Ozone is present only in small amounts in the atmosphere. Nevertheless, ozone is vital to human well-being as well as agricultural and ecosystem sustainability. Most of Earth’s ozone resides in the stratosphere, the layer of the atmosphere that is more than 10 kilometers (6 miles) above the surface. About 90% of atmospheric ozone is contained in the stratospheric “ozone layer”, which shields Earth’s surface from harmful ultraviolet radiation emitted by the Sun.
In the mid-1970s scientists discovered that some human-produced chemicals could lead to depletion of the stratospheric ozone layer. The resulting increase in ultraviolet radiation at Earth’s surface would increase incidents of skin cancer and eye cataracts, suppress the immune systems of humans, and also adversely affect agriculture as well as terrestrial and oceanic ecosystems.
Following the discovery of this environmental issue, researchers sought a better understanding of this threat to the ozone layer. Monitoring stations showed that the abundances of gases that are ozone-depleting substances (ODSs)1, such as chlorofluorocarbons (CFCs), were steadily increasing in the atmosphere. These trends were linked to growing production and use of CFCs and other ODSs for spray can propellants, refrigeration and air conditioning, foam blowing, industrial cleaning, and other applications. Measurements in the laboratory and in the atmosphere characterized the chemical reactions that were involved in ozone destruction. Computer models of the atmosphere employing this information were used to simulate how much ozone depletion was already occurring and to predict how much more might occur in the future.
By the mid-1980s observations of the ozone layer showed that depletion was indeed occurring. The most severe ozone loss, unexpected at the time of discovery, was found to be recurring each springtime over Antarctica. The loss in this region is commonly called the “ozone hole” because the ozone depletion is so large and localized. A thinning of the ozone layer also has been observed over other regions of the globe, such as the Arctic and northern and southern midlatitudes.
The work of many scientists throughout the world has built a broad and solid scientific understanding of the ozone-depletion process. With this foundation, we know that ozone depletion has been occurring and we understand why. Most importantly, we know that if the most potent ODSs were to continue to be emitted and increase in the atmosphere, the result would be ever greater depletion of the ozone layer.
In 1985, the world’s governments adopted the Vienna Convention for the Protection of the Ozone Layer in response to the prospect of increasing ozone depletion. The Vienna Convention provided a framework through which nations agreed to take appropriate measures to protect human health and the environment from activities that harm the ozone layer, including cooperation on systematic observations, research and exchange of information. In 1987, this framework led to the Montreal Protocol on Substances that Deplete the Ozone Layer (the Montreal Protocol), an international treaty designed to control the production and consumption of CFCs and other ODSs. As a result of the broad compliance with the Montreal Protocol and subsequent amendments and adjustments as well as industry’s development and deployment of “ozone-friendly” substitutes to replace CFCs, the total global accumulation of ODSs in the atmosphere has begun to decrease.
The replacement of CFCs has occurred in two phases: first via the use of hydrochlorofluorocarbons (HCFCs) that cause considerably less damage to the ozone layer compared to CFCs, and second by the introduction of hydrofluorocarbons (HFCs) that do not deplete ozone. In response, global ozone depletion has stabilized, and initial signs of recovery of the ozone layer are being observed. With continued compliance, substantial recovery of the ozone layer is expected by the middle of the 21st century. The day the Montreal Protocol was agreed upon, 16 September, is now celebrated as the International Day for the Preservation of the Ozone Layer. The Montreal Protocol has also decreased the human drivers of global warming, because many CFCs and HFCs are potent greenhouse gases (GHGs).
The amendment and adjustment process is a vitally important aspect of the Montreal Protocol, allowing the protocol to evolve and address emerging issues as our scientific understanding matures. The Protocol was amended or adjusted between 1990 and 2007 at meetings held in London, Copenhagen, Vienna, Beijing, and Montreal (see Q14). The most recent amendment was formulated at the Meeting of the Parties of the Montreal Protocol held in Kigali, Rwanda during October 2016. The Kigali Amendment phases down future global production and consumption of some HFCs to protect future climate, an important new milestone for the Montreal Protocol (see Q19). The Kigali Amendment was motivated by projections of substantial increases in the global use of HFCs in the coming decades. The control of HFCs under this amendment marks the first time the Montreal Protocol has adopted controls solely for the protection of climate.
The protection of the ozone layer and climate under the Montreal Protocol is a story of notable achievements: discovery, understanding, decisions, actions, and verification. It is a success story written by many: scientists, technologists, economists, legal experts, and policymakers, in which continuous dialogue has been a key ingredient. A timeline of milestones related to the science of stratospheric ozone depletion, international scientific assessments, and the Montreal Protocol is illustrated in Figure Q0-1.
To help communicate the broad understanding of the Montreal Protocol, ODSs, and ozone depletion, as well as the relationship of these topics to GHGs and global warming, this component of the Scientific Assessment of Ozone Depletion: 2022 report describes the state of this science with 20 illustrated questions and answers. The questions and answers address the nature of atmospheric ozone, the chemicals that cause ozone depletion, how global and polar ozone depletion occur, the extent of ozone depletion, the success of the Montreal Protocol, the possible future of the ozone layer, and the protection against climate change now provided by the Kigali Amendment. Computer model projections show that GHGs such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) will have a growing influence on global ozone in the coming decades, and in some cases may exceed the influence of ODSs on ozone by the middle of this century, given the expected future decline in the atmospheric abundance of ODSs. For each question, a brief answer is first given in highlighted text; an expanded answer then follows. The answers are based on the information presented in the 2022 and earlier Assessment reports as well as other international scientific assessments. These reports and the answers provided here were prepared and reviewed by a large number of international scientists who are experts in different research fields related to the science of stratospheric ozone and climate2.
Lead Author: Ross J. Salawitch
Coauthors: Laura A. McBride, Chelsea R. Thompson, Eric L. Fleming, Richard L. McKenzie, Karen H. Rosenlof, Sarah J. Doherty, David W. Fahey