Ozone depletion describes two distinct but related phenomena observed since the late 1970s: a steady decline of about 4% in the total amount of
ozone in
Earth's
stratosphere (the
ozone layer), and a much larger springtime decrease in stratospheric ozone around Earth's polar regions. The latter phenomenon is referred to as the
ozone hole. In addition to these well-known stratospheric phenomena, there are also springtime polar
tropospheric ozone depletion events.