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Ozone depletion refers to two related changes observed since the late 1970s: (1) a lowered total amount of ozone in Earth’s upper atmosphere (the stratosphere) and (2) a much larger springtime decrease in stratospheric ozone over polar regions, called the ozone hole. A key additional phenomenon is springtime polar tropospheric ozone depletion events. The main driver is manufactured halogenated chemicals—ozone-depleting substances (ODS) such as chlorofluorocarbons (CFCs), HCFCs, halons, and related compounds—which reach the stratosphere and release reactive chlorine and bromine atoms that catalyze ozone breakdown. The ozone layer is important because it blocks harmful ultraviolet radiation (especially UVB). When ozone thins, increased UVB can lead to higher risks of skin cancer, sunburn, cataracts, and other biological harms, as well as damage to plants and animals. Public concern and scientific evidence helped drive international action, most notably the Montreal Protocol (1987), which bans production of major ODS. Since then, ozone levels have stabilized and begun to recover, with the ozone hole expected to return toward pre-1980 levels later in the 21st century. Two related events highlighted in the topic are: (a) the general decline in total stratospheric ozone and (b) the formation of the Antarctic ozone hole (a pronounced springtime polar stratospheric ozone decrease). Both are linked to the same underlying chemistry and increasing emissions of halocarbons, and both have been tracked through satellite and ground-based observations.
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The ozone hole is the pronounced seasonal thinning of stratospheric ozone over Earth’s polar regions, especially Antarctica. It is one part of ozone depletion, which also includes the broader long-term reduction in total ozone throughout the upper atmosphere. The term does not describe a completely empty region but rather a substantial decrease in the total column of ozone, commonly measured in Dobson units. The most severe polar ozone loss occurs during the Antarctic spring, when ozone levels can be 40–70% lower than pre-ozone-hole values. Cold polar stratospheric clouds provide surfaces that convert chlorine and bromine from human-made ozone-depleting substances, such as CFCs and halons, into highly reactive forms. When sunlight returns in spring, these chemicals catalytically destroy ozone. The Arctic can also experience springtime depletion, although it is generally more variable and less severe than in Antarctica.
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