International Ozone Protection Day, or World Ozone Day, is marked every year on 16 September – the date the Montreal Protocol on Substances that Deplete the Ozone Layer (the Montreal Protocol) was signed in 1987. This HillNote provides an overview of the progress made through the Montreal Protocol to reverse damage to the ozone layer caused by human-made synthetic substances. The Montreal Protocol is often cited as an example of the progress that can be made on environmental issues through international cooperation.
The Ozone Layer
The ozone layer is the Earth’s “sunscreen.” Found in the stratosphere some 10 to 50 kilometres above the Earth’s surface, the ozone layer, as its name suggests, is made of ozone, a gas that contains three oxygen atoms. When exposed to sunlight, ozone molecules form and break down in a continuous cycle. In an unpolluted atmosphere, the amount of ozone produced is usually comparable to the amount being destroyed, leaving the total concentration relatively stable.
Ozone concentrations in the atmosphere vary naturally by season and latitude. Under normal circumstances, the ozone layer is thicker over the poles and thinner around the equator. Over Canada, the ozone layer is normally thicker in winter and early spring.
The sun produces three types of ultraviolet (UV) radiation that are more energetic than visible light. UVC rays are the most energetic and have the shortest wavelength, UVA rays are the least energetic and have the longest wavelength, and UVB rays fall in between the two. The ozone layer absorbs all UVC rays and most UVB rays, but it does not absorb UVA rays (see Figure 1).
Figure 1 – Absorption of ultraviolet radiation by the ozone layer
Source: Figure prepared by the Library of Parliament using data from Environment Canada, “Chapter 6 – Ultraviolet radiation,” Skywatchers guide to weather, p. 6-1.
The ozone layer protects the Earth’s surface from the damaging effects of UV radiation, including:
- cataracts, leading to cloudy vision or blindness;
- skin cancers; and
- damage to natural materials, like wood, and to synthetic materials, like plastic.
Damage to the ozone layer
Ozone-depleting substances (ODSs) are human-made and contain bromine or chlorine atoms, or both. Once an ODS reaches the stratosphere, it absorbs UV radiation, eventually causing it to decompose and release chlorine or bromine atoms, or both. These released atoms repeatedly react with ozone molecules and destroy them. When ODS concentrations are high, the ozone layer is destroyed faster than natural processes can create it.
Scientific discoveries in the 1970s and 1980s first identified the negative impacts of one type of ODS, chlorofluorocarbons (CFCs), on the ozone layer. In the 1980s, scientists reported that a recurring severe depletion of the ozone layer above Antarctica was caused by CFCs. This depletion has often been referred to as a “hole” in the ozone layer, but is more accurately characterized as a thin patch.
At the time, CFCs were widely used in refrigerators, air conditioners, blown insulation foams and aerosol cans, including hairspray, antiperspirants and cleaning supplies. Other ODSs were used for fire protection and to fumigate fruits and vegetables for pest control.
A depleted ozone layer means more harmful UV radiation is reaching the Earth’s surface. Increased exposure to UVB radiation increases everyone’s risk for skin cancer, but more specifically, people with fair skin living in sunny environments are at highest risk.
The Montreal Protocol
In response to these risks, the global community took action to control the use of ODSs and stop ozone depletion. Nations that were party to the Vienna Convention for the Protection of the Ozone Layer (the Vienna Convention) which took effect in 1988, agreed to exchange information about the effects of human activities on the ozone layer and committed to taking concrete action against activities likely to deplete it.
The Montreal Protocol, a supplementary agreement to the Vienna Convention, lays out the concrete actions to be taken to protect the ozone layer. Signed in 1987, it came into force internationally on 1 January 1989 and in Canada on 1 April 1989. In 2009, the Vienna Convention and the Montreal Protocol became the first United Nations treaties to be ratified by every member country at the time.
The Montreal Protocol regulates the production and use of over 100 ODSs. It calls on parties to gradually phase out the production and use of the listed ODSs over different time periods for each group of ODSs. For developed countries, completely eliminating the production and consumption of most groups of ODSs was scheduled for the 1990s, while reducing the consumption and production of one group of ODSs – hydrochlorofluorocarbons – continued until 2020.
Under article 5 of the Montreal Protocol, developing countries with low levels of ODS consumption are entitled to follow different timelines. These countries are allowed a grace period of up to 10 years to comply, and they receive financial and technical support through a multilateral fund set up under article 10 of the same protocol.
The Kigali Amendment to the Montreal Protocol, which came into force in 2019, calls for reducing the use of hydrofluorocarbons (HFCs). HFCs were introduced as cost-efficient alternatives to ODSs. While HFCs do not deplete the ozone layer, they are powerful greenhouse gases that contribute to climate change.
Parties to the Montreal Protocol submit annual reports on their production and consumption of ODSs and HFCs. They also meet once a year to decide how to continue implementing the protocol.
Canada implements the Montreal Protocol through the Canadian Environmental Protection Act, 1999 and associated regulations. Federal, provincial and territorial legislation control various elements of the ODS and HFC life cycles.
Recovery of the Ozone Layer
Compared to 1990 levels, the countries party to the Montreal Protocol have phased out 98% of ODSs. The ozone layer is also on the path to recovery. In 2024, the thin patch in the ozone layer detected over the Antarctic was smaller than in the early 2000s. The success of the Montreal Protocol shows the progress that can be achieved through international cooperation.
The ODSs already in the atmosphere will continue to degrade the ozone layer over the coming decades. However, if the controls set out in the Montreal Protocol are followed, the ozone layer should recover and return to its 1980 values by 2066 over the Antarctic, 2045 over the Arctic and 2040 over the rest of the world.
According to projections, without the Montreal Protocol, the continued emissions of ODSs would have caused much more ozone depletion. Higher UV radiation would have led to millions more cases of cataracts and skin cancers, damaged aquatic food chains and reduced crop production. Increased UV radiation would also have led to a more rapid deterioration of natural and synthetic building materials, increasing construction costs and reducing reliability. The Montreal Protocol and the Kigali Amendment also contribute to lowering greenhouse gas emissions, since HFCs and most ODSs are also greenhouse gases.
Further Reading
Government of Canada. Ozone layer. Series of informational websites on the ozone layer and related regulations.
World Meteorological Organization. Scientific Assessment of Ozone Depletion 2022, Ozone Research and Monitoring, GAW Report No. 278.
United Nations Environment Programme. Environmental Effects Assessment Panel – Update Assessment 2024: Environmental consequences of interacting effects of changes in stratospheric ozone, ultraviolet radiation and climate, Ozone Secretariat.
By Natacha Kramski, Library of Parliament
Categories: Agriculture, environment, fisheries and natural resources, Science and technology