LONG-TERM VARIATIONS IN ULTRAVIOLET SUNLIGHT REACHING THE BIOSPHERE - CALCULATIONS FOR THE PAST 3 DECADES

The long-term data base on atmospheric ozone combined with a set of radiative transfer calculations provides estimates of the variability in ultraviolet (UV) sunlight that should have 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-degrees-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 erythemal irradiance shows an upward trend of +2.1 +/- 1.2% per decade based on all ozone data at latitudes from 40 to 52-degrees-N. No trends exist in lower (30-39-degrees-N) and higher (53-64-degrees-N) 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.