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Case study: Ozone layer depletion and the Montréal Protocol

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The ozone layer is an important part of the global atmosphere and climate system. It limits the amount of ultraviolet (UV) radiation from the sun to levels necessary for life on Earth. A depleted ozone layer may likely cause serious consequences including higher rates of sunburn, skin cancer, eye damage and other diseases, as well as reducing plant growth.

Manufactured chemical compounds are the main cause of ozone layer depletion. These are compounds such as chlorofluorocarbons (CFCs) and halons, among others. In the past, these compounds were commonly used in refrigerators, air conditionners and fire-retardant chemicals. In general, when atmospheric ozone falls 1%, it is equivalent to an increase of 1%–2% in UV radiation at ground level.

State of the ozone layer

Since 1979, stratospheric ozone has decreased over the entire globe—between 4% and 6% per decade in mid-latitudes and between 10% and 12% per decade in higher southern latitudes. The levels dropped to record lows following the June 1991 volcanic eruption of Mount Pinatubo in the Philippines. However, the effects of this natural disaster have diminished, and levels have returned to values closer to the long-term downward trend.

Potential effects of ozone depletion

Stratospheric ozone depletion leads to increases in UV radiation reaching the earth's surface. High levels of UV radiation are known to slow plant growth. They may also lead to skin cancers, cataracts and immunosuppressive diseases in humans and other animals. At mid-latitudes—for example, where Toronto is located—under clear skies, a 1% decrease in the thickness of the stratospheric ozone layer results in about a 1.1% to 1.4% increase in UV-B at ground level. This varies according to the season. In Canada, about 200 species of crops and trees are, to some degree, sensitive to increased levels of UV-B.


As one of the original parties to the 1987 Montréal Protocol, Canada has taken a leadership role both in understanding the science behind ozone depletion and in acting to eliminate its causes. The production of ozone-depleting substances (ODS) in Canada has dropped from a high of 27.8 kilotonnes in 1987 to 1.0 kilotonne in 1996. At the global level, 1995 production of CFCs was 77% lower than its peak in 1988. Canada accounted for less than 1% of global production.

Despite this progress, there are still concerns. First, scientists cannot be certain that, even with current elimination targets of ODSs, the ozone layer will return to its previous thickness. The concentration of known ODSs in the stratosphere is declining, although there may be other substances that are contributing to ozone depletion. Second, developing countries now represent the biggest threat to the recovery of the ozone layer, as their production and use of CFCs has grown in recent years. Third, there is a 'loss of momentum' in developed countries, including Canada, because of the perception that the problem has been resolved. The 1997 report of the Auditor General of Canada indicates that current stocks of ODSs, those in existing equipment and quantities stored for future use, are at risk of being released to the atmosphere. This may happen unless more stringent inspections and safeguards of those stocks improve the recovery of the ozone layer by more than 10%.

The problem of ozone layer depletion became prominent in the 1980s. Scientific measurements began to show significant global decreases in ozone. Some of the general results follow.

  • For mid-latitudes, Europe and North America, annual ozone losses of 2% to 4% over the 1980s were reported.
  • For Australia, ozone losses during the 1980s ranged from 0.5% to 5%.
  • For Antarctica, the ozone hole has become a regular feature of the southern hemisphere with total ozone losses of 60% to 70% reported each spring since 1985.

The seriousness of the problem has led to global agreement to reduce and control the production of ODSs. In 1987, 149 countries gathered in Montréal and signed an agreement to reduce the use of ODSs. The decisions taken were the following:

  • Freeze consumption of CFC-11 at 1986 levels by 1989.
  • Reduce consumption of CFC-12 by 20% by 1 July 1993.
  • Make an effort to meet Montréal targets as seen below.
Table 1: Ozone depleting substances
  Ozone depletion potential1 Canada's phase-out date Lifetime in atmosphere
Halons 3.0 to 10.0 Jan. 1, 1994 up to 65 years
Carbon tetrachloride 1.1 Jan. 1, 1995 up to 42 years
CFCs 0.6 to 1.0 Jan. 1, 1996 from 50 to 1,700 years
Methyl chloroform 0.1 Jan. 1, 1996 6 years
Methyl bromide 0.6 Jan. 1, 20052 up to 2 years
HCFCs3 0.001 to 0.52 Jan. 1, 2020 up to 19 years