Hovering some 10 to 16 kilometres above the planet’s surface, the ozone layer filters out dangerous ultraviolet (UV) radiation from the sun, thus protecting life on Earth. Scientists believe that the ozone layer was formed about 400 million years ago, essentially remaining undisturbed for most of that time. In 1974, two chemists from the University of California startled the world community with the discovery that emissions of man-made chlorofluorocarbons (CFCs), a widely used group of industrial chemicals, might be threatening the ozone layer.
The scientists, Sherwood Rowland and Mario Molina, postulated that when CFCs reach the stratosphere, UV radiation from the sun causes these chemically-stable substances to decompose, leading to the release of chlorine atoms. Once freed from their bonds, the chlorine atoms initiate a chain reaction that destroys substantial amounts of ozone in the stratosphere. The scientists estimated that a single chlorine atom could destroy as many as 100,000 ozone molecules.
The theory of ozone depletion was confirmed by many scientists over the years. In 1985 ground-based measurements by the British Antarctic Survey recorded massive ozone loss (commonly known as the “ozone hole”) over the Antarctic, providing further confirmation of the discovery. These results were later confirmed by satellite measurements.
The discovery of the “ozone hole” alarmed the general public and governments and paved the way for the adoption in 1987 of the treaty now known as the Montreal Protocol on Substances that Deplete the Ozone Layer. Thanks to the Protocol’s rapid progress in phasing out the most dangerous ozone-depleting substances, the ozone layer is expected to return to its pre-1980s state by 2060–75, more than 70 years after the international community agreed to take action. The Montreal Protocol has been cited as “perhaps the single most successful international environmental agreement to date” and an example of how the international community can successfully cooperate to solve seemingly intractable global environmental challenges.
|The extent of ozone depletion for any given period depends on complex interaction between chemical and climatic factors such as temperature and wind. The unusually high levels of depletion in 1988, 1993 and 2002 were due to early warming of the polar stratosphere caused by air disturbances originating in mid-latitudes, rather than by major changes in the amount of reactive chlorine and bromine in the Antarctic stratosphere.
The ozone layer over the Antarctic has been thinning steadily since the ozone loss predicted in the 1970s was first observed in 1985. The area of land below the ozone-depleted atmosphere increased steadily to encompass more than 20 million square kilometres in the early 1990s, and has varied between 20 and 29 million square kilometres since then. Despite progress achieved under the Montreal Protocol, the ozone “hole” over the Antarctic was larger than ever in September 2006. This was due to particularly cold temperatures in the stratosphere, but also to the chemical stability of ozone-depleting substances – it takes about 40 years for them to break down. While the problem is worst in the polar areas, particularly over the South Pole because of the extremely low atmospheric temperature and the presence of stratospheric clouds, the ozone layer is thinning all over the world outside of the tropics. During the Arctic spring the ozone layer over the North Pole has thinned by as much as 30 per cent. Depletion over Europe and other high latitudes has varied from 5 to 30 per cent.
|Stratospheric ozone, tropospheric ozone and the ozone “hole”
Ozone forms a layer in the stratosphere, thinnest in the tropics and denser towards the poles. The amount of ozone above a point on the earth’s surface is measured in Dobson units (DU) – it is typically ~260 DU near the tropics and higher elsewhere, though there are large seasonal fluctuations. Ozone is created when ultraviolet radiation (sunlight) strikes the stratosphere, dissociating (or “splitting”) oxygen molecules (O2) into atomic oxygen (O). The atomic oxygen quickly combines with oxygen molecules to form ozone (O3).
The ozone hole is defined as the surface of the Earth covered by the area in which the ozone concentration is less than 220-Dobson units (DU). The largest area observed in recent years covered 25 million square kilometres, which is nearly twice the area of the Antarctic. The lowest average values for the total amount of ozone inside the hole in late September dropped below 100 DU.
At ground level, ozone is a health hazard – it is a major constituent of photochemical smog. Motor vehicle exhaust and industrial emissions, gasoline vapors, and chemical solvents as well as natural sources emit NOx and volatile organic compounds (VOCs) that help form ozone. Ground-level ozone is the primary constituent of smog. Sunlight and hot weather cause ground-level ozone to form in harmful concentrations in the air.
|#1a. Knowing that ozone depletion will not return to pre-1980 levels until 2060 or 2070, what do scientists anticipate will be the impacts on human health?
#1b. Scientists have been conducting research in Antarctica for years. Have any studied the effects that the “ozone hole” has had/is having on the ecology of Antarctica?
#1c. Arctic warming is being described as attributable to climate change. To what extent is ozone depletion a contributing factor? What impacts do scientists working in the Arctic think that ozone depletion in the Arctic may be having on Arctic biodiversity? Or on residents of, e.g., Greenland?