There has been increased interest in understanding the variability and trends in ultraviolet (UV) radiation since it was realized that the world's protective ozone layer was at risk from a build-up of anthropogenic trace gases in the atmosphere. Good progress has been made through improvements in instrumentation, calibration procedures, and data quality assurance. The widespread adoption of a standardized metric for reporting UV radiation risk—namely erythemally weighted UV (UVEry1, in W m-2), or the UV Index (WHO, 2002) (UVI = 40 x UVEry), has also facilitated meaningful comparisons.

Assessments of our understanding of UV radiation and its effects on the environment are updated regularly. The most recent of these assessments predicts that although the ozone layer will gradually recover over the next few decades, the outlook for future UV is less certain (UNEP, 2006; 2007; WMO, 2007). Despite the progress in instrumentation, any changes in UVEry attributable to ozone depletion have been difficult to detect, because of: (1) uncertainties in UV measurement, (2) a relatively low sensitivity of UVEry to changes in ozone, and (3) the effects of other changes in atmospheric composition (e.g., changes in aerosols and clouds).

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