Ozone depletion

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Ozone depletion consists of two related events observed since the late 1970s: a steady lowering of about four percent in the total amount of ozone in Earth's atmosphere,^{[citation needed]} and a much larger springtime decrease in stratospheric ozone (the ozone layer) around Earth's polar regions.^[1] The latter phenomenon is referred to as the ozone hole. There are also springtime polar tropospheric ozone depletion events in addition to these stratospheric events.

The main causes of ozone depletion and the ozone hole are manufactured chemicals, especially manufactured halocarbon refrigerants, solvents, propellants, and foam-blowing agents (chlorofluorocarbons (CFCs), HCFCs, halons), referred to as ozone-depleting substances (ODS).^[2] These compounds are transported into the stratosphere by turbulent mixing after being emitted from the surface, mixing much faster than the molecules can settle.^[3] Once in the stratosphere, they release atoms from the halogen group through photodissociation, which catalyze the breakdown of ozone (O₃) into oxygen (O₂).^[4] Both types of ozone depletion were observed to increase as emissions of halocarbons increased.

Ozone depletion and the ozone hole have generated worldwide concern over increased cancer risks and other negative effects. The ozone layer prevents harmful wavelengths of ultraviolet (UVB) light from passing through the Earth's atmosphere. These wavelengths cause skin cancer, sunburn, permanent blindness, and cataracts,^[5] which were projected to increase dramatically as a result of thinning ozone, as well as harming plants and animals. These concerns led to the adoption of the Montreal Protocol in 1987, which bans the production of CFCs, halons, and other ozone-depleting chemicals.^[6] Over time, scientists have developed new refrigerants with lower global warming potential (GWP) to replace older ones. For example, in new automobiles, R-1234yf systems are now common, being chosen over refrigerants with much higher GWP such as R-134a and R-12.

The ban came into effect in 1989. Ozone levels stabilized by the mid-1990s and began to recover in the 2000s, as the shifting of the jet stream in the southern hemisphere towards the south pole has stopped and might even be reversing.^[7] Recovery was projected to continue over the next century, with the ozone hole expected to reach pre-1980 levels by around 2075.^[8] In 2019, NASA reported that the ozone hole was the smallest ever since it was first discovered in 1982.^[9][10] The UN now projects that under the current regulations the ozone layer will completely regenerate by 2045.^[11][12] The Montreal Protocol is considered the most successful international environmental agreement to date.^[13][14]