Ozone Depletion

Ozone Depletion

Ozone depletion is a chemical thinning of the OZONE LAYER in the stratosphere or upper atmosphere. Scientists determined in the 1970s that it is caused by ozone depleting substances (ODSs), particularly by the use of chlorofluorocarbons (CFCs). The thinning of the ozone layer has caused an increase in the ultraviolet-B radiation (radiation with wavelengths 290-320 nanometres in length and are biological active) reaching Earth's surface. This enhanced UV-B radiation may cause biological effects including the reduction of crop yields, the slower growth of forests and reduced fishery yields. As well, the increased UV-B radiation likely causes increased skin CANCER rates and other health effects in humans. The CFCs have a very long lifetime (up to 100 years) in the atmosphere and so these effects are very long-lasting.

International Initiatives

In 1985, the United Nations adopted the Vienna Convention for the Protection of the Ozone Layer, which came into force in 1988 and has been signed by nearly 200 countries. The Montreal Protocol on Substances that Deplete the Ozone Layer was adopted in Montréal on 16 September 1987. Under this protocol, the signatory nations agreed to reduce CFC usage to 50% of the 1986 production levels by the year 2000. Revisions to this protocol at London, England (1990) and Copenhagen, Denmark (1992) tightened the schedule of the protocol such that the production of many CFCs ceased by 1 January 1996 in industrialized countries. Amendments in Montréal (1997) and Beijing, China (1999) enacted further restrictions.

Monitoring

Satellite, balloon and ground-based instruments are used to measure ozone and related gases. In the ground-based network, 80 Dobson instruments and 50 Brewer ozone spectrophotometers (a Canadian invention) are used around the world and report total ozone amounts in the atmosphere to the World Ozone and Ultraviolet Radiation Data Centre (WOUDC). Small balloons carrying ozone monitors are launched regularly around the world to provide information on the vertical distribution. In Canada, Environment Canada has a network of approximately 15 stations to measure total ozone amount and approximately eight locations where balloon launches are regularly carried out.

Satellite instruments have measured ozone from space since the launch of the Solar Backscatter Ultraviolet Radiometer (SBUV) instrument on Nimbus 4 in 1970. The Total Ozone Mapping Spectrometer (TOMS) instrument launched on Nimbus 7 in 1978 is notable both for its long life (1978-1993) and its role in the measurement of the Antarctic Ozone hole. In 2004 the Aura satellite was launched carrying the Ozone Monitoring Instrument (OMI). OMI measures ozone globally daily in daylight parts of the planet. The best estimate of the ozone over planet Earth is obtained by combining the wide coverage of the satellite instruments with the high accuracy of local measurements.

UV-B radiation is monitored at many sites across Canada. Information is regularly supplied to the public as the UV Index to enable individuals to protect themselves from excess exposure.

Ozone Holes

In 1985, British scientists reported a new phenomenon that has come to be known as the Antarctic ozone hole. This is a deep depletion of the ozone layer over a wide area of Antarctica in the spring (in the southern hemisphere this is October). Studies of satellite and other data records suggest that this phenomenon did not happen before the early 1980s. Depletions grew in magnitude and extent throughout the later years of the 20th century, but appear to have stabilized in the first decade of the 21st century, although there are large year-over-year variations.

Studies have been conducted in the North to determine whether a possible Arctic ozone hole exists. The analysis of satellite data and the ground-based monitoring network indicates that the ozone layer over the Arctic has declined by 5-10% but also with significant spring depletion events. Several significant depletion events were seen from 1990 to 2010 and the largest Arctic depletion occurred in 2011.

The Future

It is expected that the ozone layer reached a minimum between 2000 and 2010, and will slowly recover as nature breaks down existing CFCs in the atmosphere. Thus, the ozone is expected to remain lower than normal and UV-B radiation levels higher until the middle of the 21st century. However, changes in chemical emissions and the global climate may slow or advance that recovery. There are also links between ozone and CLIMATE CHANGE since ozone concentrations depend upon temperature in a complex manner and ozone, along with many ODSs and their replacements, are greenhouse gases.