evidence: Changes in solar radiation and increases in stratospheric aerosols (small particles) from volcanic eruptions both affect the abundance of stratospheric ozone. Global total ozone in the early 1990s decreased by about 5% when compared to the 1964–1980 average, and is now about 2 to 3% below this value (see Q12). The longterm depletion of ozone is primarily attributed to increases in halogen source gases, with additional depletion in the early 1990s associated with the volcanic eruption of Mount Pinatubo. Equivalent effective stratospheric chlorine (EESC) (see definition in Q15) is often used as a measure of the potential of reactive and reservoir halogen gases to deplete ozone. Comparisons of the long-term changes in solar radiation, stratospheric volcanic aerosol, and EESC are useful in evaluating the contribution of these factors to long-term changes in total ozone. [...] The maximum UVI at Palmer Station is now larger by about a factor of 2.5 compared with the pre-ozone-hole period. The highest UVIs observed in spring now exceed those measured in spring and early summer in San Diego, despite San Diego’s much lower latitude. The large levels of UV radiation reaching the surface due to the Antarctic ozone hole have had an adverse effect on the microscopic plants and animals at the base of the food chain in the high-latitude marine environment.