The Ozone Hole and the Montreal Protocol
CFCs, the 1985 Discovery, and the Treaty That Reversed It
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The international response came through the Montreal Protocol, signed in 1987, which banned the production of many ozone-depleting substances. The treaty's success has been measured by satellite data showing the ozone layer beginning to recover. Scientists like Susan Solomon helped confirm the connection between CFCs and ozone loss, while researchers continue monitoring the healing process.
This book explains how a scientific discovery led to global cooperation on environmental protection. It covers the science of ozone depletion, the role of international treaties, and how climate change relates to ozone layer damage. Anyone interested in environmental policy or atmospheric science will find this account both accessible and important.
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Overview
In 1987, nations came together to sign the Montreal Protocol, formally called the Montreal Protocol on Substances That Deplete the Ozone Layer, a treaty aimed at protecting the ozone layer by phasing out substances responsible for its destruction. It went into effect on January 1, 1989. Since then, it has been amended several times, with changes agreed upon in 1990, 1992, 1995, 1997, 1999, 2007, 2016, and 2018. The agreement has been ratified by 198 parties—197 countries and the European Union—making it the first universally accepted treaty in United Nations history. Former UN secretary-general Kofi Annan called it “perhaps the single most successful international agreement to date.” As a result, the ozone hole over Antarctica is slowly healing, with climate projections showing the ozone layer will return to 1980 levels by 2040 globally and 2066 over Antarctica.
Terms and purposes
The Montreal Protocol set out specific terms to control ozone-depleting substances, including timelines for phasing them out, methods for reviewing progress, and rules for handling countries that didn’t comply. The agreement grouped these substances by type and listed exact compounds in its annexures. Article 5 gave developing nations extra time to meet the Protocol’s goals, recognizing their different circumstances. Article 10 created a financial support system to help those countries comply with the treaty's requirements.
Chlorofluorocarbons (CFCs) Phase-out Management Plan
The Montreal Protocol commits signatory nations to control ozone-depleting substances, recognizing serious health and environmental risks. It sets specific limits on production and consumption of chemicals in Group I of Annex A, including CFC-11, CFC-12, CFC-113, CFC-114, and CFC-115. Non-Article 5 countries were to freeze production at 1986 levels by July 1, 1989, with a 75% reduction by 1994 and full phase-out by 1996. Article 5 countries had a later start, freezing at the average of 1995–97 levels, with a 50% cut by 2005 and complete elimination by 2010. Substances like carbon tetrachloride and 1,1,1-trichloroethane were addressed individually. Less harmful HCFCs began phasing out in 1996, with a final phase-out set for 2030. Halons in Group II—halon-1211, -2404, and -1301—were frozen at 1986 levels for non-Article 5 countries by January 1, 1992, with full phase-out by 1994. Article 5 countries had a later freeze on January 1, 2002, with a 50% cut by 2005 and complete phase-out by 2010. Exceptions were made for essential uses where substitutes weren’t available, such as asthma inhalers and fire suppression systems in submarines and aircraft. The Protocol requires decisions to be based on scientific assessments conducted by the Scientific Assessment Panel (SAP), with reports published in 1989, 1991, 1994, 1998, and 2002. In 1990, a Technology and Economic Assessment Panel (TEAP) was formed to advise on alternatives and evaluate exemptions. TEAP’s annual reports help inform the Parties’ decisions. Numerous organizations have catalogued alternatives for sectors like refrigeration, fire protection, and aerospace.
Hydrochlorofluorocarbons (HCFCs) Phase-out Management Plan (HPMP)
Under the Montreal Protocol, parties agreed to freeze HCFC consumption and production in 2013 for developing countries, with developed nations beginning reductions in 2004 and 2010, respectively, aiming for full phase-out by 2020. Developing countries started reducing their use in 2015, with a target of 100% elimination by 2030. These transitional substances, known as HCFCs, are man-made compounds used in refrigeration, aerosols, foam production, and air conditioning. Though less harmful to the ozone layer than CFCs—having an ODP of 0.01–0.5 compared to CFCs’ 0.6–1.0—they still pose a significant greenhouse gas threat, with GWPs ranging from 76 to 2,270, much lower than CFCs’ 4,680–10,720.
Hydrofluorocarbons (HFCs)
On 1 January 2019, the Kigali Amendment to the Montreal Protocol began taking effect, requiring countries to cut HFC use by more than 80% over 30 years. By December 2018, 65 nations had ratified it, and by October 2024, 160 states and the European Union had joined. HFCs, produced mostly in developed countries, replaced CFCs and HCFCs, not harming the ozone layer but being powerful greenhouse gases. A 2009 study showed phasing them down could prevent up to 8.8 gigatons of CO2 equivalent in emissions by 2050, potentially avoiding as much as 0.5°C of warming by 2100. The idea came from the Federated States of Micronesia in 2009, with support from the U.S., Canada, and Mexico in 2010. After seven years of talks, the amendment was adopted in October 2016 in Kigali. It commits signatories to reduce HFC production and consumption by at least 85% from the average levels between 2011 and 2013, with developing countries like China, Brazil, and South Africa having until 2045 to comply, and others such as India, Iran, Iraq, Pakistan, Saudi Arabia, and Kuwait until 2047. Sweden's ratification on 17 November 2017 pushed the amendment over the threshold needed for it to enter into force.
History
In 1974, chemists Frank Sherwood Rowland and Mario Molina at UC Irvine discovered CFCs could break down in the stratosphere, releasing chlorine atoms that might destroy ozone. Their theory was confirmed by the U.S. National Academy of Sciences in 1976. By 1982, 24 countries met in Stockholm to discuss protecting the ozone layer. The following year, the U.S. and others proposed banning nonessential CFC use in spray cans. In 1985, British scientists Joe Farman, Brian Gardiner, and Jon Shanklin reported unusually low ozone above Halley Bay, sparking global concern. NASA images of the ozone hole helped push negotiations forward. That same year, 20 nations signed the Vienna Convention. Just 18 months later, in Montreal, Canada, the Montreal Protocol was agreed upon. Mostafa Kamal Tolba was called the "father of the Montreal Protocol" for his role. In 1986, a major assessment predicted severe ozone loss and related health impacts. The CFC industry resisted, with DuPont arguing the science was uncertain. But in 1986, the Alliance for Responsible CFC Policy changed course, saying future CFC increases would be unacceptable. Three months before the Montreal negotiations began, U.S. industry supported new international controls.
Multilateral Fund
The Multilateral Fund was created to help developing countries comply with the Montreal Protocol, especially those where people consume less than 0.3 kilograms of ozone-depleting substances per capita. Currently, 147 of the 196 parties qualify as Article 5 countries. The fund reflects the principle from the 1992 UN Conference on Environment and Development that all nations share responsibility but with different capacities. An executive committee manages it, with equal representation from industrialized and developing countries elected annually by the Meeting of the Parties. Four agencies carry out its work on the ground: UNEP, UNDP, UNIDO, and the World Bank. Contributions are replenished every three years, with pledges totaling $3.1 billion from 1991 to 2005. The money helps convert manufacturing processes, train workers, pay for new technologies, and set up national ozone offices.
Parties
As of October 2022, the original Montreal Protocol had been ratified by 198 parties, including all United Nations Member States, the Cook Islands, Niue, the Holy See, and the European Union. The State of Palestine was the final party to ratify the agreement, bringing the total to 198. Of those, 197 have also accepted the London, Copenhagen, Montreal, and Beijing amendments.
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Overview
Ozone depletion in the stratosphere refers to a drop in ozone levels, especially over the South Pole where it became known as the ozone hole. This happens because of human-made chemicals like CFCs and halons, which are ozone-depleting substances. These compounds rise into the upper atmosphere and break down ozone molecules through photodissociation. The problem grew as emissions increased, leading to more UVB radiation reaching Earth, which raises cancer and blindness risks. In response, the international community adopted the Montreal Protocol in 1987, banning production of these chemicals. Controls took effect in 1989, and by the mid-1990s, ozone levels had stabilized and began recovering. Scientists predict full recovery by 2045, with the ozone hole expected to return to 1980 levels around 2075. In 2019, NASA reported the smallest ozone hole since its discovery in 1982. The Montreal Protocol is seen as the most successful global environmental agreement ever.
Ozone cycle overview
Ozone forms in the stratosphere when ultraviolet light breaks apart oxygen molecules, creating atomic oxygen that combines with O2 to make O3. This ozone then absorbs UVB radiation, splitting back into oxygen and atomic oxygen in a continuous cycle. The process is maintained by a balance between production and recombination. But ozone can be destroyed by free radicals—especially chlorine and bromine atoms, which are released from compounds like CFCs after traveling unchanged through the troposphere to the stratosphere. A single chlorine atom can destroy up to 100,000 ozone molecules before being removed from the cycle, making these substances extremely damaging to the ozone layer.
Observations on ozone layer depletion
The ozone hole is measured by reductions in total column ozone in Dobson units, with the most dramatic drops seen in the Antarctic lower stratosphere. Observations using instruments like TOMS showed a 70 percent decrease beginning in 1985, first reported by Farman et al., and these levels remained low through 2010. While the Antarctic ozone hole has shown signs of healing since 2016, with 2017 marking the weakest since 1988, full recovery is expected around 2070. Arctic ozone loss is more variable, peaking at up to 30 percent in winter and spring. Polar stratospheric clouds enhance depletion, which is why the Antarctic hole formed first and was deeper. Mid-latitude regions saw declines before 1996, with northern mid-latitudes rebounding slightly after 1996 as regulations took effect. Tropical ozone levels have remained stable, as halogen compounds haven't broken down enough to release chlorine and bromine there yet. Large volcanic eruptions like Mt. Pinatubo in 1991 also caused measurable ozone loss. Ozone depletion explains much of the cooling seen in the stratosphere and upper troposphere, since ozone absorbs UV radiation that heats the stratosphere. Predictions have improved over time, with the World Meteorological Organization's Report No. 44 supporting the Montreal Protocol, though a UNEP 1994 Assessment had overestimated losses during 1994-1997.
CFCs and related compounds
Chlorofluorocarbons, or CFCs, are synthetic chemicals invented by Thomas Midgley Jr. in the 1930s, mainly used as refrigerants, aerosol propellants, and cleaning agents for electronics. No natural sources of these compounds have ever been found— their presence in the atmosphere comes entirely from human production. When released into the stratosphere, CFCs break down under UV light, releasing chlorine atoms that destroy thousands of ozone molecules. A single CFC molecule can take five to seven years to reach the upper atmosphere and may linger there for up to a century, destroying up to 100,000 ozone molecules during that time. One such chemical, CFC-113a, was newly discovered in the atmosphere by researchers at the University of East Anglia; its levels are still rising, though its source remains unknown. In 2013, scientists found that emissions from northeastern China were releasing large quantities of another banned chemical, CFC-11, which could delay the recovery of the ozone layer by a decade.
Aluminum oxide
Satellites burning up upon re-entry into Earth's atmosphere release aluminum oxide nanoparticles that stay in the air for decades. In 2022, scientists estimated about seventeen metric tons of these particles were produced, with each satellite contributing roughly thirty kilograms of nanoparticles per every two hundred fifty kilograms of spacecraft. As more and more satellites are launched, especially in large groups, this could eventually cause serious damage to the ozone layer.
Very short-lived substances (VSLS)
Very short-lived substances are a group of ozone-depleting chemicals permitted under the Montreal Protocol, and they break down in less than six months. Ninety percent of these substances occur naturally—for instance, bromine-based compounds released by seaweed and phytoplankton. The remaining ten percent are manmade, like dichloromethane.
Computer modeling
Scientists figured out that the hole in the ozone layer was caused by human-made chemicals called CFCs, using computer models alongside real measurements. These models, like SLIMCAT and CLaMS, mix data about gases and weather with known chemical reaction speeds. By doing this, they trace how CFCs break down in the stratosphere and release substances that destroy ozone. The models show exactly how these chemicals move through the atmosphere and where they come into contact with ozone molecules. This work helped confirm the link between CFCs and ozone depletion, leading to global action.
Ozone hole and its causes
The Antarctic ozone hole drops ozone levels to 33 percent of pre-1975 values each spring September to early December. Strong westerly winds create a polar vortex trapping extremely cold air that allows polar stratospheric clouds (PSCs) to form. These PSCs made of nitric acid trihydrate, water-ice, or nacreous particles provide surfaces for chemical reactions converting chlorine reservoir compounds into reactive forms like Cl and ClO. During dark Antarctic winter, no sunlight drives these reactions, but spring UV light triggers ozone destruction. Denitrification removes nitrogen dioxide from stratosphere, preventing ClO from recycling back into reservoirs. As temperatures rise near end of spring, vortex breaks apart, warm air flows in, PSCs disappear, and ozone hole closes. Most destruction occurs in lower stratosphere, unlike smaller upper stratosphere depletion through gas-phase reactions.
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Overview
Chlorofluorocarbons, or CFCs, are chemical compounds made of carbon, hydrogen, chlorine, and fluorine, and they're created from methane, ethane, and propane. A well-known example is dichlorodifluoromethane, also called R-12 or Freon, which was widely used in refrigeration, aerosols, fire suppression, and cleaning. Because CFCs damage the ozone layer, their production was gradually stopped under the Montreal Protocol. They’re now being replaced by alternatives like hydrofluorocarbons and hydrofluoroolefins, including R-410A, R-134a, and R-1234yf.
Structure, properties and production
CFCs, or chlorofluorocarbons, have a tetrahedral structure like simpler alkanes, but because chlorine and fluorine atoms are larger and more charged than hydrogen, the molecular shape isn't perfectly symmetrical. These compounds are volatile, though less so than their parent alkanes, due to polarity from the halogen atoms that creates intermolecular forces. For example, methane boils at minus 161 degrees Celsius, while fluoromethanes boil between minus 51.7 and minus 128 degrees Celsius. CFCs are useful as solvents and refrigerants because of their boiling points and low flammability, which comes from fewer C–H bonds and the flame-suppressing effect of halides. Their densities are higher than corresponding alkanes, increasing with the number of chlorine atoms. Most CFCs are made through halogen exchange reactions starting with chlorinated methanes and ethanes. One example is making chlorodifluoromethane from chloroform using hydrogen fluoride. Brominated versions are created by replacing C–H bonds with C–Br bonds, as seen in the production of the anesthetic halothane.
Applications
CFCs and HCFCs found their way into many everyday uses because of how safe and stable they are—low toxicity, not very reactive, and non-flammable. Scientists looked at every possible mix of fluorine, chlorine, and hydrogen based on methane and ethane, and most were turned into commercial products. There are also versions with more carbon atoms and even compounds with bromine. These chemicals served as refrigerants, blowing agents for foam, propellants in medical sprays, and solvents for cleaning. One specific compound, chlorodifluoromethane, is made in billions of kilograms each year, mainly to create tetrafluoroethylene, which becomes Teflon.
Numbering system
A special numbering system identifies fluorinated alkanes like CFCs, Freons, and HCFCs, using prefixes such as Freon-, R-, CFC-, and HCFC-. For example, Freon-12 shows a methane derivative with two fluorine atoms and no hydrogen, written as CCl₂F₂. The numbers in the name correspond to atomic counts: the rightmost digit is fluorine atoms, the next is hydrogen atoms plus one, and the leftmost is carbon atoms minus one. Another method adds 90 to the number—so CFC-12 becomes 102, meaning 1 carbon, 0 hydrogens, 2 fluorines, and 2 chlorines. This second approach makes it easier to identify carbon count. Some compounds include bromine and are marked with four numbers; isomers of ethane or propane derivatives use letters after the number sequence.
Reactions
When CFCs reach the upper atmosphere, they break apart under sunlight, splitting a carbon-chlorine bond. This creates a chlorine radical, written as Cl•, which is very different from the chlorine molecule found in everyday life. The chlorine radical lives long enough in the stratosphere to start a chain reaction that destroys ozone. Ozone normally blocks harmful UV-B rays from the sun, so when it's depleted, more of this dangerous radiation reaches Earth. Bromine atoms are even better at destroying ozone than chlorine, which is why brominated CFCs are also controlled under the treaty.
Impact as greenhouse gases
CFCs were banned under the Montreal Protocol because of their role in destroying the ozone layer, but they also act as powerful greenhouse gases. These chemicals absorb heat in the atmosphere, especially in a range of wavelengths from 7.8 to 15.3 micrometers, known as the "atmospheric window." Because CFCs are present in low concentrations, their impact on global warming is much greater per molecule than that of carbon dioxide. Unlike CO2, which has limited infrared absorption bands and is already close to saturation, CFCs have a stronger greenhouse effect that increases linearly with their amount. NASA reported in 2018 that the ozone hole is healing due to these bans, yet research from 2019 revealed a troubling rise in CFC levels, likely from unregulated use in China.
History
In the 1920s, refrigerators used dangerous gases like ammonia and chloromethane, which led to tragic accidents. That’s when Frigidaire, General Motors, and DuPont joined forces to find a safer option. Thomas Midgley Jr. of General Motors created the first chlorofluorocarbons, or CFCs. The company Frigidaire got the first patent for the formula on December 31, 1928. In 1930, Midgley showed off the gas by inhaling it and blowing out a candle. By 1935, over 8 million refrigerators were using R-12, and in 1932, Carrier introduced the first self-contained air conditioner using R-11. CFCs became the standard for public buildings because they were non-toxic. Sales grew rapidly, peaking at over a billion dollars a year. Then, in 1974, chemists F. Sherwood Rowland and Mario Molina discovered that CFCs were harming the ozone layer. That finding sparked global action that eventually led to the Montreal Protocol.
Commercial development and use in fire extinguishing
In the 1960s, fluoroalkanes and bromofluoroalkanes emerged as highly effective fire-fighting agents. Much early research with Halon 1301 was done under the US Armed Forces, while Halon 1211 was mainly developed in the UK. By the late 1960s, these substances became standard in places like computer rooms, laboratories, and museums where water or dry powder could damage sensitive equipment. In the 1970s, bromofluoroalkanes were also used on warships and in confined spaces for rapid fire suppression with little risk to personnel. By the early 1980s, they were common not only in those settings but also on aircraft, ships, and large vehicles. However, concerns began to grow about their impact on the ozone layer. The Vienna Convention did not restrict bromofluoroalkanes at that time, since emergency discharge was seen as too small to harm the ozone and too vital for safety to limit. Instead, consumption was frozen at 1986 levels.
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Overview
Susan Solomon is an American atmospheric chemist who has spent most of her career at NOAA and now teaches at MIT, where she holds the Ellen Swallow Richards Professorship in Atmospheric Chemistry & Climate Science. In 2011, she joined the faculty at MIT, and with her colleagues, she was the first to propose the reaction mechanism involving chlorofluorocarbon free radicals that causes the Antarctic ozone hole. She also wrote a book in 2024 called Solvable: how we healed the earth, and how we can do it again. Solomon is a member of several prestigious academies, including the U.S. National Academy of Sciences, the European Academy of Sciences, and the French Academy of Sciences. In 2002, Discover magazine named her one of the 50 most important women in science, and in 2008, Time magazine selected her as one of the 100 most influential people in the world. She also serves on the Science and Security Board for the Bulletin of the Atomic Scientists.
Early life
Susan Solomon grew up in Chicago, Illinois, where her fascination with science first took hold, sparked by watching The Undersea World of Jacques Cousteau. As a teenager, she placed third in a national science competition. She earned a B.S. in chemistry from the Illinois Institute of Technology in 1977, followed by an M.S. in chemistry in 1979. She completed her Ph.D. in atmospheric chemistry at the University of California, Berkeley in 1981.
Work
Susan Solomon led the Chemistry and Climate Processes Group at the National Oceanic and Atmospheric Administration’s Chemical Sciences Division until 2011. That year, she began working as a faculty member in the Department of Earth, Atmospheric and Planetary Sciences at MIT.
The Ozone Hole
Susan Solomon, working with colleagues at NOAA Earth System Research Laboratories, proposed that the Antarctic ozone hole formed through reactions between ozone and chlorofluorocarbon radicals on ice particle surfaces in high-altitude clouds. In 1986 and 1987, she led the National Ozone Expedition to McMurdo Sound, where her team gathered evidence confirming accelerated reactions. She was the sole leader and only woman on the expedition, finding chlorine oxide levels 100 times higher than expected from ultraviolet radiation breaking down CFCs. Solomon later showed volcanoes could accelerate these reactions, worsening ozone layer damage. Her work helped form the U.N. Montreal Protocol, an international agreement to regulate harmful chemicals, which she also suggested was having a positive effect. For her role in saving the ozone layer, Solomon shared the 2021 Future of Life Award with Joe Farman and Stephen O. Andersen. Jim Hansen said, "In Farman, Solomon and Andersen we see the tremendous impact individuals can have on the course of human and planetary history." Professor Guus Velders called Solomon a deserving recipient, noting she explained the ozone hole's formation while bridging science and policy in the Montreal Protocol.
The Coldest March
Susan Solomon, known for her work on the ozone hole, also wrote about the 1911 Antarctic expedition led by Robert Falcon Scott in a book titled The Coldest March: Scott's Fatal Antarctic Expedition. She used research from Scott’s journey to defend him against claims made by British journalist Roland Huntford, who argued that Scott was overly proud and poorly prepared. Solomon stood by Scott, saying that “modern data side squarely with Scott,” and described the weather conditions in 1911 as unusual.
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Overview
Human activity has caused major changes to the environment, affecting ecosystems, biodiversity, and natural resources. These impacts include global warming, ocean acidification, mass extinction, and ecological collapse. Some of the causes are population growth, neoliberal economic policies, rapid economic development, overconsumption, pollution, and deforestation. The term "anthropogenic" describes effects caused by humans, first used technically by Russian geologist Alexey Pavlov and later in English by British ecologist Arthur Tansley. Atmospheric scientist Paul Crutzen introduced the term "Anthropocene" in the mid-1970s, referring to human influence on Earth’s systems. Many environmental problems stem from burning fossil fuels for energy, transportation, manufacturing, and heating, as well as deforestation.
Overconsumption
Overconsumption happens when we use up natural resources faster than the Earth can replace them. The Global Footprint Network reports that our demand exceeds what ecosystems can renew by 70%. This pattern leads to environmental damage and resource loss. The New York Times noted in 2008 that people in developed nations consume about 32 times more resources than those in developing countries, even though the latter make up most of the world’s population. Human activity has already caused the loss of 83% of wild mammals and half of all plants. Chickens now outweigh wild birds, and cattle and pigs surpass wild mammals by a ratio of 14 to 1. Meat consumption is expected to rise by as much as 76% by 2050, driven by population growth, further threatening biodiversity and increasing greenhouse gas emissions.
Population growth and size
Some scholars see human population growth as a key driver of environmental problems, with a 2017 warning from over 15,000 scientists calling it the "primary driver behind many ecological and even societal threats." The 2019 UN Global Assessment Report said population growth is a major factor in biodiversity loss, and a 2021 Frontiers in Conservation Science report linked it to soil degradation and pollution. Scientists like E. O. Wilson have expressed concern about how human biomass now exceeds that of all large land animals by over 100 times, while a 2026 study suggested reducing the population to 4 billion by 2200 could help decrease emissions and defaunation. Still, others argue that blaming overpopulation oversimplifies things, especially since population growth is slowing, and global resources may still meet demand. Some experts believe that overconsumption, not just population size, must be addressed, with advocates like Paul R. Ehrlich suggesting fertility reduction should focus on the wealthy and middle classes to shrink "the scale of the human enterprise."
Fishing and farming
Agriculture affects the environment in many ways, and those effects depend on how farmers actually farm. Different farming methods lead to different outcomes, and it's not just about what happens on the farm itself — things like rain and temperature also play a role. Scientists measure these impacts using two kinds of signs: one shows what farmers do, like how much fertilizer they use, and the other shows what happens because of those actions, such as nitrate leaking into groundwater. The effects of farming reach into soil, water, air, plants, animals, and even the food we eat. Issues like climate change, deforestation, pollution, and soil loss are all connected to how we grow our food.
Fishing
Fishing affects the planet in two key ways: it impacts fish populations through overfishing and unsustainable practices, and it damages other parts of the ocean like coral reefs and marine habitats. Overfishing is now seen as the main cause of mass extinctions in the oceans, according to a major report. In 2006, a study in the journal Science predicted that if current trends continued, the world would run out of wild-caught seafood by 2048. That forecast sparked debate, but countries like Tonga, the United States, Australia, and New Zealand, along with international groups, have started managing marine resources more carefully. The UN's Food and Agriculture Organization reported in 2018 that global fish catch levels have stayed flat over the last twenty years, yet 33% of fisheries are now being overfished unsustainably. At the same time, aquaculture—farmed fish—has grown from 120 million tonnes per year in 1990 to more than 170 million tonnes in 2018. Shark and ray populations have dropped by 71% since 1970, with more than three-quarters of their species now threatened with extinction due to overfishing.
Irrigation
Irrigation changes how water and soil behave, affecting rivers, groundwater, and ecosystems. When water is diverted from rivers for irrigation, less flows downstream, evaporation increases, and the water table rises. These direct effects can lead to waterlogging and soil salinization, which harm crops and natural habitats. In some cases, irrigation helps fix salinization by washing salts away. But when groundwater is pumped out instead, it can cause the water level to drop, leading to land subsidence and saltwater intrusion near coastlines. Large-scale irrigation uses powerful pumps, dams, and pipelines that drain rivers, lakes, and aquifers. This massive use of freshwater stresses ecosystems and contributes to the decline of aquatic species. While irrigation offers benefits, its negative impacts are often ignored.
Agricultural land loss
By 2024, UNESCO reported that 75% of soils were degraded, affecting 3.2 billion people, with the share potentially rising to 90% by 2050 if current trends continue. During 2015–2019, about 100 million hectares of soil were lost each year. Lal and Stewart estimated a global loss of 12 million hectares annually due to degradation and abandonment. Scherr, referencing GLASOD under the UN Environment Programme, noted that since the mid-1940s, 6 million hectares of agricultural land had been lost yearly to soil degradation, a figure similar to earlier estimates by Dudal and Rozanov et al. Such losses stem from erosion, salinization, nutrient loss, acidification, compaction, waterlogging, and subsidence. Oldeman estimated that about 19 million square kilometers of global land were degraded, while Dregne and Chou found 36 million square kilometers degraded in dry regions. Despite these losses, the amount of arable land used for crop production globally increased by about 9% from 1961 to 2012. Global average soil erosion rates are high, with conventional cropland losing more soil than is naturally produced. In the US, erosion on cropland was estimated at 10.7 tons per hectare annually in 2010, a 34% reduction since 1982. No-till practices have become common and may support sustainable agriculture by reducing erosion rates closer to soil production. Land degradation is defined as any human-induced change to the land perceived as deleterious, excluding natural hazards, and it affects agronomic productivity, the environment, and food security, with up to 40% of agricultural land seriously degraded.
Meat production
Meat production has major environmental effects, using fossil fuels, water, and land while generating greenhouse gases. The Food and Agriculture Organization found that livestock are responsible for 18% of global greenhouse gas emissions, with meat accounting for 26% of all livestock products in 2011. Enteric fermentation in cattle produces about 27% of methane emissions, though methane's impact on warming is relatively small compared to carbon dioxide from fuel use and nitrous oxide from fertilizers. Water use is high, especially for feed crops, with most beef production relying on "green water" from rainfall. While the U.S. livestock industry generally complies with environmental laws, pollution from large operations can still occur. By 2007, U.S. beef production used less fuel, water, and land than in 1977, even as output rose. Some argue meat production helps reduce waste and supports biodiversity through grazing and manure use, but studies also link growing meat demand to deforestation and habitat loss, with livestock using 26% of Earth's land surface.
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Overview
Ozone depletion and climate change are linked environmental issues scientists have studied closely, especially regarding global regulation. By 1994, legal debates about handling climate change, ozone depletion, and air pollution were called "monumental." While both involve long-lasting gases that are hard to reverse, international responses differed: the Vienna Convention and Montreal Protocol succeeded in tackling ozone depletion, whereas the Kyoto Protocol on climate change largely fell short. Ozone does two key things for Earth's temperature — it heats the stratosphere by absorbing UV radiation and traps heat in the troposphere by absorbing infrared radiation. Although ozone depletion had a cooling effect, human-made increases in the lower atmosphere more than made up for that. Chlorofluorocarbons and other halocarbons that caused ozone loss are also powerful greenhouse gases, contributing more to warming than the net change in ozone levels. Projections show they will account for 4–10% of total expected greenhouse warming by 2100.
Policy approach
The response to ozone depletion was different from the approach to climate change, even though both involve atmospheric concerns. The Vienna Convention came before scientific agreement on the ozone hole, while climate regulation efforts like the Kyoto Protocol struggled despite global ambitions. The Montreal Protocol began with just 43 nations in 1986, but by December 2012, nearly all UN members had ratified it, including the European Union and several small states. The halocarbon industry later supported limits on CFCs, with DuPont acting faster than European companies. Germany’s shift helped the EU move toward stricter rules, though France and the UK initially defended their CFC industries even after the treaty was signed. Sir Robert Watson played a key role in unifying scientific assessments.
Policy and consensus
In 1996, Aant Elzinga noted how the Intergovernmental Panel on Climate Change had tried to build consensus in its first two reports. Stephen Schneider and Paul N. Edwards pointed out in 1997 that after the IPCC’s Second Assessment Report, groups like the Global Climate Coalition tried to discredit it by highlighting skeptical voices. Reiner Grundmann, writing in 2007, contrasted U.S. inaction with Europe’s efforts, suggesting that media coverage in the U.S. gave undue weight to climate skeptics, while Germany acted more boldly. That earlier, in 1986, the German Parliament had formed the Enquetekommission ‘Vorsorge zum Schutz der Erdatmosphäre’ to assess climate risks and propose emissions targets. Michael Oppenheimer later admitted limitations in the IPCC’s consensus model, calling for smaller, focused assessments instead of large, repeated summaries every six years.
Public opinion
The public’s understanding of ozone depletion and climate change differs sharply, especially when it comes to basic science and policy. People knew little about global warming and often confused it with the ozone hole. The ozone issue gained stronger support because it was communicated through relatable metaphors like “ozone shield” and “ozone hole,” which connected to everyday fears such as skin cancer and damage to plants. These images made the problem feel urgent and personal, unlike climate change, which many saw as distant or irrelevant. Americans even stopped using aerosol sprays on their own before laws were passed, showing how effectively the ozone message resonated. The public viewed the ozone hole as a serious threat, while global warming failed to inspire similar concern or comprehension.
Personal risk assessment and knowledge
Sheldon Ungar, a Canadian sociologist, notes that while specialized knowledge grows, scientific ignorance among the public often increases. People don’t connect climate change to immediate events, like the 1988–89 North American drought, even when scientists like James Hansen tried to use them as warnings. Unlike climate change, which feels abstract and future-oriented, the ozone problem resonated more with everyday understanding. Metaphors like “ozone shield” and “ozone hole” made it easier for people to grasp, especially when linked to popular culture, such as Star Wars imagery. The CFC regulation in the late 1980s used these simple ideas effectively. In 1985 and 1987, President Ronald Reagan’s skin cancer removal added personal urgency to the issue, something climate change lacked in public perception.
Cost-benefit assessments and industry policy
The United States accepted the Montreal Protocol but rejected the Kyoto Protocol, with Cass Sunstein suggesting cost-benefit analyses were key. Daniel Magraw argued motivations beyond costs and benefits mattered more. Peter Orszag and Terry Dinan viewed climate action as driven by fears of dire consequences. DuPont lost interest in defending CFCs after their Freon patent expired in 1979, while a citizen boycott led to a 1978 U.S. spray can ban. France and the UK tried to protect CFC industries, but the EU changed course after Germany withdrew support. DuPont moved faster than European rivals, fearing lawsuits over skin cancer risks, especially after the EPA released a 1986 study predicting 40 million additional cases and 800,000 deaths in the U.S. Greenpeace's marketing of "Greenfreeze" had strong impact in Europe and Asia. The Kyoto Protocol struggled, though Angela Merkel helped prevent failure by using 1990 as the baseline year for emissions. Eastern European industry decline allowed for strong commitments, but global emissions still rose.
Science background
Ozone depletion and climate change are linked in complex ways, with policy experts noting the need to connect efforts to protect both. Ozone acts as a greenhouse gas, and human activity has reduced it in the stratosphere while increasing it in the troposphere, creating opposing climate effects. These changes produce radiative forcing, though estimates vary due to ozone's uneven distribution and incomplete historical data. Many ozone-depleting substances are also powerful greenhouse gases; their rise contributed about 0.34 watts per square meter of forcing, or roughly 14% of total greenhouse gas impacts. The 1987 Montreal Protocol banned these chemicals, and one model suggests that without the ban, global temperatures in 2100 could be 2.5°C higher—1.7°C from direct greenhouse effects and 0.8°C from increased CO2 due to UV damage to vegetation. Drew Shindell's climate models show that while past research focused on CFCs' impact on ozone, future concerns will center on how climate change affects ozone and vice versa. The stratospheric ozone layer shields life from harmful UV radiation, and only human-made chlorine, not natural sources like sea spray, causes significant depletion. The same CO2-driven cooling of the stratosphere is expected to increase ozone loss, especially in polar regions, with a radiative forcing of around −0.15 watts per square meter.
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Overview
Every year on September 16, the United Nations celebrates the International Day for the Preservation of the Ozone Layer, a date chosen to mark the signing of the Montreal Protocol in 1987. That same year, nations came together to address the growing problem of ozone depletion caused by substances like CFCs. The treaty was designed to phase out these harmful chemicals, and its effects have been long-lasting, with the ozone layer expected to recover fully only after 50 to 100 years. In 2000, the UN General Assembly officially designated this day to honor the protocol’s impact. The first major sign of recovery was seen 30 years later, in 2017, when scientists observed the closing of the ozone hole over Antarctica.
Substance regulation and phase-out
Under the Montreal Protocol, countries agreed to cut back on using and making ozone-depleting substances. The treaty set a plan to get rid of key chemicals like CFCs, HCFCs, and halons. In 2016, the Kigali Amendment added new rules to reduce HFCs, which are strong greenhouse gases but don’t hurt the ozone layer.
Global impact and ratification
The Montreal Protocol stands as a landmark achievement in global environmental cooperation, celebrated for its universal acceptance among nations. Since its implementation, it has played a key role in healing the ozone layer and helping to slow climate change. Scientists credit its success to the way it was built to evolve, with regular updates and changes made possible by new scientific findings and technology. This adaptable framework has allowed the treaty to stay relevant and effective over time.
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Overview
The ozone layer is a part of Earth's stratosphere that blocks most of the Sun's harmful ultraviolet radiation, thanks to a high concentration of ozone (O3) gas. French physicists Charles Fabry and Henri Buisson discovered it in 1913 after noticing that sunlight reaching Earth was missing certain wavelengths, which matched the absorption pattern of ozone. Later, British meteorologist G. M. B. Dobson developed a device called the Dobsonmeter to measure ozone from the ground and set up a global monitoring network between 1928 and 1958. The unit used to measure ozone, the Dobson unit, is named after him. In 1985, scientists found that industrial chemicals like chlorofluorocarbons (CFCs) were destroying the ozone layer, leading to international action and a ban on these substances. The United Nations now marks September 16 as the International Day for the Preservation of the Ozone Layer. Venus also has a thin ozone layer at about 100 kilometers above its surface.
Ultraviolet light
The ozone layer protects life on Earth by absorbing harmful UV radiation, despite containing only a small amount of ozone. While nitrogen blocks shortest UV wavelengths, the remaining UV-A, UV-B, and UV-C are divided among three types. UV-C is entirely stopped by dioxygen and ozone, primarily around 35 kilometers up. UV-B, which causes skin damage, cataracts, and cancer, is mostly absorbed by the ozone layer though some reaches the surface, especially at longer wavelengths. This exposure is necessary for human vitamin D production. UV-A, largely unaffected by ozone, makes up most ultraviolet radiation reaching Earth and is less DNA-damaging but can still cause skin damage and aging.
Distribution in the stratosphere
The ozone layer's thickness varies by location and time, being thinner near the equator and thicker near the poles. Its upper edge fades gradually where air gets too thin for UV light to make much ozone, and its lower edge is where ozone blocks most incoming UV light. Ozone forms mainly in the tropics, then moves toward the poles via wind patterns called the Brewer–Dobson circulation. In the northern hemisphere, this causes the ozone layer to peak in spring and drop in fall. The U.S. sees highest ozone levels in April and May, lowest in October. Northern latitudes have more concentrated ozone than southern ones, with spring readings sometimes topping 600 DU compared to about 400 DU in Antarctica before human-caused depletion. These differences stem from the northern hemisphere's stronger circulation and mountain ranges, which have become more pronounced since the 1970s due to the ozone hole. The Arctic shows its peak ozone in March and April, while the Antarctic hits its lowest levels in September and October.
Depletion
The ozone layer can be broken down by certain chemicals, especially chlorine and bromine radicals from man-made compounds like CFCs and bromofluorocarbons. These substances rise into the stratosphere where UV light frees the radicals, each capable of destroying over 100,000 ozone molecules. The Antarctic ozone hole was first reported in 1985 by Joe Farman, Brian Gardiner, and Jonathan Shanklin. The U.S. Clean Air Act set standards for ozone pollution, and the Montreal Protocol, signed by all 197 countries, capped CFC production starting in 1986, with full phase-outs by 1995 in developed nations and later in developing ones. By 2003, scientists said global ozone depletion might be slowing, and recovery to 1980 levels is expected by the middle of the 21st century.
Implications for astronomy
Because the ozone layer blocks most harmful ultraviolet radiation from reaching Earth, scientists must study this light from space using satellites. One key mission was the Galaxy Evolution Explorer, or GALEX, a space telescope launched on April 28, 2003, and which operated until early 2012. Since young, hot stars emit much of their energy in ultraviolet wavelengths, studying these lights helps astronomers understand how galaxies form. This work happens above the atmosphere, where the ozone layer no longer shields us from such radiation.
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