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Total column 03 (Dobson units)

FIGURE 13.17 Radiation amplification factor weighted by DNA action spectrum as a function of total column 03 at Palmer Station, Antarctica, during Austral springs 1988 and 1990 (adapted from Lubin et al, 1992).

Calculations using satellite measurements of total column ozone indicate that there should be a trend in surface UV at midlatitudes as well, with increases in UV being higher for the higher latitudes. For example, Sabziparvar et al. (1998) have calculated that the daily integrated radiation in the 280- to 320-nm range may have increased by as much as 60% in October and 20% in April at high latitudes since preindustrial times. Figure 13.18 shows calculated monthly changes in the effective UV dose weighted for DNA damage (see Fig. 13.15) as a function of latitude and month. Clearly, substantial increases in UV are expected, based on the ozone depletion data.

In midlatitudes, the actual measured trends in UV at the earth's surface are not so clear-cut (e.g., see Madronich et al., 1998). Some studies report no trend or even a decreasing trend in UV at the earth's surface, whereas others report an increase. For example, in one of the first studies of this issue, no increase was detected over the period f 974 to 1985 using data from the Robertson-Berger network (RB) at National Weather Stations in the United States (Scotto et al., 1988a), although the effects of urban air pollution were suggested to possibly have offset any increases in UV due to ozone depletion (Grant, 1988; Scotto et al., 1988b). Weatherhead and co-workers (1997) have carried out an in-depth analysis of the RB data and concluded that instrumental problems such as calibration difficulties render this data set unsuitable for the detection of long-term trends (see also Basher et al., 1994; and Krzyscin, 1996).

The first convincing evidence of an upward trend in UV was reported by Kerr and McElroy (1993). They monitored radiation at wavelength intervals of 0.5 nm between 290 and 325 nm with a resolution of approximately 0.5 nm in Toronto, Canada. Between 1989 and 1993, UV at 300 nm increased by about 35% in the winter and 7% in the summer, but there was no such

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FIGURE 13.18 Calculated monthly changes in UV exposure weighted by the DNA action spectrum over the period 1979-1992 (adapted from Herman et al., 1996).

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FIGURE 13.18 Calculated monthly changes in UV exposure weighted by the DNA action spectrum over the period 1979-1992 (adapted from Herman et al., 1996).

increase in the 320- to 325-nm range where light absorption by 03 is weak. Figure 13.19 shows their reported trends in UV-B radiation over this period of time as a function of wavelength; the dependence is what one would expect due to decreased column ozone, with larger decreases at shorter wavelengths. However, some caution is prudent in extrapolating this to much longer time scales (e.g., see Michaels et al., 1994; and Kerr and McElroy, 1994). It has been suggested that the relationship between UV and total 03 can be used to estimate total 03 at sites where UV is measured but column 03 is not (Fioletov et al., 1997).

Because one of the major impacts of increased UV at the earth's surface is expected to be an increase in skin cancer, several countries now include UV forecasts as part of their weather reports (e.g., see Kerr, 1994), and estimates of skin cancer increases due to ozone depletion have been made. Figure 13.20, for example, shows the estimated number of excess cases of skin cancers for the United States and for northwestern Europe if no controls had been imposed on CFCs and halons and those expected with the Copenhagen Amendments (Slaper et al., 1996). Clearly, there is expected to be a major impact of the control strategies on the incidence of skin cancer, although the number of excess cases will still be about 33,000 per year in the United States and 14,000 per year in northwestern Europe at the projected worst-case year of 2050.

Seckmeyer et al. (1997) showed that even though the monthly mean spectral UV over Germany in 1995 was not significantly different from that in previous years, periods of very high and very low irradiance levels

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