The biologically effective ultraviolet irradiance at the earth's surface varies with the elevation of the sun, the atmospheric ozone amount, and with the abundance of scatterers and absorbers of natural and anthropogenic origin. Taken alone, the reported decrease in column ozone over the Northern Hemisphere between 1969 and 1986 implies an increase in erythemal irradiance at the ground of four percent or less during summer. However, an increase in tropospheric absorption, arising from polluting gases or particulates over localized areas, could more than offset the predicted enhancement in radiation. Any such extra absorption is likely to be highly regional in nature and does not imply that a decrease in erythemal radiation has occurred on a global basis. The Antarctic ‘ozone hole’ represents a special case in which a portion of the earth has experienced ultraviolet radiation levels during spring that are far in excess of those which prevailed prior to the present decade.

The long-term data base on atmospheric ozone combined with a set of radiative transfer calcu!ations provides estimates of the variability in ultraviolet (UV) sunlight that should ha\ e occurred over the period 1957-1988 under clear, pollution-free skies. Results refer to the earth's surface at specific locations in the Northern Hemisphere and to averages over collections of sites located in three latitude bands from 30 to 64"N. For any one year the annually integrated solar irradiance, weighted by the action spectrum for erythema, typically lies within 3-4% of the 32-year mean. No statistically significant trends span the entire 32-year time frame. However, over the shorter time period 1970-1988 the annually integrated erythema1 irradiance shows an upward trend of +2.1 2 1.2% per decade based on all ozone data at latitudes from 40 to 52"N. No trends exist in lower (30-39"N) and higher (53-64'") latitude bands. We caution that a trend line provides a very simple index of the variability in UV sunlight, and these results should not necessarily be extrapolated into the future.J. E. FREDERICK et al.