Temps de lecture :4 minutes
Ozone is a gas naturally present in the atmosphere, but in very low quantities. It is colorless and has a very strong odor that gave it its name (from Greek ozein, odor). About 90% of the atmospheric ozone is located in the stratosphere (region between an altitude of about 10 and 50 km) and comprises what one calls the ozone layer. It absorbs a large part of ultraviolent rays which are very dangerous in heavy doses, and thus protects life on Earth. (1)
The protective ozone layer must not be confused with the ozone present at low altitude (in the troposphere) and linked to urban pollution. It is only loosely linked to stratospheric ozone. Gases due to pollution can not plug up the hole in the ozone layer.
The hole in the ozone layer
The destruction of the ozone layer was observed at the beginning of the 1980s, when satellite imaging showed a decrease of almost 50% of the presence of this gas in the Antarctic. (2) What has since been called the hole in the ozone layer has continued to grow. In October 2007, it was spread over 24 million km2, or a surface area equivalent to North America’s. (3)
A similar phenomenon has been observed in the North Pole since 1994, in smaller proportions. At medium altitudes, the loss of ozone is less significant: on the order of 5 or 6%. (4)
The hole of the ozone layer is caused by chemical reactions involving a whole group of gases, called halogens. Specifically, the chloro-fluoro-carbons (CFCs) and hydro-chloro-fluoro-carbons (HCFCs). These molecules come from human activity. They have been used since the 1960s as refrigerants, industrial solvants, propellants for aerosols or for manufacturing expanded polystyrene.
One CFC molecule can destroy 10,000 ozone molecules, in some cases, millions. The phenomenon is more particularly noticeable at the Poles in springtime (in October in the Southern Hemisphere) because this meteorological phenomena— the formation of polar stratospheric clouds— promotes a destructive chemical reaction.
The Montreal Protocol
The ozone story is the most successful example of an international response to an ecological problem. (DEBAT) In 1987, barely 2 years after the publication of studies on the hole in the ozone, 46 countries, including the main producers of CFCs, agreed to reduce their consumption and their production of the gases most destructive to the ozone layer. They adopted the Montreal Protocol, which is today signed by more than 190 countries. The progressive stop to the production and consumption of less destructive gases should continue until 2030.
The reason for such a success is without a doubt that substitute products were already present at the time and that the modification of industrial practices could have been executed at a small cost. The risk -maintaining life on Earth -and the absence of scientific controversy did facilitate a decision to be made.
A slow-acting effect
Despite the application of the protocol, the hole in the ozone layer is continuing to grow. It is only a paradox in appearance. Because it is due to the long life span of halogen gases that can stay in the atmosphere for about a century. There is therefore a gap between the moment when the gas production stops and when its concentration in the atmosphere decreases. And therefore, likewise when the ozone layer’s hole stops increasing in size.
In 2005, the total combined concentrations of anthropic gases (from humans) ozone destructors present in the atmosphere were less than 8 to 9% compared to the levels in 1992-1994, and even from 90% for methyl-chloroform which has a short lifespan. But specialists do not expect the ozone layer to revert back to its normal level before 2050.
On the other hand, as total prohibition nears, a production and illegal trade of gases that are destructive to the ozone layer is developing. [Debate]
Impact on human health
As long as the state of the ozone layer has not been completely restored, ultraviolent rays that reach Earth represent an increasing danger. There are three types: UVA, UVB, and UVC. The ozone layer absorbs all UVCs and 95% of UVBs, most UVAs not being filtered. In humans, moderate exposure to UVs is important for the synthesis of vitamin D, but overexposure is harmful to the health: sunburns, skin cancer, cataracts, immune system diseases, etc. The decrease in ozone layer reduces the protection against ultraviolet rays, namely against UVBs. The OMS estimates that a decrease of 10% of the concentration of stratospheric ozone causes 300,000 skin cancers, 4,500 melanomas, and between 1.60 and 1.75 million more cases of cataracts in the world. (5)
The countries closest to the « hole » are the most affected: Australia and southern Argentina or Chile.
Impact on other living organisms
In 2007, a study directed by the United Nations established the consequences of UV exposure on different organisms. Those located in Polar Regions and in lake regions, where the ozone layer is very thin, are the most vulnerable. Phytoplankton, essential element of the food chain, would also be affected, as well as the development of urchin embryos and several fish species. They would also be at the source of the malformation and mortality of amphibians.
Ozone and climate change
It is a common mistake to associate global warming with the hole in the ozone layer. The two phenomena involve different gases functioning largely independently: carbon dioxide in the first case and ozone in the second one.
There are however some indirect relationships. Some ozone gas destroyers are also Greenhouse gases: they contribute to warming the atmosphere. Their elimination should therefore improve the situation. On the other hand, if UVs harm the phytoplankton, it could become less plentiful. Meaning, it plays an important role in the carbon fixation by the oceans. Less ozone would mean thus more warming.