projected decreases in cloud cover and aerosols in some regions.⁴ Current high UV radiation levels also occur in certain high-altitude and low latitude locations.⁸

UV-B-mediated shifts in the allocation of resources (such as sugars, mineral ions, and water) within the plant may have important consequences for agriculture. Redirection of these resources away from defense mechanisms and toward processes for rapid growth in response to attenuation of UV-B radiation in high-density stands¹⁶⁸ may have negative consequences for plant health in agricultural settings.¹⁶⁹ Further research on the molecular connections between the perception of UV-B radiation and the regulation of plant immunity may provide clues to improve the resistance of crop plants to pests and pathogens under the conditions of high-density agricultural practices.

The use of pesticides in agriculture continues to be a concern. It is known that UV radiation facilitates the breakdown of pesticides^170,171 and may in some cases increase the toxicity of certain pesticides and/or their degradation products (see section 5.5). The detrimental effects to the plant may result in an increase in the permeability of membranes leading to increased uptake of pesticides when plants are exposed to elevated levels of UV radiation.^172,173 Because of the potentially harmful effects of pesticides on the environment, more selective biological methods of control are being developed. The use of certain fungi (entomopathogenic) constitute one biological control measure, although unfortunately they also can be very sensitive to UV-B radiation. This will decrease their usefulness as biological controls in regions of enhanced UV-B radiation.^174,175

3.2 In terms of plant adaptation, there are several implications for food production and altered plant attributes as a consequence of exposure to UV radiation and a rapidly changing and variable climate

While plant response can indicate mechanisms of adaptation, some of these adaptations are also of interest from an agricultural, horticultural, and nutritional perspective (ref. 163, 176, 177, and references therein) and can be managed by changing certain practices of crop management for beneficial outcomes. For example, stimulation by UV radiation of polyphenolics can be used to increase the nutritional quality of plant products and plant tolerance to stress conditions by manipulating the exposure of crops to UV radiation through e.g., opening of the plant canopy or the use of filters.^162,176,178

Other examples include the modulating effect of UV radiation on the growth of grape vines and subsequent alterations in the composition of the berry fruit and resulting juice or wine. These changes can be used by growers to experimentally manipulate conditions to alter the taste, quality of the berry, and appearance of the wine, and cross-tolerance to environmental stressors^179–181 including fungal infections,^182,183 and temperature.¹⁷⁹ In many geographical regions, a concern arising from increasing temperature is that certain crops, including grape vines, show more rapid ripening that affects quality (e.g. increased alcohol content in wine¹⁸¹). However, when multiple, interacting environmental variables are taken into account, such as exposure to increasing temperature and elevated carbon dioxide levels, UV-B radiation may also alleviate oxidative damage as well as offset the hastening of ripening by the increased carbon dioxide levels and temperature.¹⁸¹

3.3 Some male and female plants have been reported to respond differently to temperature and UV radiation

Changes in sex ratios in these plants as a result of exposure to UV-B radiation and temperature may modify ecosystems and the spatial distribution of plant habitats. For example, in certain trees and shrubs, male (pollen-producing) plants establish and grow more rapidly than females (seed producing), perhaps because less energy is allocated to reproduction than in female plants. Tolerance of environmental stresses, such as UV-B radiation, water-deficiency, increased salinity, and temperature, also appears to differ in populations of male and female plants resulting in the ability for gender proportions across a population to be changed. Male plants may be more tolerant of stresses than females.^184–187 This greater stress tolerance can result from increased temperature and UV-B radiation.¹⁸⁸ However, the temperature-driven enhanced production of protective flavonoid compounds in female plants¹⁸⁷ further illustrates differences in the sexes. Thus modifications at certain life stages in male and female plants reflect complex interactions between exposure to enhanced UV-B radiation and rising temperature, and may therefore alter the balance of plant gender and the establishment of a species.

3.4 Solar UV radiation can increase rates of decomposition of plant litter via photodegradation, especially in ecosystems with arid and semi-arid climates (i.e., grasslands, savannas, and deserts)

Litter from different plant species varies considerably in its rate of degradation and the precise chemical changes of litter resulting from photodegradation remain unclear.¹⁷⁷ Lignin is thought to be a primary molecular target of photodegradation,^177,189,190 but variation among species in the susceptibility of litter to photodegradation appears not to be solely related to lignin levels¹⁹¹ or optical properties that influence the penetration of UV radiation.¹⁹² Results from a meta-analysis¹⁹¹ suggest that the density of litter and thus the surface area of litter exposed to UV radiation can be an important attribute influencing the photodegradation of litter (see section 5.3 below). UV radiation directly degrades lignin structures¹⁹³ and indirectly alters other cellular constituents, such as cellulose and hemicellulose, and these chemical changes may then enhance microbial decomposition of litter (ref. 190, 194 but see also ref. 195).

3.5 UV radiation influences the decomposition of litter through several interacting and possibly synergistic abiotic and biotic mechanisms

The indirect enhancement effects of UV radiation on litter can apparently persist for some time and potentially carry over from seasonal dry periods (i.e., summer when UV radiation levels are typically high) to cooler, moist periods (winters) when the temperature and moisture conditions are more favourable for microbial decomposition.¹⁹⁶ This enhanced stimulation of microbial decomposition through UV radiation also appears to extend to partially decomposed soil organic matter.¹⁹⁷ Results from a recent meta-analysis¹⁹⁸ suggest that the combined effect of UV radiation on abiotic (photodegradation) and biotic (microbial) decomposition processes contributes to a greater mass loss of litter than photodegradation alone; and lignin loss is greater when photodegradation interacts with microbial decomposition (see also section 5). An increased understanding of the mechanisms of the effects of UV radiation on the decomposition of litter will increase our ability to predict how climate change will affect this process.

3.6 Climate change can potentially mediate the effects of UV radiation on the decomposition of litter through various direct and indirect pathways

However, few studies have experimentally examined the interactive direct effects of climate change and UV radiation on litter decomposition (also see Section 5.3 below). Results from an experimental study of Mediterranean grasslands (see also section 5.3 below) indicate that reduced rainfall and increased temperature decreased the rates of decomposition in both the standing and ground litter of a perennial grass species (Stipa tenacissima). In addition, ambient UV radiation increased the rate of decomposition but only under dry, warm conditions.¹⁹⁹ These findings suggest that, at least in these ecosystems, the relative importance of UV-driven photodegradation may increase with climate change with potential consequences for carbon cycling and sequestration. Climate change may also influence litter decomposition indirectly via changes in vegetation composition, structure, and fire (see section 3.10 below).

3.7 The molecular mechanisms by which terrestrial plants respond to changes in UV-B radiation are becoming better understood

Since the mechanism of perception of UV-B radiation by the UVR8 photoreceptor protein was elucidated in plants,²⁰⁰ there has been substantial progress in the understanding of the downstream signaling events that lead to physiological responses, including the activation of protection mechanisms (reviewed in ref. 201). This improved understanding of the perception of UV-B radiation and signaling in plants has important implications for biotechnology. Biotechnological applications of this knowledge include the enhancement of desirable responses to UV-B radiation in crops (such as the accumulation of pigments and antioxidants in fruits, vegetables and other crops)^177,202 and the development of novel tools that allow the manipulation of cellular activities with light.²⁰³

3.8 There is further evidence that plants use solar UV-B radiation as a signal of light availability and the intensity of competition from other plants

Although high levels of UV-B radiation can be harmful for some organisms, it is becoming increasingly clear that terrestrial plants have effective protective mechanisms against the damaging effects of UV-B radiation. These mechanisms include the accumulation of protective UV-absorbing pigments in epidermal cells and the cell walls of plants,^{177,204–209} the activation of antioxidant systems²¹⁰ and increased expression of genes involved in DNA repair mechanisms. In certain species, the UV-absorbing pigments can be adjusted on a diurnal basis in response to changes in solar UV radiation.^211,212 Many of these protective responses are regulated by the UV-photoreceptor, UVR8.²¹³ In addition, plants use UVR8, along with other photoreceptor proteins, to sense changes in the light environment caused by the proximity of other plants^168,214). Under low light conditions (shade) that inactivate UVR8, resources are redirected from defense to rapid growth.¹⁶⁸ This strategy helps the plant to compete for light with its neighbours, but also makes it more vulnerable to the attack of pathogens and pests.

3.9 High sensitivity to UV-B radiation has been demonstrated in some invertebrates

Terrestrial arthropods (such as insects, spiders, and millipedes) can be affected by UV-B radiation via effects on their food sources (mainly plants, e.g., ref. 202, 215 and 216). It has usually been assumed, without much hard evidence, that the exoskeleton of arthropods affords good protection against the direct effects of UV-B radiation for most terrestrial arthropods. However, high sensitivity to UV-B radiation occurs in aphids^217–219 and mites.^220–222 Some arthropods also sense and respond to solar UV-B radiation with avoidance behaviour.²²³

Adult damselflies are able to coat the mouthparts of ectoparasitic water mites with melanin (invertebrate melanotic encapsulation) as an effective response to deter these parasites. However, if damselfly larvae are exposed to UV-B radiation, they produce more melanin in their exoskeleton for protection from UV-B damage. As a result, the adults are smaller and have a reduced immunity response (30% reduction in melanotic encapsulation). This constitutes the first evidence for a UV-driven impaired immune response in an insect.²²⁴

3.10 Ozone depletion is changing the global climate (wind patterns, temperature, and precipitation) across the globe with consequences for agriculture, ecosystems, and human health

These ozone-related changes are manifested at the Earth's surface in the Southern Hemisphere during summer (December–February) and in the Northern Hemisphere in spring (April–May),^{29,31,177,189,225–229} with widespread implications for terrestrial and aquatic ecosystems, human health and food security, some of which are detailed below.

Changing wind patterns and intensity. More intense mid-latitude winds linked to ozone depletion could reduce or enhance the ability of the oceans to sequester carbon.^189,227,230 Enhanced wind-driven upwelling of carbon-rich deep water would result in less uptake of atmospheric carbon dioxide (CO₂) by the oceans, reducing their potential to absorb carbon.^177,189 However, winds can also transport more dust from drying areas, such as South America, into the oceans, enhancing fertilisation by iron and resulting in more plankton and greater carbon uptake.^189,227,231 These stronger more southerly winds have also been shown to have positive impacts on some foraging sea birds^232–234 with the sub-Antarctic wandering albatross showing a 15% increase in both body weight and breeding success.²³² However, for the flightless southern rockhopper penguins, the poleward shift of the jet stream is predicted to reduce the number of days with westerly winds in the future, leading to fewer days with favourable foraging conditions.²³⁵

Changing temperatures. Ozone depletion at the South Pole is associated with the cooling of East Antarctica (see ref. 227), while ozone depletion at the North Pole is associated with springtime cooling over China, Greenland and north-eastern Canada,²²⁸ but northern Europe and Siberia show enhanced springtime warming. These changes in temperature have likely reduced melting of the ice sheet in polar regions and resulted in changed phenology (e.g., mismatches between plants and their pollinators). The depletion of Antarctic ozone has possibly offset a substantial portion of the summer warming that would otherwise have occurred in Eastern Australia, Southern Africa and South America, due to increasing amounts of greenhouse gases (ref. 30 and section 1.9 above). As ozone concentrations recover, this amelioration may reduce with potential implications for the populations of these regions.

Changing precipitation. Ozone depletion is similarly associated with changing precipitation across the globe.^{225–229,237} More springtime precipitation occurs at high latitudes in the North Atlantic Ocean (70°W to 20°E) and in Siberia (50–120°E), associated with extremely low polar stratospheric ozone. Regional changes in precipitation will likely alter the levels and transparency of water and hence the exposure to UV radiation in aquatic ecosystems by the mechanisms mentioned in section 4.1. As detailed in our last assessment,¹⁷⁷ terrestrial ecosystems in the Southern Hemisphere are being affected by these ozone-related climate changes. Wetter summers have seen increased tree growth in eastern New Zealand²³⁶ and the expansion of agriculture in south-eastern South America.²³⁷ Conversely, in Patagonia and East Antarctica, declining growth of trees and moss beds has been linked to a reduced availability of water.^177,227,238

Changes in wind patterns, temperature, ocean salinity and precipitation are known to have an impact on agriculture and food security, human health, marine and terrestrial ecosystem health.²³⁹ However, there have been relatively few studies (i.e., none for the Northern Hemisphere) that directly consider these climate-related effects of ozone depletion. Ecological and health studies aimed at revealing such effects would probably be most effective if they focused on regions with statistically significant correlations between the spring-time depletion of ozone and either summertime surface temperature (in the Southern Hemisphere) or springtime surface temperature (in the Northern Hemisphere).

4 Aquatic ecosystems: the effects of solar UV radiation and interactions with climate change

4.1 Climate change is altering the exposure of aquatic ecosystems to UV radiation through a variety of mechanisms

Incident UV radiation is no longer increasing due to stratospheric ozone depletion, and a slow return to pre-1980 levels of stratospheric ozone is expected in the coming decades. However, exposure to UV radiation in inland, coastal, and open ocean waters continues to be altered by changes in ice and snow cover, cloud cover, increases in thermal stratification of the water column induced by warmer air temperatures, and by heavy precipitation that reduces the UV transparency of water.

Climate change is altering the concentrations of UV-absorbing substances such as terrestrially-derived dissolved organic matter (DOM) in inland and coastal waters through altering the amount, timing, and type of precipitation and the frequency of extreme precipitation and drought events.^240,241 Increases in precipitation, including extreme events and flooding, increase the concentrations of DOM and the turbidity in inland and coastal aquatic ecosystems, reducing their UV transparency (Fig. 2).²⁴²

Fig. 2 Plume of turbid water entering the east basin of New York City's Ashokan Reservoir following an extreme runoff event. The increase in extreme precipitation events in many regions of the world will decrease UV transparency, the potential for solar disinfection of parasites and pathogens, and increase the cost of effective disinfection of drinking water sources. See the text for details. Photo credit: Randall Hurlbert.

An 18-year study in Sagami Bay, Japan, found that seasonal reductions in UV transparency and shallower mixing depths are related to increases in DOM concentrations and to weather patterns related to the Pacific Decadal Oscillation index (an ocean-based oscillation that influences regional weather, as does El Niño).²⁴³ Increases in DOM and changes in UV transparency can alter microbial food webs²⁴⁴ and the distribution and abundance of zooplankton.^245–249 Dietary photoprotective compounds can decrease the threat of damage by UV radiation and reduce the need for behavioural responses in zooplankton.²⁴⁵

The fate of DOM in inland waters is also important for the global carbon cycle, because inland waters release large amounts of CO₂ (∼2 petagrams (Pg) per year), much of which is derived from DOM.²⁵⁰ While microbial activity has generally been thought to be the primary mechanism of breakdown of DOM in aquatic ecosystems, recent data suggest that solar UV radiation may be responsible for 70–95% of the breakdown of this substantial pool of organic carbon in aquatic ecosystems in the Arctic.^250,251 This sunlight-driven degradation of DOM has important implications for biogeochemical cycles and greenhouse gas production, as discussed in more detail in section 5.2 below.

The seasonal duration and thickness of ice cover in aquatic ecosystems is decreasing globally, increasing exposure to UV radiation.²⁵² In the Arctic, the areal extent and thickness of sea ice continue to decline. Recent modeling efforts estimate that exposure to the UV-B wavelength range in the surface waters of the Arctic Sea may increase by as much as tenfold between 1950 and 2100 due to the melting of sea ice.²⁵³ Copepods, which are the dominant grazers in the oceans, ascend to shallower waters around the time of ice-out in the Arctic, which has the potential to increase their exposure to UV radiation. Increases in photoprotective compounds in the copepods at this time of year reduces the potential for damage by UV radiation.²⁵⁴ In the Antarctic there is a different pattern of sea ice. The areal extent of Antarctic Sea ice has been increasing by about 1% per year,²⁵⁵ but the thickness of the ice has been decreasing at an accelerating rate with as much as 18% loss within two decades in some regions.²⁵⁶ Recent modeling indicates that polar stratospheric ozone depletion may alter the areal extent of sea ice; however, the direction of this effect is still a matter of debate.^31,255 In contrast, other modeling has shown that while ozone depletion is altering the temperature and salinity of the Southern Ocean, effects on Antarctic Sea ice are not likely.³¹ In the Arctic, models that compare the increase in seasonal loss of sea ice in the late twenty-first versus the late twentieth century predict that climate change-induced sea ice loss will reduce ozone by 13 Dobson Units during the spring.²⁸ This small percentage loss in ozone is not likely to lead to important feedbacks between ozone depletion and sea ice. These potential feedbacks between ozone and sea ice are covered in more detail in sections 1.2, 1.8, and 1.9 above.

4.2 Increasing air temperatures have led to a warming of surface waters in lakes and oceans, increasing the strength of thermal stratification and exposure of surface-dwelling organisms to UV radiation

Mean annual temperatures of oceanic surface water have become warmer by about 1 °C over the last 125 years with increases in thermal stratification and a reduction in the depth of the upper mixed layer, as well as deep ocean warming.²⁵⁷ Since most of the primary producers are found in this sunlit zone, they are effectively exposed to higher levels of visible light and UV radiation. Stronger stratification also hampers the upwelling of nutrients from deeper water, reducing productivity and altering the species composition of primary producers.²⁵⁸ There is also some evidence that, by reducing the average exposure of algae to UV radiation, greater concentrations of DOM may thus reduce the photoprotection and UV tolerance of algae. Under some conditions these algae may be circulated into the near surface waters where they are exposed to high UV radiation and subsequently more readily damaged.²⁵⁹

4.3 In contrast to the warming of most ocean waters, there is a significant cooling in the North Atlantic between Greenland and Ireland

This is due to a weakening of the Gulf Stream that heats the North Atlantic, the American east coast, and Northern Europe,²⁶⁰ and a decline in the Atlantic Meridional Overturning Circulation (AMOC).²⁶¹ These changes in oceanic currents and surface temperatures alter thermal stratification and thus the exposure of marine organisms to UV radiation.

Warming of the climate is altering the strength, depth, and duration of thermal stratification in inland water bodies as well. This will change the exposure to UV radiation with the response varying with the depth, surface area, and water clarity of the ecosystem.^262,263 Deeper, warmer lakes (e.g., tropical lakes and reservoirs) are increasingly becoming stratified faster and remain so for longer than shallower, cooler lakes.²⁶³ The transparency of water modulates the amount of solar UV radiation penetrating the water column and is also a primary modifier of thermal stratification.²⁶⁴ While lower transparency reduces exposure to UV radiation on a depth-averaged basis, it can also warm surface waters, cool bottom waters, and increase stratification. Reduced transparency in clear-water systems substantially increases the duration of stratification and delays the onset of ice cover,²⁶⁴ thereby extending the ice-free season. Longer ice-free periods and increased thermal stratification increase the exposure of surface-dwelling organisms to UV radiation, alter vertical habitat gradients of food and predation in the water column, and modify nutrient cycling and gas exchange rates including those for greenhouse gases. These changes in aquatic ecosystems have the potential to reduce biodiversity through increased exposure to UV radiation, increase greenhouse gas production, and create mismatches between consumers and their food resources that can decrease consumer abundance.

4.4 Acidification of the oceans and associated increases in the concentrations of CO₂ in the oceans interact with the effects of UV radiation, climate change, and other stressors in affecting marine organisms

A recent review highlighted the fact that acidification of the oceans is exacerbated by inputs of CO₂ derived from the degradation of DOM as well as from the atmosphere.²⁶⁵ In coastal regions, increased runoff containing DOM and nutrients associated with climate-driven extreme precipitation events can further increase the concentrations of CO₂ in seawater and accelerate acidification beyond that expected from elevated atmospheric CO₂ alone.²⁶⁵

The rates of growth and/or photosynthesis in ecologically important algae can be inhibited by interactions between the effects of UV radiation and acidification²⁶⁶ as well as by interactions between these two stressors and limitation of nutrients.²⁶⁷ In contrast, the elevated concentrations of CO₂ associated with acidification may ameliorate the negative effects of photoinhibition by UV radiation in some marine diatoms by increasing photosynthesis as long as exposure to UV radiation is not so high as to cause damage.²⁶⁸

4.5 An increase in the prevalence of water-borne parasites is a likely response to reduced solar disinfection from decreased penetration of UV radiation in aquatic ecosystems induced by increases in precipitation and runoff from terrestrial ecosystems

It has long been recognised that viral, bacterial, and protozoan pathogens and parasites are inactivated by exposure to solar UV radiation. Decreases in the UV-transparency of inland and coastal waters related to increases in inputs of DOM suggest that inactivation of parasites and pathogens by solar radiation is becoming progressively less effective.¹⁶³ Outbreaks of water-borne pathogens are frequently preceded by high precipitation events.²⁶⁹ While mobilisation of pathogens from terrestrial ecosystems to inland waters is thought to account for a substantial portion of these outbreaks, the sensitivity of many pathogens to UV radiation coupled with reductions in solar disinfection related to decreases in the UV transparency of inland and coastal waters may also be a contributing factor.

A variety of environmental factors as well as the characteristics of parasites and pathogens may influence the effectiveness of solar disinfection by UV radiation in surface waters (see also section 5.5 below). Pathogenic bacteria²⁷⁰ and fungi²⁷¹ are inactivated by natural and simulated solar radiation, and this inactivation may be reduced by a shading of UV radiation due to DOM and algal blooms.²⁷² Hydrodynamic modeling based on data from Lake Tahoe indicates that variations in the depth of the thermocline and vertical mixing alter the exposure to UV radiation and are thus an important element in the effectiveness of the solar disinfection of parasites.²⁷³

In contrast to the UV-disinfection of surface waters, exposure to high levels of UV radiation can either stress or suppress the immune system of hosts, making them more susceptible to infection. For example, high levels of UV radiation combined with warm temperatures may increase the infection of freshwater fish by the protozoan parasite that causes white spot disease.²⁷⁴ Similarly, exposure of larval damselflies to UV radiation can suppress the immune response of adults, reducing their ability to defend against parasitic mites (see section 3.9 above).²²⁴

4.6 The effects and transport of pollutants in aquatic ecosystems are altered by UV radiation

Our general knowledge of the interactions between UV radiation and pollutants is limited because they have not been studied for many compounds. UV radiation can either enhance or reduce the effects of pollutants (see also section 5.5 below). Studies of the interaction of UV radiation and four toxic compounds on the growth and metabolism of algae revealed both synergistic and antagonistic effects.²⁷⁵ The mechanism of action of UV radiation may involve more than just photosynthesis. For example, the uptake and thus toxicity of copper in aquatic plants can be enhanced by exposure to UV radiation.²⁷⁶ Pesticides may also inhibit the repair of UV-induced DNA damage in embryos of African clawed frogs,²⁷⁷ as well as decrease the ability of their tadpoles to behaviourally avoid UV radiation.²⁷⁸

The toxic effects of oil spills are enhanced by UV radiation due to radical formation, alone or in combination with warming of the climate. In contrast, UV radiation has been shown to alleviate the negative effects of the combination of oil and ocean acidification on bacterial communities,²⁷⁹ and to play a role in photodegrading antibiotics in the environment.²⁸⁰

Mercury is an important aquatic pollutant and the toxicity and transport in the food web depends on the form of the mercury compound, with toxicity being especially severe for organic methylmercury. In nature there is a balance between the production of methylmercury and its breakdown by either bacterial or photochemical processes. Photochemical degradation of methylmercury is assumed to be the more dominant of these two pathways in lakes. UV-B radiation is the most active component of the solar spectrum in the photodegradation of methylmercury in Norwegian lakes;²⁸¹ likewise in laboratory studies.²⁸²

Anthropogenic sunscreens (commercially produced sun lotions) contain a variety of chemicals that include organic compounds such as oxybenzone (benzophenone-3, or BP-3) that absorb damaging UV radiation, or inorganic compounds such as titanium dioxide or zinc oxide that reflect UV radiation. With the hundreds of millions of tourists that crowd beaches and use these sun lotions each year, there is the potential for these chemicals to accumulate to concentrations that may damage inland and coastal aquatic ecosystems. The organic compound BP-3 is toxic to coral planula larvae as well as to the cells of corals.²⁸³ When exposed to BP-3, even in the μg L⁻¹ range, the larvae of coral lose their motility and their algal symbionts, and suffer substantial DNA and tissue damage, which leads to increased mortality. Since BP-3 is photo-toxic, these negative effects of BP-3 are worse in the sunlit waters where coral larvae are found.²⁸³ In the presence of solar UV radiation, titanium dioxide nanoparticles produce the strong oxidising agent hydrogen peroxide. While these nanoparticles are coated to prevent human skin damage in sun lotions, the coatings dissolve in natural waters and thus become more reactive. Combined laboratory experiments and field sampling at a beach in the Mediterranean Sea demonstrated that the levels of hydrogen peroxide produced in the presence of UV radiation by titanium dioxide in sun lotions can damage phytoplankton.²⁸⁴ These nanoparticles and other chemical compounds may concentrate in the surface microlayer where critical processes such as gas exchange and solar flux are regulated, as well as accumulate in the sediments where they may harm sediment-dwelling organisms.^284,285 Other sun lotions release inorganic compounds that include not only toxins, but also phosphorus and nitrogen that are important nutrients that may increase primary productivity.²⁸⁵ While our knowledge of the effects of sun lotions on aquatic ecosystems is very limited, the available data suggest that the chemicals released by sun lotions may damage corals and alter phytoplankton dynamics, leading to negative effects on aquatic ecosystems and their food webs.

4.7 Several recent advances have been made in our understanding of how exposure to UV radiation can alter species interactions from phytoplankton to invertebrates and vertebrates in aquatic ecosystems, and also how some responses to UV radiation are temperature-dependent

Even closely related species may differ in their sensitivity to UV-B radiation owing to variation in the type and amounts of UV-absorbing compounds and detoxifying enzymes that they each have.²⁸⁶ Photoprotective defense mechanisms differ widely among species of phytoplankton²⁸⁷ and invertebrates.²⁸⁸ UV-induced changes in the composition of species at one trophic level can have consequences for other trophic levels. For example, concentrations of fatty acids in sub-Antarctic phytoplanktonic algae, important to grazing zooplankton, increase when these algae are grown without UV-B radiation.²⁸⁹

The role of UV radiation in the vision, communication, and performance of vertebrates is becoming more widely recognised. For example, it was recently found that hooded seals have enhanced vision in the UV wavelength range.²⁹⁰ The development of UV markings used in communication among coral reef fish is absent in captivity and is dependent on exposure to UV radiation on the reef.²⁹¹ When exposed to UV radiation, spring-born mosquitofish reared at temperatures higher or lower than their 28 °C optimum exhibit decreases in swimming performance and higher metabolic rate accompanied by increased damage to proteins and lipids in their tail muscles. The responses of fish born later in the year are less temperature dependent.²⁹² These new findings indicate that the effects of UV radiation are altered by the ability of aquatic organisms to detect, respond to, and defend against solar UV radiation, and that these responses can be temperature-dependent.

5 Biogeochemical cycles: the effects of solar UV radiation and interactions with climate change

5.1 Climate change could enhance the production of short-lived halogens that cause depletion of ozone in the stratosphere and troposphere

Short-lived halocarbons (with lifetimes <6 months; e.g., methylene chloride and bromoform) that cause depletion of ozone are produced in marine environments;^293,294 for example, at the sea surface microlayer,²⁹⁵ and when terrestrial dissolved organic matter (DOM) is introduced into seawater.²⁹⁶ These short-lived brominated and iodinated halocarbons are formed via reactions associated with the natural breakdown of DOM in marine systems.^297,298 Some of these products can reach the stratosphere where they participate in the destruction of ozone.²⁹⁸ Climate-induced increases in runoff of DOM from land to the ocean²⁹⁹ are likely to enhance the formation of these short-lived halocarbons with potentially important consequences for the depletion of ozone in both the stratosphere and troposphere.^293,294

5.2 Ultraviolet radiation stimulates the production of carbon dioxide from terrestrial dissolved organic matter due to melting permafrost, and contributes to a significant positive feedback on global warming

Terrestrial permafrost contains nearly 1500 petagrams (1 Pg = 10¹⁵ g) of organic carbon.^300,301 The release of CO₂ from thawing permafrost in the Arctic is known to be increasing²⁹⁹ and is considered to be a major positive feedback in climate change by amplifying global warming.³⁰⁰ A range of processes is responsible for this release of CO₂, but central in this change is the increased transport of terrestrial dissolved organic matter (DOM) into streams, rivers and lakes due to melting permafrost.^250,301 Recent evidence has highlighted the role of solar radiation in the degradation of terrestrial DOM to produce CO₂.^250,251 In parallel, the mechanisms and organic precursors underlying the photodegradation of terrestrial DOM are becoming better understood.^302,303 Photodegradation accounts for 70–95% of the total breakdown of DOM in Arctic lakes and rivers, and is therefore a major factor driving the release of CO₂ from thawing permafrost.

5.3 Land-use and climate change alter vegetative cover and affect UV-mediated carbon turnover of plant litter in terrestrial ecosystems

Substantial differences in vegetative cover occur in terrestrial ecosystems due to influences from natural climatic factors coupled with human activities such as agriculture, deforestation, or the intentional planting of exotic species.³⁰⁴ Plant cover is a major determinant of the amount of UV radiation and total solar radiation reaching the soil surface,³⁰⁵ which means that plant cover modulates the effect of UV radiation on carbon turnover.^306,307 Reductions in plant-cover due to decreased rainfall increase the direct photochemical degradation of carbon compounds in soil and may indirectly enhance microbial litter decomposition.^196,199 Afforestation (the planting of woody species in areas that were previously unforested), which increases plant cover, dramatically reduces carbon loss and photodegradation in arid land ecosystems.³⁰⁵ In ecosystems with marked seasonality of rainfall, UV radiation in particular has been shown to stimulate litter decomposition (see section 3.4 above).^192,194

5.4 Increases in the frequency, intensity, and extent of fires due to anthropogenic climate change and other human activities alter the effects of UV radiation on carbon cycling in terrestrial ecosystems

Estimates of the severity of extreme events highlight drought, and the consequent increased fire frequency, as potentially having the largest impact on the carbon cycle in the coming decades.³⁰⁸ Fires (Fig. 3) can interact with the effects of UV radiation on carbon cycling in a number of contrasting ways. The burning of above-ground biomass produces CO₂, methane and other greenhouse gases and increases intercepted solar irradiance at the soil surface, all of which lead to increased fluxes of carbon into the atmosphere. In contrast, fires also generate short-lived aerosols that reduce UV and visible radiation and act as cloud-condensation nuclei (see sections 1.6 above and 6.8 below). Fires can increase diffuse radiation that would enhance the uptake of CO₂ by plants.³⁰⁹ Charred biomass on the land surface includes a broad spectrum of organic constituents (including black carbon), which have different photoreactivities,³¹⁰ and which can emit gaseous products from photodegradation. This charred organic matter can then run off into streams, lakes, and coastal inland waters, where photochemical mineralisation and biological degradation can occur. Taken together, the interaction of UV radiation and fires is complex, but suggests that an increased frequency of fires, particularly large-scale intense fires, will amplify the importance of UV-driven carbon losses due to changes in exposure and the characteristics of post-fire organic matter.

Fig. 3 Climate change is leading to an increased frequency and extent of high-latitude fires that interact in various ways with changing UV radiation. Photo credit: Brian J. Stocks, Canadian Forest Service, Sault St. Marie, Ontario, Canada.

5.5 The fate and transport of pesticides, pharmaceuticals, heavy metals, nanomaterials, and pathogens are affected by UV-induced photochemical processes

UV radiation induces photoreactions that dissipate pollutants and pathogens. For example, protection from UV radiation by overlying plastics enhances the residues of some photosensitive pesticides in water and on crops.^311,312 Pharmaceuticals, heavy metals, nanomaterials, organic pollutants, and pathogens enter aquatic environments where they are degraded through exposure to UV radiation. This photodegradation involves both direct photolysis and photoreactions sensitised by terrestrially-derived DOM^{189,313–316} (and see section 4.5 above). UV-initiated photoreactions play a key role in the release of nanomaterials from their composites with polymers in commercial products,³¹⁷ carbon nanotubes,³¹⁸ and graphene oxide.³¹⁹ Future concentrations of certain environmental contaminants will thus depend on their release into the environment coupled with climate change events and photodegradation by UV radiation.

5.6 The production of methane and other trace gases from the foliage of a wide variety of plant species under realistic levels of ultraviolet radiation has been confirmed to be negligible for the global methane budget

Leaves from a wide range of species of plants were evaluated for emissions of greenhouse gases under environmentally relevant levels of UV-B radiation.³²⁰ All had measurable, but small, emissions of methane, carbon monoxide, and ethane. These fluxes are consistent with previously published studies,³²¹ thus confirming that the global contribution of methane from this source is small (≤1 Teragram [1 Tg = 10¹² g] y⁻¹), representing less than 0.2% of global sources.³²⁰

6 Tropospheric air quality, composition, and processes: the effects of solar UV radiation and interactions with climate change

6.1 Ground-level ozone concentrations may increase substantially over large geographic regions due to a combination of stratospheric ozone recovery and climate change in the coming decades

Model simulations indicate that ground-level ozone would have decreased in northern mid-latitudes due to stratospheric ozone depletion, but this decrease was offset by changes in atmospheric circulation.³²² As stratospheric ozone recovers, future background (non-polluted) ground-level ozone concentrations are expected to increase due to slower destruction by decreased UV radiation,³²³ and from increased transport of ozone from the stratosphere to ground-level. These increases in background ozone are likely to have negative implications for agriculture and ecosystem health. In addition, significant increases in ground-level ozone are expected due to increased storm-associated lightning,³²⁴ although the magnitude of this effect is uncertain due to a lack of understanding of the process for the production of nitrogen oxides (NO_x) by lightning. These changes in ground-level ozone are in addition to any increases due to anthropogenic³²⁵ and biogenic emissions that are expected to be temperature-dependent.³²⁶ In urban areas, high emissions of NO_x may counteract some of these effects and urban ozone concentrations may decrease.

6.2 Significant positive long-term trends in the concentrations of ozone at the Earth's surface have been observed over the last 25 years at most locations³²⁷

The exceptions are for Europe since 2000, and the east coast of the USA since the 1990s, where the trends were negative.³²⁷ In the Southern Hemisphere the increase was small but statistically significant. It remains unclear what is driving these trends. Model simulations for the west coast of the USA suggest that there has been an increase in ozone due to the export of NO_x from Asia during 2005–2010, offsetting about half of the tropospheric column ozone reductions attained by reductions in local emissions.³²⁸ However, stratospheric-tropospheric ozone transport can also contribute to this trend.

6.3 An analysis of records of ground-level ozone in the UK concludes that the magnitude of estimated long-term trends depends strongly on the metrics used to quantify ozone patterns and amounts³²⁹

The trends depend on how ozone metrics are computed, e.g., from ozone peak levels, averages, or other algorithms (SOMO10, SOMO35).³³⁰ The choice of metric has implications for assessing the effectiveness of regulations and air quality improvements, and their relationship to background ozone concentrations that are beyond local/national regulation.

6.4 Increasing evidence indicates that the known sources and sinks of hydroxyl (˙OH) radicals are insufficient to explain concentrations of ˙OH actually observed or inferred in the atmosphere

UV-generated ˙OH radicals perform a critical function by accelerating the removal of many compounds from the atmosphere. Global observations of chemicals removed by ˙OH (e.g., methyl chloroform) indicate little or no difference in OH concentrations between the Northern and Southern Hemisphere, while models predict ∼25% more ˙OH in the Northern Hemisphere due to larger emissions of nitrogen oxides (NO_x).³³¹ Direct ˙OH measurements in locations with high amounts of biogenic hydrocarbons (e.g. isoprene) and low anthropogenic pollution (low NO_x) also show larger amounts of ˙OH than predicted – likely produced through complicated radical recycling chemistry recently identified in laboratory experiments.^332–336 Additional sources include the photolysis of ozone on the surface of cloud droplets³³⁷ and the reaction of methyl peroxy radicals with bromine monoxide.³³⁸ Measurements also show that the total rate of removal of ˙OH (the reactivity of ˙OH) is larger than predicted, further indicating the occurrence of recycling of ˙OH or other sources.^339–343 Better quantification of the chemical reactions of ˙OH will improve our ability to predict future atmospheric compositions in response to changing emissions of critically important gases, e.g., methane, sulfur and nitrogen oxides, as well as HFCs and HCFCs.

6.5 Halogens contribute to the atmospheric oxidation (self-cleaning) capacity in marine environments, and new data suggest that they also contribute inland

Nitryl chloride (ClNO₂) has been identified in the nocturnal atmosphere in a range of environments including over the coast and well inland. While stable in the dark, ClNO₂ is photolysed at sunrise releasing Cl atoms that are significant oxidants at that time.³⁴⁴ Natural and anthropogenic emissions of short-lived halogens appear to supplement ˙OH in the atmospheric removal of critically important gases (methane, sulfur, and nitrogen oxides, HFCs and HCFCs; see section 5.6 above).

6.6 A new study demonstrates a clear linkage between the tropospheric oxidation capacity and stratospheric UV transmission even when modulated by factors other than ozone

Simulations of the 1991 eruption of Mt. Pinatubo confirmed that SO₂ gas and sulfate particles injected into the stratosphere by the volcano caused a substantial reduction in tropospheric UV radiation, leading to lower ˙OH and higher methane concentrations over much of the globe for several months after the eruption.³⁴⁵

6.7 UV radiation is known to be a critical driver of the formation of photochemical smog, e.g. ozone and aerosols. UV radiation may also play a role in the destruction of aerosol particles

A new modeling study suggests that UV radiation also destroys some organic aerosols, a removal process which is comparable with hitherto recognised losses such as removal by rain.³⁴⁶ Such processes, if confirmed, could have a profound effect on the spatial and temporal distribution of aerosols, their effects on human health, and their response to UV radiation and other environmental changes.

6.8 Air pollution, which includes UV-generated ground-level ozone and particulate matter, is expected to worsen in many locations and improve in others, with direct consequences for human health

A recent global burden of disease study using a business-as-usual emission scenario projected that premature mortality due to outdoor air pollution could double by 2050, mostly in Asia.³⁴⁷ In the USA and EU, regulators have concluded that adverse health effects in some cases occur at air pollution concentrations lower than those of the currently established standards.^348,349 However, recent studies have questioned the strength of the evidence supporting causal relationships between short- or long-term exposures to ozone, and e.g., cardiovascular effects.^350–352 The problem with all of these analyses lies in their complexity: choices of databases, models and statistical methods can substantially affect the conclusions.³⁵³

6.9 Based on current data, the amount of trifluoroacetic acid (TFA) formed from HCFCs and HFCs in the troposphere is too small to be a risk to the health of humans and the environment

The fate and effects of TFA in the environment have been critically assessed.³⁵⁴ In summary, the formation of TFA from the degradation of HCFCs and HFCs continues to be a concern, in part because of its very long environmental lifetime. TFA is produced naturally and synthetically and is widely used in the chemical industry. In addition, TFA is a potential environmental breakdown product of a large number of chemicals (> one million), many of which are in commerce. These chemicals include pharmaceuticals, pesticides, and polymers. While the contribution to global amounts of TFA from HCFCs and HFCs is well understood, that from these other chemicals is very uncertain. TFA salts are stable in the environment and will accumulate in terminal sinks such as playas, salt lakes, and oceans where the only process for the loss of water is evaporation. The total contribution to the existing amounts of TFA in the oceans as a result of the continued use of HCFCs, HFCs, and HFOs up to 2050 was estimated to be a small fraction (<7.5%) of the ≈0.2 μg of acid equivalents per L estimated to be present at the start of the millennium. TFA, as an acid or as a salt, is of moderate to low toxicity to a range of tested organisms. Based on the worst-case exposure scenarios for the salts of TFA, risks to mammals, plants growing in soil, and aquatic organisms is de minimis. The risks are potentially greater for plants exposed directly to the acid form of TFA but the short times of exposure in most natural environments will likely mitigate adverse effects.

7 Materials: the effects of solar UV radiation and interactions with climate change

7.1 In addition to UV radiation, air pollutants in urban areas may contribute very significantly to the discoloration of plastics routinely exposed to outdoor conditions even in the absence of direct solar radiation

In the weathering of materials outdoors, solar UV radiation is generally the most important agent responsible for discoloration and the loss of mechanical strength.³⁵⁵ In a 2-year outdoor exposure study of 17 types of plastics, where direct solar radiation and precipitation were excluded, diffuse light and pollutants, especially NO₂ and O₃, were found to be the dominant secondary agents of damage. These were more important than temperature or relative humidity in causing the yellowing of plastics in art and heritage uses.³⁵⁶ The concentrations of traffic-generated air pollutants need to be taken into account in weathering studies, especially in urban areas.

7.2 Carbon black pigment, when used together with some UV stabilisers, enhances their stability synergistically

Carbon black (CB) is a good UV stabiliser in plastics^357,358 as well as in rubber products. Recently, synergistic stabilisation of polyethylene using a combination of 1.5% CB and 0.1 to 0.2% (w/w) of the popular hindered-amine light stabilisers (HALS) was reported.³⁵⁹ In accelerated weathering studies indicators of weathering such as the gel content, carbonyl index, elastic modulus and elongation at break were the lowest for the HALS/CB system. The mechanism of the synergism has not as yet been fully elucidated. With dark-coloured plastic products, CB can be used to extend service lifetimes outdoors.

7.3 Polypropylenes containing recycled plastic show changes in bulk morphology that results in a reduced outdoor service life

Replacing a part of the virgin resin with recycled polypropylene (PP) in molding products is commonly practiced and is both economical and environmentally desirable. The mechanical properties of thick (>3 mm) molded PP with a recycled PP content of up to 40% are not very different from those of samples made from virgin PP. However, the outdoor durability of the former was shown to be much poorer when exposed to solar UV radiation, based on decreased mechanical properties,³⁶⁰ than that for virgin PP. Also, after 12 hours of accelerated UV irradiation, the decrease in toughness of molded laminates was larger by 13% for PP with a 30% recycled content compared to that for virgin PP. Despite the ‘green’ advantage of using a higher recycled content, at least for PP, it reduces the service life of the product.

7.4 In cable-jackets with the new aluminum-based fire retardants, initial degradation by UV radiation yields a filler-rich surface layer that screens the underlying polymer from further degradation

There is a growing trend in the industry to phase out polybrominated fire retardant additives. Ethylene vinyl-acetate co-polymer (EVA), with aluminum hydroxide (∼40% w/w) filler as the fire retardant, is used in cable jacketing. During exposure to UV radiation, the material undergoes heterogeneous degradation with the filler progressively migrating to the surface. This yields a protective surface layer that is enriched with aluminum oxide.^361,362 Where the underlying material is thick enough to provide the needed mechanical integrity to the cable, this phenomenon provides incidental UV stability at no additional cost.

7.5 Some residual additives may enhance the degradation by UV radiation of light-sensitive polymer layers in organic photovoltaic cells

Emerging organic photovoltaic technology, including that of polymer photovoltaic (PV) cells, is expected to pose a strong challenge to the silicon counterparts. However, improving their durability to ensure a long service life (>20 years) is critical for their commercialisation. The mechanisms of photodegradation have been studied for the light-sensitive polymers in organic PV cells^363–365 as well as their plastic encapsulants.^366–368 In a recent report, residual additives left in the light-sensitive polymer during manufacturing, which are confined and sandwiched between plastic laminates, were shown to promote accelerated degradation of these polymers.³⁶⁹ Complete removal of these additives prior to sealing the light-sensitive layers during fabrication of the cells may help avoid this complication and extend service life.

7.6 Colour changes in wood exposed to solar UV radiation correlate well with the extent of lignin photodegradation

UV-induced degradation of the lignin in wood exposed to solar radiation results in yellowing.³⁷⁰ The surface yellowness index and the extent of lignin degradation (measured spectroscopically) during solar UV-induced degradation in several hardwood species were found to be correlated.^371–373 Since colour measurements are more convenient, they could be used as a good surrogate for detecting changes in the surface chemistry of weathered wood.

7.7 Surface treatments, especially nanomaterial-based coatings, show continuing promise as UV stabilisers for wood

Nanomaterial UV stabilisers perform exceptionally well in a variety of wood species. The high specific surface area of nanoparticles provides efficient light-shielding at low weight fractions of the filler. Graphene,³⁷⁴ zirconium dioxide,³⁷⁴ iron oxide,³⁷⁵ titania,³⁷⁶ and cerium oxide³⁷⁷ can control UV-induced yellowing in several wood species. Similarly, surface modification of wood with nanocellulose crystals³⁷⁸ and epoxidised soybean oil³⁷⁹ also result in good UV stabilisation. There can be adverse environmental implications of the large-scale use of nanofiller technologies as the particles can be released during use. However, these approaches are yet to be developed for commercial use and the effects on the environment and human health from the potential release of these nanoparticles due to weathering are not yet known.

7.8 The efficacy of thermal surface modification of wood to control solar UV-induced damage remains unclear

Thermal modification of wood was developed as a means of improving the structural properties and biodegradation of wood outdoors. As the wood is used outdoors, how UV stability is affected by the heat treatment has been of recent interest. Two reports^380,381 indicated an increase in the rate of discoloration and degradation of lignin in heat treated hard- and soft-wood species upon UV irradiation; others^382,383 found the opposite. The stabilisation might be either species-specific or process-dependent. This is a promising environmentally-friendly technique for the protection from UV radiation of selected types of wood.

7.9 Modifying textile fibres with nanoparticles effectively improves their UV protective function

Nanoparticles used as fillers in textile fibres dramatically decrease the transmission of solar UV radiation. The incorporation of nanoparticulate titanium oxide,³⁸⁴ cerium oxide,³⁷⁷ and silver^385,386 into natural textile fibres at <5% (w/w), lead to very significant reductions in the transmission of solar UV-A radiation through the fabrics. The service life of these treated fabrics is not reliably known (see section 2.11 above).

7.10 The protection against exposure to UV radiation provided by cotton fabrics can be increased by an order of magnitude through treatment with graphene nanoplates

Cotton fabric typically used in garments allows ∼15% of solar UV radiation to pass through.³⁸⁷ Good UV radiation absorbers coated on the fibre or fabric can significantly reduce this value. Surface modification of cotton fabric with graphene nanoplates increases the ultraviolet protection factor (UPF) 10-fold³⁸⁸ for 0.4% (w/w) graphene. Using a chitosan dispersant with 1% (w/w) graphene enhances it 60-fold,³⁸⁹ With the growth in graphene technology the technique has commercial potential. However, the thickness and weave of the fabric also strongly determines the level of protection from UV radiation. As is the case for fabrics treated with certain nanoparticles, the service life of these treated fabrics is not reliably known.

Acknowledgements

The Environmental Effects Assessment Panel gratefully acknowledges the following: Generous contributions by UNEP/Ozone Secretariat and the World Meteorological Organisation for the convened author meeting were much appreciated. Participation for the following authors is also acknowledged: Prof. Carlos Ballaré, Drs Pieter Aucamp, Amy Austin, Krishna Pandey, and Halim Redhwi were supported by UNEP. Prof. Sharon Robinson was supported by the Centre for Sustainable Ecosystem Solutions, University of Wollongong, and self-funded. Prof. Robyn Lucas was supported by the Australia National Health and Medical Research Council Career Development Fellowship, the Telethon Kids Institute and the Australian National University. Prof. Stephen Wilson was supported by the Centre for Atmospheric Chemistry, University of Wollongong. Prof. Lars Olof Björn was supported by the South China Normal University. Prof. Janet Bornman was supported by Curtin University, Western Australia. Prof. Alkiviadis Bais was supported by the Greek General Secretariat for Research and Technology (contract 100380/2014). Dr Kevin Rose was supported by the Rensselaer Polytechnic Institute. Prof. Donat-P Häder was supported by the Bundesministerium für Umwelt Naturschutz und Reaktorsicherheit. Drs Antony Andrady, Paul Barnes, Germar Bernhard, Janice Longstreth, Sasha Madronich, Craig Williamson were supported by the U.S. Global Change Research Program. PB was also supported by the U.S. National Science Foundation (DEB 0815897) and Loyola University J.H. Mullahy Endowment for Environmental Biology. GB was also supported by the U.S. National Science Foundation (ARC-1203250) and Biospherical Instruments Inc. CW was also supported by the U.S. National Science Foundation DEB-1360066. Prof. Nigel Paul was supported by the UK Department for Environment, Food and Rural Affairs (DEFRA), Dr Richard McKenzie was sponsored by the New Zealand Government's Ministry for the Environment through the Ministry of Business, Innovation and Employment's research contract C01X1008. Dr Rachel Neale was funded by a fellowship from the National Health and Medical Research Council. Prof. M. Shao was supported by the Natural Science Foundation of China for Outstanding Young Scholars (Grant No. 41125018). Prof. Keith Solomon's participation was self-funded. Prof. Yukio Takizawa's participation was self-funded. Dr Ayako Torikai's participation was self-funded. Prof. Sten-Åke Wängberg was supported by the Swedish Agency for Marine and Water Management. Dr Antony Young's participation was self-funded. Dr Richard Zepp was supported by the National Exposure Research Laboratory, Exposure Methods & Measurement Division, U.S. Environmental Protection Agency.

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Footnote

† List of contributing authors in alphabetical order: Anthony Andrady, Pieter J. Aucamp, Amy T. Austin, Alkiviadis F. Bais, Carlos L. Ballaré, Paul W. Barnes, Germar H. Bernhard, Lars Olof Björn, *Janet F. Bornman (Co-Chair), David J. Erickson (passed away, 16th November 2015), Frank R. de Gruijl, Donat-P. Häder, Mohammad Ilyas, Janice Longstreth, Robyn M. Lucas, Sasha Madronich, Richard L. McKenzie, Rachel Neale, Mary Norval, Krishna K. Pandey, Nigel Paul (Co-Chair), Halim Hamid Redhwi, Sharon A. Robinson, Kevin Rose, Min Shao (Co-Chair), Rajeshwar P. Sinha, Keith R. Solomon (Secretary), Barbara Sulzberger, Yukio Takizawa, Ayako Torikai, Kleareti Tourpali, Craig E. Williamson, Stephen R. Wilson, Sten-Åke Wängberg, Robert C. Worrest, Antony R. Young, and Richard G. Zepp. *Corresponding author: E-mail: janet.bornman@curtin.edu.au

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