This is Section 5.1 of the Arctic Climate Impact Assessment .
Lead Authors: Betsy Weatherhead, Aapo Tanskanen, Amy Stevermer; Contributing Authors: Signe Bech Andersen, Antti Arola, John Austin, Germar Bernhard, Howard Browman,Vitali Fioletov,Volker Grewe, Jay Herman, Weine Josefsson, Arve Kylling, Esko Kyrö, Anders Lindfors, Drew Shindell, Petteri Taalas, David Tarasick; Consulting Authors: Valery Dorokhov, Bjorn Johnsen, Jussi Kaurola, Rigel Kivi, Nikolay Krotkov, Kaisa Lakkala, Jacqueline Lenoble, David Sliney
Ultraviolet radiation levels reaching the surface of the earth are directly influenced by total column ozone amounts and other geophysical parameters. In the Arctic, UV radiation is of particular concern, particularly during the spring and summer when the region experiences more hours of sunshine compared to lower latitudes. Goggles found in archaeological remains suggest that indigenous peoples had developed protection from sunlight long before the onset of anthropogenic ozone depletion. Although systematic measurements of UV radiation levels have been performed for little more than decade, analysis of fossil pigments in leaf sediments suggests that past UV radiation levels in the Arctic may have been similar to modern-day (predepletion) levels. In recent years, however, Arctic ozone depletion (which has sometimes been severe) has allowed more UV radiation to reach the surface. In the years since ozone depletion was first observed over the Arctic, UV radiation effects such as sunburn and increased snow blindness have been reported in regions where they were not previously observed.
Less than 10% of the solar energy reaching the top of the atmosphere is in the UV spectral region, with wavelengths between 100 and 400 nm. The shortest wavelengths (100–280 nm) are referred to as UV-C radiation. Radiation at these wavelengths is almost entirely absorbed by atmospheric oxygen and ozone, preventing it from reaching the surface.Wavelengths between 280 and 315 nm comprise the UV-B portion of the spectrum (while some communities use 320 nm to mark the division between UV-B and UV-A radiation, it is the convention in this report to use 315 nm). Ultraviolet-B radiation is absorbed efficiently but not completely by atmospheric ozone.Wavelengths between 315 and 400 nm are referred to as UV-A radiation. Absorption of UV-A radiation by atmospheric ozone is comparatively small.
Although the intensity of solar UV-B radiation is low, the energy per photon is high. Due to this high energy, UV-B radiation can have several harmful impacts on human beings (i.e., DNA damage, skin cancers, corneal damage, cataracts, immune suppression, aging of the skin, and erythema), on ecosystems, and on materials. These effects are discussed in detail in sections 7.3, 7.4, 8.6, 9.4, 14.12, 15.3.3, 16.3.1, and 18.104.22.168. Ultraviolet-B radiation also affects many photochemical processes, including the formation of tropospheric ozone from gases released into the environment by motor vehicles or other anthropogenic sources.
The amount of UV radiation reaching the surface of the earth is expressed in terms of irradiance, denoting the radiant power per unit area reaching a surface. Figure 5.1 shows typical spectral irradiance in the UV-A and UV-B wavelengths for the Arctic. The values were obtained using a radiative transfer model with a solar zenith angle of 56.5º, total column ozone of 300 Dobson units (DU), and surface albedo of 0.6.
The exposure necessary to produce some biological effect, such as erythema (skin reddening), at each wavelength in the UV spectral region is given by an action spectrum. In general, shorter UV-B wavelengths have greater biological effects than longer UV-A wavelengths, and action spectra account for this wavelength dependence. The action spectra are used to provide biological weighting factors to determine sensitivities to UV radiation exposure. The action spectrum often used to estimate human health effects is the McKinlay- Diffey erythemal response spectrum. This curve is shown in Fig. 5.1 and represents the standard erythemal action spectrum adopted by the Commission Internationale de l’Eclairage (CIE) to represent the average skin response over the UV-B and UV-A regions of the spectrum.
The biological response is determined by multiplying the spectral irradiance at each wavelength by the biological weighting factor provided by the action spectrum. As ozone levels decrease, the biological response increases (see Fig. 5.1). Integrating the product of the spectral irradiance and the biological weighting factor over all wavelengths provides a measure of the biologically effective UV irradiance, or dose rate, with units W/m2. This dose rate is scaled to produce a UV index value, which is made available to the public to provide an estimate of the level of UV radiation reaching the surface in a particular area at a particular time. Summing the dose rate over the exposure period (e.g., one day) results in a measure of the biologically effective radiation exposure, or dose, with units J/m2. In the Arctic, the extended duration of sunlight during the summer can result in moderately large UV radiation doses.When considering biological impacts, it is important to distinguish that the definition of dose presented here differs slightly from that used by biologists, who refer to dose as the amount actually absorbed by the receptor. In addition, for some biological effects the cumulative dose model outlined above is too simple, because dose history also plays a role. In many cases, repair mechanisms cause the dose received over a longer time period to have less effect than a single, intense exposure.
Although some exposure to UV radiation can be beneficial, increases in surface UV radiation doses can have detrimental effects on humans and organisms in the Arctic. The levels of UV radiation reaching the surface are affected not only by total column ozone and solar zenith angle, but also by cloudiness, surface reflectance (albedo), and atmospheric aerosol concentrations.
Climate change is likely to affect both future cloudiness and the extent of snow and ice cover in the Arctic, in turn leading to local changes in the intensity of solar UV radiation. It is very likely that climate change is already influencing stratospheric dynamics, which are very likely to in turn affect ozone depletion and surface UV radiation levels in the future. This chapter addresses some of the factors influencing total column ozone and surface UV irradiance, and describes both observed and projected changes in arctic ozone and UV radiation levels.
Chapter 5: Ozone and Ultraviolet Radiation
5.2. Factors affecting arctic ozone variability
5.3. Long-term change and variability in ozone levels
5.4. Factors affecting surface ultraviolet radiation levels in the Arctic
5.5. Long-term change and variability in surface UV irradiance
5.6. Future changes in ozone
5.7. Future changes in ultraviolet radiation
5.8. Ozone and Ultraviolet Radiation in the Arctic: Gaps in knowledge, future research, and observational needs
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^Leavitt, P.R., R.D.Vinebrooke, D.B. Donald, J.P. Smol and D.W. Schindler, 1997. Past ultraviolet radiation environments in lakes
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- ^e.g., UNEP, 1998. Environmental Effects of Ozone Depletion. United Nations Environment Programme. UNEP, 2003. Environmental Effects of Ozone Depletion: 2002 Assessment. Photochemistry and Photobiology, 2:1–72. United Nations Environment Programme
- ^McKinlay, A.F. and B.L. Diffey, 1987. A reference action spectrum for ultraviolet induced erythema in human skin. CIE (Commission International de l’Éclairage) Research Note, 6(1):17–22.
- ^CIE, 1998. Erythema Reference Action Spectrum and Standard Erythema Dose. Joint ISO/CIE Standard, ISO 17166:1999/CIE S007–1998, Commission Internationale de l’Eclairage.
- ^WHO, 2002. Global Solar UV Index: A Practical Guide. A joint recommendation of the World Health Organization,World Meteorological Organization, United Nations Environment Programme, and the International Commission on Non-Ionizing Radiation Protection.