ozone layer

The destruction of the ozone layer by chlorofluorocarbons (CFCs). CFCs discharged into the atmosphere break down in sunlight releasing chlorine, which breaks down the ozone to form chlorine monoxide and a free oxygen atom. These products react together to form oxygen and chlorine, leaving the chlorine to break down another ozone molecule, and so on.

Thin layer of the gas ozone in the upper atmosphere which shields the Earth from harmful ultraviolet rays. A continent-sized hole has formed over Antarctica as a result of damage to the ozone layer. This has been caused in part by chlorofluorocarbons (CFCs), but many reactions destroy ozone in the stratosphere: nitric oxide, chlorine, and bromine atoms are implicated.

It is believed that the ozone layer is depleting at a rate of about 5% every ten years over northern Europe, with depletion extending south to the Mediterranean and southern USA. However, ozone depletion over the polar regions is the most dramatic manifestation of a general global effect. Ozone levels over the Arctic in spring 1997 fell over 10% since 1987, despite the reduction in the concentration of CFCs and other industrial compounds which destroy the ozone when exposed to sunlight. It is thought that this may be because of an expanding vortex of cold air forming in the lower stratosphere above the Arctic, leading to increased ozone loss. It is expected that an Arctic hole as large as that over Antarctica could remain a threat to the northern hemisphere for several decades.

The size of the hole in the ozone layer in October 1998 was three times the size of the USA, larger than it had ever been before. In autumn 2000, the hole in the ozone layer was at its largest ever. Observers had hoped that its 1998 level was due to El Niño and would not be exceeded.

In April 1991, satellite data from the US space agency NASA revealed that the ozone layer had depleted by 4-8% in the northern hemisphere and by 6-10% in the southern hemisphere between 1978 and 1990. Between 1989 and 1996 the ozone layer over the Arctic diminished by around 40%. The ozone layer above the USA was reduced by 12.6% in 1993; that above the UK was reduced by almost 50% in 1996. By September 1996, levels in the upper atmosphere had reached a record low.

In 1987, an international agreement called the Montréal Protocol was established. The protocol was one of the first global environmental treaties and banned the use of chemicals responsible for ozone damage, such as chlorofluorocarbons (CFCs) in aerosols and refrigerants (see ozone depleter). Data collected since then by scientists using instruments in satellite and ground-based devices showed that some degree of success has been established in slowing down the rate of ozone depletion in the upper atmosphere. Since 1994, ozone depletion rates have fallen from 8% to 4% per annum. However, even at current rates, the ozone layer will not be healed until 2050.