Ultraviolet radiation and its impacts on agriculture and forests

Although ultraviolet radiation (400–280 nm) comprises only a small portion of the solar spectrum, it has a disproportionately large photobiological effect on both plants and animals due to its absorption by important biological molecules such as proteins and nucleic acids. This is particularly true in the UV-B (320–280 nm). Ultraviolet radiation has shaped life and its evolution on Earth for as long as that life has existed and in a sense there has always been a UV-B problem, particularly when species moved to locales where they were not adapted to the UV-B regime. Scientific knowledge and data that is pertinent to UV-B radiation might be said to have begun in the 1880s with the discovery that there is essentially no radiation at the Earth’s surface below a wavelength of 293 nm. The knowledge at that time that ozone absorbed strongly at short wavelengths, and especially below 293 nm suggested the presence of an ozone layer in the atmosphere. In the 1920s, G.M.B. Dobson developed an instrument to measure the total column ozone (TOC) in the atmosphere, and a network of these instruments (Dobson spectrophotometers) was instituted. Instruments of the Dobson type are still operated in an international network of about 80 stations in 40 countries today (http://www.chmi.cz/meteo/ozon/dobsonweb/ network.htm#Dobson). Several papers in this issue use the common measure of TOC, the Dobson unit, or DU, which is the depth, measured in units of 10−6 m, that all the atmospheric ozone would take up if it were brought together in a layer at sea level pressure and a temperature of 0 ◦C. In midlatitudes TOC typically ranges from 250 to 450 DU. The longest continuous Dobson record, beginning in 1926, is from Arosa, Switzerland. A graph of annual averages of these data from 1926 to 1997 shows the considerable variability of ozone over one location that makes detection of long-term trends difficult; however, a trend of decreasing ozone since the early 1970s can be discerned in the record (Heisler and Grant, 2000b). Measurements by Dobson instruments led to the discovery of the Antarctic “ozone hole” in 1985, and associated measurements confirmed that the hole could be attributed primarily to anthropogenic emissions, especially chlorofluorocarbons (CFCs). Satellite measurements (e.g. Total Ozone Mapping Spectrometer (TOMS), http://www.toms.gsfc. nasa.gov/ery uv/euv.html) indicate trends in ozone and UV-B and their distribution over the Earth’s surface. During the 1997–2000 period, TOMS measurements confirmed that minimum ozone concentrations in the Antarctic were 40–50% lower than the pre-1980 values. In the Arctic, ozone has been more variable than in Antarctic, with depletion depending strongly on temperatures in the stratosphere. Maximum depletion of ozone in the Arctic has reached 30% following cold winters, but has been slight in warm winters. Atmospheric dynamics also cause variations of ozone depletion in the Antarctic. The maximum area of the Antarctic ozone hole (defined as the area with TOC less than 220 DU) peaked in 2000 at about 29 km2; it decreased in 2001 to about 25 km2; and then in 2002 the hole was only about 20 km2 and was much shorter in duration than in recent years. The smaller hole in 2002 is not likely a sign of dramatic permanent ozone recovery, but rather a reflection of a sudden stratospheric warming event (Varotsos, 2003). In the northern midlatitudes (35–60◦N), ozone depths, relative to pre-1980 values, are about 4% lower in winter and spring and 2% lower in the summer