Climate of extremes - Antarctica: Global Science from a Frozen Continent - page 131

Figure 4.25 Map of ozone concentration showing low ozone

values (red) over the Antarctic in September 2011. The
dated circles show the earlier extent of the ozone hole in
1981 and 1991. (Credit: NOAA)

Figure 4.26 Photo of polar stratospheric clouds (also known as nacreous clouds) over McMurdo

Sound. (Credit: John Cassano)

At the end of winter, when sunlight returns, the molecular chlorine is broken
down by the energy provided in ultraviolet radiation to form atomic chlorine. This
atomic chlorine then rapidly depletes ozone leading to the formation of the ozone
hole. The minimum amount of ozone observed over the Antarctic had decreased
by a factor of two from the early 1980s until the early 2000s, but there is hope
that this trend will reverse.
Upon realising that CFCs were responsible for depleting stratospheric ozone an
international agreement, known as the Montreal Protocol, was signed in 1987 to