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The projected large increases in damaging ultraviolet radiation as a result of global emissions of ozone-depleting substances have been forestalled by the success of the Montreal Protocol. New challenges are now arising in relation to climate change. We highlight the complex interactions between the drivers of climate change and those of stratospheric ozone depletion, and the positive and negative feedbacks among climate, ozone and ultraviolet radiation. These will result in both risks and benefits of exposure to ultraviolet radiation for the environment and human welfare. This Review synthesizes these new insights and their relevance in a world where changes in climate as well as in stratospheric ozone are altering exposure to ultraviolet radiation with largely unknown consequences for the biosphere.

In the early 1970s, Molina and Rowland proposed that chlorofluorocarbons, widely used as refrigerants and propellants, would reach the stratosphere and catalyze the destruction of ozone molecules there¹. In 1985 evidence of an 'ozone hole' over Antarctica was first published² and its progression over the ensuing years has been captured in images that have become symbols of human influences on the global environment.

Large-scale depletion of stratospheric ozone and high levels of ultraviolet (UV) radiation have been avoided by the unprecedented success of the Montreal Protocol on Substances that Deplete the Ozone Layer, signed in 1987. The Montreal Protocol remains the only treaty ever ratified by all members of the United Nations. This unusual consensus on an environmental issue was driven by concerns that life on Earth was at risk, a concern that is supported by recent analyses of the 'world avoided' scenario of what could have happened without the Montreal Protocol^{3, 4}. The actions taken under the protocol have also made the single largest contribution to the mitigation of climate change so far, because many of the ozone-depleting substances (ODS) are also greenhouse gases (GHGs)⁵.

Solar radiation is essential to life on Earth, but its UV component may also damage both living organisms and non-living matter. UV radiation is usually divided into three wavelength bands: UV-A (315–400 nm), UV-B (280–315 nm) and UV-C (100–280 nm). UV-C radiation is potentially the most damaging, but is completely filtered out by the Earth's atmosphere and does not reach the surface. The Earth's surface is also largely protected from the most damaging short wavelength UV-B radiation due to absorption by stratospheric ozone. UV-A radiation passes through the atmosphere with little attenuation and thus is the largest component of ground-level solar UV radiation. Although generally less harmful than UV-B radiation, UV-A radiation has important effects on tropospheric chemistry, air quality, and aquatic and soil processes, as well as being mutagenic and causing immune suppression in humans⁶.

Implementation of the Montreal Protocol has drastically curtailed production of chlorofluorocarbons and other ODS⁷. It has thus successfully reduced depletion of stratospheric ozone and associated increases in ground-level UV-B radiation. However, the long lifetimes of many ODS in the atmosphere mean that substantial ozone depletion still occurs over the Antarctic, and is expected to continue for several more decades⁸. Stratospheric ozone loss has also been observed over the Arctic⁹, with 2011 showing the largest depletion ever recorded¹⁰. This major depletion event was caused by a combination of unusually low stratospheric temperatures, ODS-derived chlorine in the stratosphere and a change in circulation patterns that delayed the seasonal transport of ozone from the tropics¹⁰.

During the twenty-first century, upper stratospheric ozone is projected to increase due to the reduction in ODS and continued cooling from the increasing concentrations of GHGs. In the lower stratosphere, ozone is projected to decrease¹¹, offsetting the effect of upper stratospheric cooling. The net effect of these changes on terrestrial UV radiation is complex, as additional factors, such as increasing concentrations of carbon dioxide (CO₂) and other GHGs, begin to play an ever-increasing role in determining levels of stratospheric ozone and cloud cover. For example, by 2100, models predict that UV radiation will have increased in the tropics (where the current UV radiation is already intense), and to have decreased at polar latitudes (where the current UV radiation is generally less intense)¹².

A different world has evolved after 26 years of the Montreal Protocol. The phase-out of ODS is projected to lead to recovery of stratospheric ozone. However, additional climate-related changes in the incident UV radiation at Earth's surface may result from changes in cloud, snow and ice cover, land-use, and atmospheric and oceanic circulation, and will vary regionally. Circulation patterns, such as the North Atlantic Oscillation, account for a high proportion of the variability in the total ozone column¹³. Such patterns are predicted to be altered by the accumulation of GHGs with subsequent changes in UV-B radiation levels at Earth's surface. These changes will, in turn, alter sinks and sources of CO₂ and other trace gases that will affect future climate warming.

The unequivocal warming of the climate system¹⁴ may have important impacts on future stratospheric ozone depletion independently of the concentration of ODS in the atmosphere. Increasing concentrations of GHGs cause a radiative cooling in the stratosphere, and extremely cold polar stratospheric winters are responsible, in part, for the Antarctic and Arctic spring ozone depletions^{15, 16}. Denitrification of the chlorine reservoir (chlorine nitrate, ClONO₂) occurs on the surfaces of polar stratospheric clouds and this process is a major reason for the observed 2011 Arctic spring ozone loss^{10, 16}. The response to global warming is particularly rapid in the Arctic¹⁷. Moreover, global warming may also affect stratospheric ozone by increasing the atmospheric water content and its rate of transport through the cold tropopause (the troposphere–stratosphere boundary)¹⁸. Water vapour is a key component of stratospheric chemistry and may influence stratospheric temperatures and winds. It is involved in ozone destruction by accelerating the gas-phase hydrogen oxides (HO_x) catalytic cycle, and by increasing the surface area of stratospheric aerosol particles on which ozone-depleting halogen molecules can be activated.

Models suggest that in the first half of the twenty-first century, levels of UV radiation at Earth's surface will be determined by the recovery of stratospheric ozone, while in the second half, changes in UV radiation will be dominated by changes in clouds and GHG-induced transport of ozone¹². These climate-driven changes are projected to markedly influence the amount of UV radiation received at Earth's surface. For example, by 2050, sunburning or erythemal UV irradiance (primarily in the UV-B region of the spectrum) is projected to decrease by 2–10% at mid-latitudes, and by up to 20% at northern and 50% at southern high latitudes, relative to 1980 levels. By the end of the twenty-first century, erythemal UV irradiance is projected to remain below 1960 levels at mid-latitudes, be reduced at high latitudes (particularly in the Arctic) by 5–10% due to increases in clouds¹⁹, but to increase in the tropics by between 3 and 8% due to decreases in clouds and ozone, caused by increasing GHGs¹² (Fig. 1). Improvements in air quality, especially reductions of aerosols, may in the future result in higher UV radiation levels at Earth's surface. In the Arctic, there may be increases in sea-salt aerosols from the larger open-ocean area, as well as reductions in surface albedo due to the loss of sea ice^{20, 21}, resulting in lower surface UV irradiance.

Figure updated with permission from ref. 33.

Solar UV radiation has a profound influence on the chemical composition of the atmosphere, contributing both to cleaning of the atmosphere and to the generation of photochemical smog. These seemingly opposite effects are actually two aspects of the same chemical system. At its essence, atmospheric cleaning relies on increasing the reactivity of emitted pollutants to shorten their lifetimes. However, the higher reactivity also means that these transient compounds are often more toxic to humans and ecosystems.

UV radiation initiates this chemistry by breaking some relatively stable molecules into highly reactive fragments, and subsequent reactions involving oxygen and water generate hydroxyl (OH) radicals. These strongly oxidizing OH radicals have a beneficial cleaning effect as they remove many of the gases emitted at Earth's surface, including some important GHGs. The lifetimes and atmospheric quantities of these gases are controlled by the concentrations of OH radicals²⁵, which are in turn sustained by the UV radiation transmitted through the stratosphere to the troposphere²⁶. This coupling between stratospheric and tropospheric photochemistry is a powerful mechanism, not only for the removal of present-day emissions, but also for maintaining the long-term stability of the atmosphere against major perturbations in emissions. Such perturbations would eventually propagate to the stratosphere, where they would probably decrease ozone and increase transmission of UV radiation, thus increasing the production of tropospheric OH and ultimately accelerating the removal of the pollutants, re-establishing the global oxidation capacity²⁷.

On shorter temporal scales, the partly oxidized intermediates of this UV-initiated chemistry constitute photochemical smog, a complex mixture of gases and condensed particles (aerosols) that reach concentrations detrimental to health in many urban areas. Poor outdoor air quality causes increased hospitalizations²⁸, with several million premature deaths globally in 2010 (ref. 29), as well as damage to crops³⁰. Apart from ozone and NO₂, photochemically produced pollutants of major concern include particles containing nitrate, sulphate and various organics. Higher levels of both UV-A and UV-B radiation may intensify local and regional photochemical smog episodes, even while cleaning the global atmosphere more effectively.

The interactions of the tropospheric photo-oxidation system with the physical climate are numerous and complex. While OH radicals limit the abundance of some GHGs, such as CH₄ and halogenated hydrocarbons, the subsequent reactions can produce tropospheric ozone, which is itself a strong GHG. As production of ozone in the troposphere requires the presence of nitrogen oxides (NO_x), it is likely that tropospheric ozone has increased substantially since pre-industrial times³¹ and has contributed to radiative forcing. Globally averaged OH concentrations tend to increase in response to more intense UV radiation and larger NO_x emissions, but decrease in response to higher hydrocarbon emissions, so even the direction of net past (and future) changes remains uncertain³². Sulphate and organic aerosols affect solar radiation directly by absorption or scattering, or indirectly by modifying the formation, optical properties and lifetimes of clouds. Taken together, the direct and indirect effects of aerosols have been identified as one of the largest uncertainties in the radiative forcing of climate³². Increased cloudiness would generally decrease UV radiation reaching Earth's surface³³, but may enhance the radiation at higher altitudes by reflection from clouds below and to the sides³⁴.

The projected future changes in precipitation, vegetation cover and agricultural intensification will influence the balance between the detrimental and beneficial effects of UV radiation and their bidirectional interactions with climate change. This will have important implications for ecosystem processes and food production.

Globally, the negative effects on plant biomass of increases in UV-B radiation as a result of stratospheric ozone depletion have been minimal³⁵. In fact, the reduction in plant growth caused by increased UV-B radiation in areas affected by ozone decline since around 1980 is unlikely to have exceeded 6% (ref. 35). Plant acclimation and adaptation mechanisms, such as increased production of UV-screening phenolic substances and morphological changes, are likely to have contributed to the relatively small impact of changes in UV-B radiation on growth³⁵, although these responses can be species and region specific. Although plants found in naturally high UV radiation environments (for example, tropical or high alpine) produce more UV-absorbing compounds ('sunscreens'), those endemic to low UV radiation environments may be more vulnerable to damage³⁶. The mechanisms that mediate these acclimation responses in plants are being elucidated, including the identification of a specific UV-B photoreceptor³⁷.

Solar radiation, in particular UV-B, can be a positive regulator of plant defence systems against a broad spectrum of insect pests and pathogenic microorganisms³⁸. This has been demonstrated in field experiments where significant increases in the severity of attack by a wide range of invertebrate herbivores occurred when solar UV-B radiation was attenuated using filters (reviewed in ref. 35). This beneficial role of UV-B radiation in resistance to pests is sometimes caused by increased activity of hormonal pathways responsible for the coordination of plant immunity, such as the jasmonate pathway³⁹. Exposure to UV-B radiation intensifies the jasmonate immune response, so that the magnitude of defence induced by herbivore attack is increased³⁹. In other cases, resistance is conferred by secondary metabolites that the plant synthesizes in response to UV-B radiation, for example, phenolic compounds⁴⁰. Importantly, some of these UV-B-induced secondary metabolites may also have roles in human nutrition because of their antioxidant properties⁴¹.

Utilization and modification of plant defence responses, which are activated by UV-B radiation, may help to improve crop health in agricultural systems³⁸. In addition, manipulation of UV radiation in horticultural systems has provided an understanding of the potential positive effects of UV radiation, which can also be exploited to increase food production and quality. For example, UV-enhanced production of polyphenolics and other compounds can be used to enhance the nutritional quality of plant products and plant resistance to biotic stressors^{38, 42}. Pests and diseases can account for up to one-quarter of pre-harvest crop losses in modern agricultural systems⁴³, and standard chemical controls are becoming increasingly regulated due to their negative impacts on human health and ecosystems⁴⁴.

New insight into how UV radiation affects carbon and nutrient turnover has broadened our understanding of its impact in terrestrial ecosystems. For example, exposure to UV radiation can cause degradation of senescent plant material (such as leaf litter) and so stimulate the release of CO₂ and the mineralization of nutrients⁴⁵, especially in arid and semi-arid ecosystems⁴⁶. Changes in vegetative cover due to human activity or climate resulting in aridification can increase UV irradiation at the soil surface, causing decreased carbon sequestration but increased nutrient release through accelerated degradation of senescent plant material. Climate interactions through permafrost thawing can result in exposure of dissolved organic matter (DOM) to solar UV radiation and, as a consequence, release of CO₂ and methane via DOM mineralization⁴⁷. This process, coupled with other decomposition processes and increased fire incidence, can weaken the net CO₂ uptake of tundra, which is at present considered a carbon sink²⁰. Reduction of CO₂ uptake by terrestrial ecosystems due to the combined effects of UV radiation and climate change may result in an UV-mediated increase in atmospheric CO₂.

Long-term effects of interactions between UV radiation and other concurrent environmental stress factors, such as water availability and high temperature, are unknown and will vary depending on geographical location, prevailing climate, ecosystem type³⁵ and agricultural practices³⁸. Consequently, these strong stress conditions in combination may lead to decreased plant productivity and increased reliance on pesticides³⁸ as defence systems weaken⁴⁸. In addition, changes in plant species in favour of more resilient species may compromise growth of current food crops.

The extent and duration of periods of ice and snow cover on oceanic and inland waters have been decreasing in recent decades, altering the underwater light environment and potentially resulting in direct exposure of the aquatic environment to higher UV radiation⁴⁹. The Arctic Ocean is expected to be ice-free during the summer within the next 30 years^{20, URL:}