Oxygen (a gaseous compound and a substantial component of the modern atmosphere) is composed of two oxygen atoms and is a waste product of photosynthetic processes, whereas ozone (which represents a miniscule amount of the atmosphere, but a substantial contribution to the blocking of UV light) is composed of three oxygen atoms. Ozone is generated from oxygen molecules in the presence of UV radiation (three molecules of oxygen break down and form two of ozone). Ozone then absorbs UV radiation and itself breaks down (two molecules of ozone reform into three of oxygen). This process occurs when the wavelength of the radiation is 200-300 nm (the most harmful to organisms), meaning the ozone layer protects living beings on the Earth’s surface from 95-99% of ultraviolet radiation by depleting it in this manner. Precambrian life is thought to have been tolerant to this level of radiation, although all other life forms were restricted to the oceans.
Aquatic photosynthetic organisms such as cyanobacteria began producing molecular oxygen from about 3,500 million years ago; until around 2,000 million years ago, however, this oxygen was expended in producing banded iron formations (alternating layers of oxidised iron and silica, evidence of the first photosynthetic life), meaning that none escaped into the atmosphere. Once a sufficient quantity had built up there, however, a layer of ozone began to form which protected the Earth’s surface from UV radiation. As oxygen accumulated in the atmosphere and the Earth was shielded from harmful levels of UV radiation, aerobic land-dwelling organisms came into being at the beginning of the Phanerozoic eon.
At present, the ozone layer is threatened by the enormous quantities of emissions being produced by industry. The destruction of ozone (as distinct from the greenhouse effect, referred to below) is perpetrated by gases that release chlorofluorocarbons (known as ‘radicals’), which were developed in the 1930s and subsequently became widely used as air conditioner and refrigerator coolants and aerosol can propellants, as well as miscellaneous other areas. The fact that they are non-flammable, non-toxic and relatively stable ensured their broad distribution.
Such was the seriousness of ozone degradation as a result of these emissions, though, that in 1987 the Montreal Protocol (signed by 150 nations, representing 95% of global CFC production). Said degradation has been markedly reduced by the introduction of hydrofluorocarbons (c. 1994) in air conditioning systems and ongoing efforts to create more environmentally friendly alternatives. This concern is very timely as although the ‘hole’ in the ozone layer is actually a thinning it is no less frightening, for it is currently the size of Antarctica.
It bears special mention that ‘ozone destruction’ is clearly distinct from the ‘greenhouse effect’, which concerns the trapping of heat by gases (from both natural and human sources) such as carbon dioxide, water vapour, methane and nitrous oxide. These gases are described as such because they trap heat in the atmosphere, simulating the conditions of a greenhouse on the Earth’s surface. The Kyoto Protocol, designed to curb this, has been less successful.