THE OZONE HOLE
In 1985 it was discovered that there was a much lower concentration of ozone over Antartica during spring time. During winter, very low temperatures over the South Pole cause the formation of polar stratospheric clouds (PSCs) within a vortex of swirling cold air. The formation of PSCs involves the production of chlorine which in the spring, when sunlight is available, is used to destroy ozone, causing a temporary drop of about 50% of ozone concentration.
An ozone hole only occurs above Antartica because the bulk of land mass at the northern hemisphere disrupts the meteorology there, causing higher temperatures and weaker PSC formation.