Ozone depletion

Image of the largest Antarctic ozone hole ever recorded (September 2006).

Ozone is a gas that is present in the ozone layer in the stratosphere of the Earth. There, it absorbs almost all ultraviolet light which is harmful to many organisms. The term Ozone depletion can refer to a number of distinct, but related phenomena:

Since the 1970s, there has been a decrease in total volume of the Ozone in the ozone layer, of about 4%, per decade.
There are seasonal variations in the ozone layers; in springtime, there is less ozone in the ozone layer over the polar regions, esp. the south pole. This is generally known as ozone hole.
In addition to these well-known stratospheric phenomena, there are also springtime polar tropospheric ozone depletion events.

The details of polar ozone hole formation differ from that of mid-latitude thinning, but the most important process in both is catalytic destruction of ozone by atomic halogens.^[1] The main source of these halogen atoms in the stratosphere is photodissociation of man-made halocarbon refrigerants (CFCs, freons, halons.) These compounds are transported into the stratosphere after being emitted at the surface.^[2] Both types of ozone depletion were observed to increase as emissions of Halocarbons increased.

CFCs and other substances contributing to this effect are referred to as ozone-depleting substances (ODS). Since the ozone layer prevents most harmful UVB wavelengths (280–315 nm) of ultraviolet light (UV light) from passing through the Earth's atmosphere, observed and projected decreases in ozone have generated worldwide concern. As a result, the Montreal Protocol has been adopted. The treaty bans the production of CFCs, halons as and other ozone-depleting chemicals such as carbon tetrachloride and trichloroethane. The biological consequences of this change in ozone levels may include an increases in skin cancer, cataracts,^[3] damage to plants, and the reduction of plankton populations in the ocean.

Estimates of how quickly the ozone layer will regenerate vary by about two decades. NASA estimated in 2006, that it will take to 2068 for the ozone layer in Antarctica to recover to the level before human-induced thinning.^[4] According to the World Meteorological Organization, the state of the ozone layer over Antarctica will not change noticeably in the next twenty years. The state before 1980 will be reached between 2060 and 2075, 25 years later than predicted in 2002. Over the Arctic, there will be bigger losses in ozone over the next fifteen years, esp. in cold winters. For the Arctic regions, the state before 1980 will be reached around 2050.^[5]

Ozone Depletion Media

SVG form of Nuvola apps arts.png:
Diagram illustrating the ozone-oxygen cycle NOTE: the text in the final version can't be modified but earlier versions can be if you wish to translate them.
Levels of ozone at various altitudes (DU/km) and blocking of different bands of ultraviolet radiation: In essence, all UVC is blocked by diatomic oxygen (100–200 nm) or by ozone (triatomic oxygen) (200–280 nm) in the atmosphere. The ozone layer then blocks most UVB.
Global monthly average total ozone amount
Lowest value of ozone measured by TOMS each year in the ozone hole
Ozone hole in North America during 1984 (abnormally warm, reducing ozone depletion) and 1997 (abnormally cold, resulting in increased seasonal depletion). Source: NASA
NASA projections of stratospheric ozone concentrations if chlorofluorocarbons had not been banned
Ozone levels stabilized in the 1990s following the Montreal Protocol, and have started to recover. They are projected to reach pre-1980 levels before 2075.
Ozone-depleting gas trends