Peace and Environment News* June 1995
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Peace and Environment News * June 1995 |
by the Canadian Global Change Program of the Royal Society of Canada
 The process by which catalysts (X) such as chlorine, bromine, nitrogen oxide and hydrogen destroy atmospheric ozone (O3). Adapted from H. Hengeveld, Understanding Atmospheric Change (Atmospheric Environment Service, Environment Canada, 1991). |
Ozone (O3) is a regular oxygen molecule with an extra oxygen atom attached to it. When scientists normally speak of the ozone layer, they are referring to a wide band of a greater concentration of O3 molecules that is found in the stratosphere (an area of the upper atmosphere), about 12 to 25 kilometres above the surface of the earth. The ozone layer acts to shield the surface of the earth from the harmful effects of ultraviolet rays emitted by the sun.
Scientific measurements of the upper atmosphere have revealed that about 3 percent of the Earth's protective ozone layer has been depleted over Canada, the United States and other temperate latitude countries in the past two decades. This depletion is expected to worsen in the 1990s, resulting in an additional 3 percent loss by the end of the century. Meanwhile, over Antarctica scientists have measured reductions in ozone as high as 50 to 60 percent, with virtually all of the ozone depleted at altitudes of 12 to 20 kilometres. First observed in 1985, this "hole" in the ozone layer covers an area as large as Canada. It appears during the Antarctic spring, and tends to persist for two to three months before disappearing. Direct human exposure to the ozone hole is rare in the uninhabited Antarctic, but the dispersion of the ozone hole in the late spring has resulted in reductions of up to 10 percent in ozone levels over southern Australia and New Zealand for a period of two to three weeks. To date, a similar ozone hole has not appeared over the northern pole, and the atmospheric conditions over the northern arctic make the development of a large hole unlikely. However, smaller reductions in ozone concentration have been observed in early spring over the arctic.
Ozone depletion is caused by two non-toxic, non-flammable, normally non-reactive groups of gases called chlorofluorocarbons (CFCs) and halons. When they were first introduced as a replacement for ammonia in early refrigerators, they were hailed as an inexpensive and safe miracle chemical. In addition to acting as a refrigerant, CFCs have been used as propellants in aerosol spray cans, as a cleaning solvent in the manufacture of electronic circuit boards and as a blowing agent in the manufacture of foam insulation and furniture foam.
CFCs react slowly with other chemicals and can persist in the atmosphere for as long as 150 years. The CFC or halon molecule makes its way to the upper atmosphere, where levels of ultraviolet (UV) light are naturally much higher. This UV light contributes to the separation of chlorine atoms from the CFC molecule and bromine from the halon. Once released, the chlorine or bromine molecule is capable of destroying ozone molecules in a continuous, repeating chemical reaction that allows a single chlorine or bromine atom to react with as many as 100,000 ozone molecules before finally settling out below the ozone layer.
Some related chemicals, methyl chloroform, carbon tetrachloride, methyl bromide and hydrobromofluorocarbons, have also been identified as contributors to ozone layer destruction as well as global warming. While most of these will no longer be used by the end of the decade, phaseout of methyl bromide, commonly used in fumigating soil and crops, will not likely be accomplished until early in the next century.
Chlorine and bromine emitted from human activities have been confirmed as the main cause of the loss of stratospheric ozone. As a result, international cooperation to stop the production of ozone-depleting substances has crystallized on a global scale in a manner not witnessed before around an environmental issue. In 1987, an international agreement called the Montreal Protocol on Substances that Deplete the Ozone Layer was signed by sixty-eight countries and as of the beginning of 1992 had been ratified by all but seven. The Protocol calls for the elimination of the most harmful ozone-depleting substances by the year 2000 in industrialized countries and by 2010 in developing countries. Canada has committed itself to an accelerated phaseout schedule that will eliminate the production and importation of major ozone-depleting substances by the end of 1995. Less damaging chemical substitutes for most CFCs have been developed by industry and are currently being tested to ensure that they do not pose other environmental or health threats. Once this testing is completed, they can be released onto the market. For the most part, these substitutes are more expensive than the original CFCs and will necessitate modifications to the technologies in which they are used.
What are the likely impacts on Canada and the world?
A 1 percent reduction in ozone concentration in the upper atmosphere results in roughly a 2 percent increase in the amount of harmful UV-B radiation that reaches the earth's surface. This will have direct impacts on the health of many organisms that inhabit our planet, including humans.
The above text was excerpted from the publication Global Change and Canadians, prepared by the Canadian Global Change Program and published by the Royal Society of Canada (1993). Copies of the publication as well as a Teacher's Guide are available for $5.00 from the Canadian Global Change Program (tel: 991-5639; fax: 991-6996; e-mail: <cgcp@rsc.ca> ).
Converted July 7, 2000 - Lg
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