The ozone hole; over 30 years of satellite observations. 1979-2012. Ozone depletion describes two distinct but related phenomena observed since the late 1970s: a steady decline of about four per cent 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. The blue and purple colours are where there is the least ozone, and the greens, yellows, and reds are where there is more ozone.