Changes in Ultraviolet and Visible Solar Irradiance 1979 to 2008

Abstract

A description is presented of instruments and requirements for measuring ultraviolet (UV) and visible irradiance and estimating long-term changes in irradiance from the ground-based and satellite data The 30-year changes in zonal average UV irradiances are estimated from changes that have occurred in ozone amount and cloud cover as a function of latitude and season. Ozone changes have been obtained from a multiple satellite time series starting with Nimbus-7/Total Ozone Mapping Spectrometer (TOMS) in 1979 and continuing to the end of 2008 with the Solar Backscatter Ultraviolet (SBUV-2) series, Earth-Probe TOMS, and Ozone Monitoring Instrument (OMI). The changes in cloud cover have been obtained using the 340 nm reflectivity data from the same series of satellite instruments, except for Earth-Probe TOMS. The results show large increases in UV-B (280 nm–315 nm) irradiance in both hemispheres, mostly caused by changes in ozone amounts. The largest increases have occurred in the Southern Hemisphere for clear-sky conditions when compared to the same latitudes in the Northern Hemisphere. Since 1979, an increase of 5%–8% has occurred in clear-sky DNA damage action spectra weighted irradiance P DNA during most of the spring and summer, with increases ranging from 12% to 15% between 30°S and 40°S and 18% to 22% between 40°S and 50°S. Increases in erythemal irradiance are about half that of P DNA . There were only small changes in the equatorial zone (±23°), where sea level UV irradiances are largest because of naturally low ozone amounts and the nearly overhead sun.