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The ozone hole is an annual thinning of the ozone layer over Antarctica, caused by stratospheric chlorine. Other more moderate thinnings have also been called "ozone holes", such as that over the North Pole during certain weather conditions.
The most pronounced decrease in ozone has been in the lower stratosphere. However, the ozone hole is most usually measured not in terms of ozone concentrations at these levels (which are typically of a few parts per million) but by reduction in the total column ozone, above a point on the earth's surface, which is normally expressed in Dobson units. Marked decreases in column ozone in the Antarctic spring and early summer compared to the early 1970s and before have been observed using instruments such as the Total Ozone Mapping Spectrometer (TOMS). 
Cause of the ozone hole
The cause of the ozone holes is generally agreed to be CFC (chlorofluorocarbon) compounds which break down due to ultraviolet light and become free radicals containing chlorine high in the Earth's atmosphere. These radicals then break down the ozone catalytically. Ozone destruction due to chlorine radicals from CFCs can take place in the gas phase, but occurs particularly rapidly on the surface of polar stratospheric clouds (PSC), which form over the poles (particularly the south pole) during winter.
The photochemical processes involved are complex but well understood, with UV radiation being involved in both the natural production and destruction of ozone, as well as the breakdown of CFCs into free radicals and the destruction of ozone by chlorine radicals. The role of sunlight in ozone depletion is the reason why the Antarctic ozone depletion is greatest during spring; during winter, even though PSCs are at their most abundant, there is no light over the pole to drive the chemical reactions.
CFCs are a byproduct of some chemical processes, and were also used in air conditioning/cooling units. They were also used as aerosol propellants. What makes CFCs so effective in breaking down ozone is that one CFC radical acts as a catalyst and can break down many ozone molecules. Furthermore, these radicals stay in the atmosphere for a very long time.
Scientists[who?] have increasingly been able to attribute the observed ozone depletion to the increase of anthropogenic halogen compounds from CFCs, by the use of complex chemical transport models and their validation against observational data (e.g. SLIMCAT). These models work by combining satellite measurements of chemical concentrations and meteorological fields with chemical reaction rate constants obtained in lab experiments, and are able to identify not only the key chemical reactions but also the transport processes which bring CFC photolysis products into contact with ozone.
In 2014, researchers reported their discovery of another "hole", an absence of atmospheric-cleansing hydroxyl throughout the entire depth of the troposphere across a large region of the tropical West Pacific. They suggest that this hole is permitting large quantities of ozone-degrading chemicals to reach the stratosphere, and that this may be significantly reinforcing ozone depletion in the polar regions with potential consequences for the climate of the Earth. 
Increased UV due to the ozone hole
Although ozone, O3, is a minority constituent in the earth's atmosphere, it is responsible for most of the main absorption of ultraviolet (UV) radiation in the atmosphere. Correspondingly, a significant decrease in atmospheric ozone could be expected to give rise to significantly increased levels of UV near the surface.
Increases in surface UV due to the ozone hole can be partially inferred by radiative transfer model calculations, but cannot be calculated from direct measurements because of the lack of reliable historical (pre-ozone-hole) surface UV data, although more recent surface UV observation measurement programmes exist (e.g. at Lauder, New Zealand ).
Because it is this same UV radiation that creates the ozone in the ozone layer from O2 (regular oxygen) in the first place, a reduction in stratospheric ozone would actually tend to increase photochemical production of ozone at lower levels (in the troposphere), although the overall observed trends in total column ozone are still a decrease, largely because ozone produced lower down has a naturally shorter photochemical lifetime, so it is destroyed before the concentrations could reach a level which would compensate for the ozone reduction higher up.
Biological effects of increased UV
The main public concern regarding the ozone hole has been the effects of surface UV on human health. As the ozone hole over Antarctica has in some instances grown so large as to reach southern parts of South America, Australia, and New Zealand, environmentalists[who?] have been concerned that the increase in surface UV could be significant.
UVB (the higher energy UV radiation absorbed by ozone) is generally accepted to be a contributory factor to malignant melanoma (skin cancer) – for example one study showed that a 10% increase in the UVB was associated with a 19% increase in melanomas for men and 16% for women (Fears et al., Cancer Res. 2002, 62(14):3992–6).
So far, ozone depletion in most locations has been typically a few percent. Were the high levels of depletion seen in the ozone hole ever to be common across the globe, the effects could be substantially more dramatic. For example, recent research  has analyzed a widespread extinction of plankton 2 million years ago that coincided with a nearby supernova. Researchers[who?] speculate that the extinction was caused by a significant weakening of the ozone layer at that time when the radiation from the supernova produced nitrogen oxides that catalyzed the destruction of ozone (plankton are particularly susceptible to effects of UV light, and are vitally important to marine food-webs).
Aside from the direct effect of ultraviolet radiation on organisms, increased surface UV leads to increased tropospheric ozone, as noted above. Paradoxically, at ground-level increased ozone is generally recognized to be a health risk, as ozone is toxic due to its strong oxidant properties. 
Public policy in response to the ozone hole
Environmentalists[who?] assert that the CFCs have caused so much damage to the ozone layer that the use of CFCs should be banned. The full extent of this damage CFCs have caused is not known and will not be known for decades; however marked decreases in column ozone have already been observed (see above).
In 1987, the Montreal Protocol was signed, controlling the emissions of CFCs. To some extent, their role has been replaced by the less damaging hydro-chloro-fluoro-carbons (HCFCs), although concerns remain regarding HCFCs as well.
Controversy regarding ozone science and policy
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Any counter-measures which have a negative economic impact will remain controversial due to the economic interests involved, with key questions regarding whether the scientific understanding is strong enough to warrant the proposed countermeasures. In this context it is worth noting that it is commonly believed that one reason for the relative ease of introduction of the Montreal protocol was the availability of CFC replacements at little extra cost.
The consensus amongst most atmospheric physicists[who?] and chemists[who?] is that the scientific understanding has now reached a level where countermeasures to control CFC emissions are justified, although the decision is ultimately one for policy-makers and society.
Despite this general consensus, the science behind ozone depletion remains complex, and some who oppose the enforcement of countermeasures point to some of the difficulties experienced in these studies. For example:
- Initial studies of the ozone hole were hampered with difficulties. Most notably, satellite measurements showing massive depletion of ozone around the south pole were initially rejected as unreasonable by data quality control algorithms; the ozone hole was only detected in satellite data when the raw data was reprocessed with modified processing algorithms following evidence of an ozone hole in in situ observations. This, however, was simply a problem with the data-processing algorithms for the satellite data and has long been corrected, and so has no bearing on the current situation.
- Predictions of ozone remains a difficult science. The World Meteorological Organization Global Ozone Research and Monitoring Project – Report No. 44, which on balance comes out strongly in favour of the Montreal protocol, nonetheless notes that projections of ozone loss for the 1994–1997 period made in the UNEP 1994 Assessment had been an overestimate.
- Although increased UVB has been shown to constitute a melanoma risk (see above), it has been difficult for statistical studies to establish a direct link between ozone depletion and increased rates of melanoma. Although melanomas did increase significantly during the period 1970–1990, it is difficult to separate reliably the effect of ozone depletion from the effect of changes in lifestyle factors (e.g. time spent outdoors).
One prominent opponent of CFC reduction strategy has been the atmospheric scientist Fred Singer, who has noted the scientific uncertainties such as the lack of direct observations of surface UV increases (as mentioned above). However, Singer goes far beyond this to claim, for example, that "CFCs with lifetimes of decades and longer become well-mixed in the atmosphere, percolate into the stratosphere, and there release chlorine" is controversial , when there is clear evidence for it (though Singer is wrong to use the word "percolate"). Singer, who is also a leading skeptic of strategies on global warming, has consistently insisted that the remaining level of scientific uncertainty about these issues is too high to justify taking the control measures recommended by most other atmospheric scientists, given their possible economic impact.
- The Skeptics vs. the Ozone Hole
- WMO/UNEP Scientific Assessment of Ozone Depletion: 1998
- The Ozone Tour at the Centre for Atmospheric Science, University of Cambridge
- The Ozone FAQ – a superb resource (but last updated in 1997)
- The Politics of Methyl Bromide
- Jon Shanklin's Ozone pages, including a pamphlet.
- The MACC stratospheric ozone service delivers maps, datasets and validation reports about the past and current state of the ozone layer.
- "What is Ozone?". By NASA. Retrieved March 17, 2013.
- "Frequently Asked Questions About The Ozone Hole". By Weather Underground. Retrieved March 17, 2013.
- Farman, J. C.; Gardiner, B. G.; Shanklin, J. D. (1985). "Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx interaction". Nature 315 (6016): 207. doi:10.1038/315207a0.
- "Recent Ozone Loss over Antarctica". Centre for Atmospheric Science. Retrieved December 27, 2013.
- "Ozone hole shrinks to record low (February 12, 2013)". By Stephanie Pappas of LiveScience. February 12, 2013. Retrieved March 1, 2013.
- "The Ozone Hole: It's Still Up There, Changing Oceans, Maybe Climate (February 6, 2013)". By Julia Whitty of Mother Jones. Retrieved March 1, 2013.
- "Ozone Hole At Record Low: 2012 Data Shows Smallest Loss In Last Decade (February 13, 2013)". By Ryan Grenoble of The Huffington Post. February 13, 2013. Retrieved March 1, 2013.
- "NOAA, NASA: Antarctic ozone hole second smallest in 20 years (October 24, 2012)". By National Oceanic and Atmospheric Administration (NOAA). Retrieved March 1, 2013.
- [“Like a giant elevator to the stratosphere”, News Release, Alfred Wegener Institute, April 3, 2014]
- "Remember the ozone hole? (January 22, 2013)". By Simeon Tegel of GlobalPost. Retrieved March 17, 2013.