We are concerned about decreases of stratospheric ozone levels because of subsequent increases of ultraviolet (UV) radiation that may penetrate to Earth's surface. We will briefly review some biologically important aspects of UV radiation before discussing estimated trends of UV radiation. See Chapter 1 for a more detailed discussion of the effects of UV radiation on biological systems.

6.1 Different Categories of UV Radiation and Their Biological Effects

The most important solar UV wavelengths reaching Earth's surface are grouped into two broad spectral bands related to their effects on biological activity. Remember from Chapter 4 that solar radiation at shorter wavelengths is more energetic than at longer wavelengths. Therefore, the shorter wavelength radiation can do more damage to plant and animal life. Radiation in the UV-B wavelength band, defined as 290-320 nm, is strongly absorbed by ozone before reaching Earth's surface. Nevertheless, radiation at these wavelengths causes the most damage to erythema (inflammatory reddening of the skin), plants, and molecular DNA. Radiation in the UV-A wavelength band, defined as 320-400 nm, is only weakly absorbed by ozone in comparison to UV-B. Despite having longer, less energetic wavelengths, UV-A contributes significantly to biological damage, through effects such as premature aging of skin and cataract formation.

To measure biological damage from overexposure to UV radiation we can apply a normalized action spectrum. An action spectrum characterizes the relative effectiveness of radiation at each wavelength in the UV range in provoking a certain biological response, such as erythema, changes in plant growth, or changes in molecular DNA. The action spectrum is normalized to vary between 0 and 1. If there is no effect at a certain wavelength, the value of the action spectrum at that wavelength is 0. For example, human skin is more quickly damaged by incident radiation in the UV-B wavelength range than in the UV-A wavelength range. Therefore, the action spectrum for erythema is closer to 1 in the UV-B range and closer to 0 in the UV-A range. Examples of several important action spectra weighting curves are shown by Tsay and Stamnes [1992].

For a given spectrum of incident UV radiation, we can calculate the total biological dosage for a particular response by multiplying the action spectrum for that response to the spectrum of UV radiation to get the dose at each wavelength, and then add all the doses to get the total. The total dosage changes every time the spectrum of incident radiation changes. Incident UV radiation is changed by sun angle, clouds, aerosols, and ozone amounts. When the sun is high in the sky we get large amounts of incident solar radiation; when it is cloudy we get much less radiation. As the sun angle changes during the day, so does the effective biological dosage of UV radiation for each response. If we account for these sun angle changes, we can add the doses throughout the course of the day to get the daily total UV dosage for each type of response. We call this the daily exposure. For example, the erythema daily exposure for a person the sun all day is a measure of the damage done to their skin that day.

6.2 UV Trend Estimates

In this analysis, we are most interested in how the incident UV radiation, and thus the exposure, varies with varying ozone amounts, and whether there has been an increase in exposure as stratospheric ozone values decrease. By analyzing ozone values from the Nimbus 7 TOMS data and accounting for clouds, Herman et al. [1996] estimated the daily spectra of UV radiation reaching Earth's surface. They then were able to calculate the daily exposure for erythema, plant growth, and molecular DNA. The authors estimated that loss of global ozone from 1979-1992 has led to a corresponding increase of incident UV radiation in the 300-340 nm wavelength range. This leads to long-term increases of erythema, DNA, and plant UV exposures as indicated in Figure 9.11. The figure shows the long-term trends in each of the daily exposure time series as a function of season and latitude. The largest exposure increases for all three occurred in the high latitudes and the smallest exposure increases for all three occurred in the tropics. The largest exposure increases in the midlatitudes were 4-12% per decade for plant and DNA exposure and somewhat less for erythema. Analyses of the annual mean trends of exposure, shown in Figure 9.12, indicate statistically significant trends in the middle and high latitudes of both hemispheres at the 2sigma level for both plant and DNA exposure.

An important conclusion from the Herman et al. [1996] study was that despite the fact that clouds can strongly affect surface UV, they had little effect in altering long-term UV trends. This is because there is no significant long-term trend in cloud cover. That is, there is no evidence that skies are cloudier now than they were in the early 1980s. Clouds therefore cause day to day variations in the surface UV levels, but do not contribute to long-term variability of UV. This result agrees with an earlier study by Lubin and Jensen [1995] that used version 6 TOMS data and five years of Earth Radiation Budget Experiment (ERBE) cloud reflectance data.