The Montreal Protocol on Substances That Deplete the Ozone Layer is an international treaty designed to protect the ozone layer by phasing out the production of numerous substances that are responsible for ozone depletion. It was agreed on 16 September 1987, and entered into force on 1 January 1989. Since then, it has undergone nine revisions, in 1990 (London), 1991 (Nairobi), 1992 (Copenhagen), 1993 (Bangkok), 1995 (Vienna), 1997 (Montreal), 1999 (Beijing) and 2016 (Kigali). As a result of the international agreement, the ozone hole in Antarctica is slowly recovering. Climate projections indicate that the ozone layer will return to 1980 levels between 2040 and 2066. Due to its widespread adoption and implementation, it has been hailed as an example of successful international co-operation. Former UN Secretary-General Kofi Annan stated that "perhaps the single most successful international agreement to date has been the Montreal Protocol". In comparison, effective burden-sharing and solution proposals mitigating regional conflicts of interest have been among the success factors for the ozone depletion challenge, where global regulation based on the Kyoto Protocol has failed to do so. In this case of the ozone depletion challenge, there was global regulation already being installed before a scientific consensus was established. Also, overall public opinion was convinced of possible imminent risks.
The ozone layer or ozone shield is a region of Earth's stratosphere that absorbs most of the Sun's ultraviolet radiation. It contains a high concentration of ozone (O3) in relation to other parts of the atmosphere, although still small in relation to other gases in the stratosphere. The ozone layer contains less than 10 parts per million of ozone, while the average ozone concentration in Earth's atmosphere as a whole is about 0.3 parts per million. The ozone layer is mainly found in the lower portion of the stratosphere, from approximately 15 to 35 kilometers (9 to 22 mi) above Earth, although its thickness varies seasonally and geographically.
Ozone depletion consists of two related events observed since the late 1970s: a steady lowering of about four percent in the total amount of ozone in Earth's atmosphere, and a much larger springtime decrease in stratospheric ozone around Earth's polar regions. The latter phenomenon is referred to as the ozone hole. There are also springtime polar tropospheric ozone depletion events in addition to these stratospheric events.
The stratosphere is the second-lowest layer of the atmosphere of Earth, located above the troposphere and below the mesosphere. The stratosphere is composed of stratified temperature zones, with the warmer layers of air located higher and the cooler layers lower. The increase of temperature with altitude is a result of the absorption of the Sun's ultraviolet (UV) radiation by the ozone layer, where ozone is exothermically photolyzed into oxygen in a cyclical fashion. This temperature inversion is in contrast to the troposphere, where temperature decreases with altitude, and between the troposphere and stratosphere is the tropopause border that demarcates the beginning of the temperature inversion.
Ground-level ozone (O3), also known as surface-level ozone and tropospheric ozone, is a trace gas in the troposphere (the lowest level of the Earth's atmosphere), with an average concentration of 20–30 parts per billion by volume (ppbv), with close to 100 ppbv in polluted areas. Ozone is also an important constituent of the stratosphere, where the ozone layer (2 to 8 parts per million ozone) exists which is located between 10 and 50 kilometers above the Earth's surface. The troposphere extends from the ground up to a variable height of approximately 14 kilometers above sea level. Ozone is least concentrated in the ground layer (or planetary boundary layer) of the troposphere. Ground-level or tropospheric ozone is created by chemical reactions between NOx gases (oxides of nitrogen produced by combustion) and volatile organic compounds (VOCs). The combination of these chemicals in the presence of sunlight form ozone. Its concentration increases as height above sea level increases, with a maximum concentration at the tropopause. About 90% of total ozone in the atmosphere is in the stratosphere, and 10% is in the troposphere. Although tropospheric ozone is less concentrated than stratospheric ozone, it is of concern because of its health effects. Ozone in the troposphere is considered a greenhouse gas, and may contribute to global warming.
Mario José Molina-Pasquel Henríquez was a Mexican physical chemist. He played a pivotal role in the discovery of the Antarctic ozone hole, and was a co-recipient of the 1995 Nobel Prize in Chemistry for his role in discovering the threat to the Earth's ozone layer from chlorofluorocarbon (CFC) gases. He was the first Mexican-born scientist to receive a Nobel Prize in Chemistry and the third Mexican-born person to receive a Nobel prize.
Harry Wexler was an American meteorologist, born in Fall River, Massachusetts.
Jerry Mahlman was an American meteorologist and climatologist.
Susan Solomon is an American atmospheric chemist, working for most of her career at the National Oceanic and Atmospheric Administration (NOAA). In 2011, Solomon joined the faculty at the Massachusetts Institute of Technology, where she serves as the Ellen Swallow Richards Professor of Atmospheric Chemistry & Climate Science. Solomon, with her colleagues, was the first to propose the chlorofluorocarbon free radical reaction mechanism that is the cause of the Antarctic ozone hole. Her most recent book, Solvable: how we healed the earth, and how we can do it again (2024) focuses on solutions to current problems, as do books by data scientist Hannah Ritchie, marine biologist, Ayana Elizabeth Johnson and climate scientist Katharine Hayhoe.
Tropospheric ozone depletion events are phenomena that reduce the concentration of ozone in the earth's troposphere. Ozone (O3) is a trace gas which has been of concern because of its unique dual role in different layers of the lower atmosphere. Apart from absorbing UV-B radiation and converting solar energy into heat in the stratosphere, ozone in the troposphere provides greenhouse effect and controls the oxidation capacity of the atmosphere.
Ozone depletion and climate change are environmental challenges whose connections have been explored and which have been compared and contrasted, for example in terms of global regulation, in various studies and books.
Sarah Hörst is an associate professor of planetary sciences at Johns Hopkins University, who focuses on understanding planetary atmospheric hazes, in particular the atmosphere of Saturn's moon Titan.
Anne Mee Thompson is an American scientist, who specializes in atmospheric chemistry and climate change. Her work focuses on how human activities have changed the chemistry of the atmosphere, climate forcing, and the Earth's oxidizing capacity. Thompson is an elected fellow of the American Meteorological Society, American Geophysical Union, and AAAS.
Arlene M. Fiore is an atmospheric chemist whose research focuses on issues surrounding air quality and climate change.
Akkihebbal Ramaiah (Ravi) Ravishankara ForMemRS FAAAS FRSC is a scientist specializing in Chemistry and Atmospheric Sciences, and University Distinguished Professor in the Departments of Chemistry and Atmospheric Sciences at Colorado State University, Fort Collins.
Emily V. Fischer is an atmospheric chemist and an associate professor in the department of atmospheric science at Colorado State University. She earned notoriety from her work on the WE-CAN project and on PAN, specifically its role in changing the distribution of oxidants in the troposphere. She has received many honors including the prestigious James B. Macelwane Medal which is "given annually to three to five early career scientists in recognition of their significant contributions to Earth and space science." Fischer is also a role model and activist in galvanizing support for women in STEM fields.
Mian Chin is a Chinese atmospheric chemist. She is a physical scientist in the atmospheric chemistry and dynamics laboratory in the earth science division at Goddard Space Flight Center. Her research includes aerosol-cloud-chemistry-climate interactions. She received the NASA Exceptional Achievement Medal in 2005.
Anne Ritger Douglass is atmospheric physicist known for her research on chlorinated compounds and the ozone layer.
John W. Birks is an American atmospheric chemist and entrepreneur who is best known for co-discovery with Paul Crutzen of the potential atmospheric effects of nuclear war known as nuclear winter. His most recent awards include the 2019 Haagen-Smit Clean Air Award for his contributions to atmospheric chemistry and the 2022 Future of Life Award for discovery of the nuclear winter effect.
Margaret A. Tolbert is an American atmospheric chemist, specializing in polar stratospheric clouds.
