The ozone shield over the United States and Great Britain has thinned slightly during the last four years. Although nuclear explosions may not destroy ozone to the extent that has previously been thought, volcanic eruptions and solar flares may have some effect on ozone amount. These are among the conclusions of a National Oceanic and Atmospheric Administration study of ozone records from stations around the world.
Instruments at these stations look upward through the atmosphere and derive the total ozone in the entire vertical column. The study used ozone data for the period 1970–1974 to delineate recent trends, and longterm data to compare variations in the shield with the occurrence of nuclear explosions, volcanic eruptions, and solar events, all of which have been suspected as possible causes of ozone depletion.