Changes in ultraviolet-radiation reaching the earths surface

The quality and quantity of UV measurements have increased greatly in the last few years. Variations among measurements from different instruments are diminishing toward the 5% level. Long-term trend detection is still a problem, with little historical data available for baseline estimations. Enhanced UV levels are clearly associated with the Antarctic springtime ozone reductions. Measurements show that maximum UV levels at the South Pole are reached well before the summer solstice, and DNA-damaging radiation at Palmer Station, Antarctica (64 degrees S) during the springtime ozone depletion can exceed maximum summer values at San Diego, USA (32 degrees N). UV increases at mid-latitudes are smaller. However, increases associated with the record low ozone column of 1992/93 in the Northern Hemisphere are evident when examined on a wavelength-specific basis. Measurements in Argentina, Chile, New Zealand, and Australia show relatively high UV levels compared to corresponding Northern Hemispheric latitudes, with differences in both stratospheric ozone and tropospheric pollutants likely to be playing a role. Tropospheric ozone and aerosols can reduce global UV-B irradiances appreciably. At some locations, tropospheric pollution has increased since pre-industrial times, leading to decreases in surface UV radiation. However, recent trends in tropospheric pollution probably had only minor effects on UV trends relative to the effect of stratospheric ozone reductions. Global ozone measurements from satellites over the period 1979-1993 imply significant UV-B increases at high and mid-latitudes of both hemispheres, but only small changes in the tropics. Such estimates however assume that cloud cover and tropospheric pollution have remained constant over this time period. Under the current CFC phase-out schedules, global UV levels are predicted to peak around the turn of the century in association with peak loading of chlorine in the stratosphere and the concomitant ozone reductions. The recovery to pre-ozone depletion levels is expected to take place gradually over the next 50 years. (Less)