Ozone Layer Depletion

Introduction

Ozone is a gas that exists both in the Earth's upper atmosphere and at ground level. Depending on where it sits in the atmosphere, ozone can be "good" or "bad" for our health and for the environment.

Ozone is found in two layers of the atmosphere. Ground level, or "bad", ozone is an air pollutant that is hazardous to health and harms crops, trees and other vegetation. It is a major contributor to urban smog. The stratospheric, or "good", ozone layer extends upward from about 6 to 30 miles (roughly 10 to 50 km) and shields life on Earth from the sun's harmful ultraviolet (UV) rays.

Stratospheric ozone is a naturally occurring gas that filters ultraviolet radiation from the sun. When the ozone layer thins, more UV radiation reaches the Earth's surface. Overexposure to UV rays can cause skin cancer, cataracts and weakened immune systems in humans. Increased UV can also reduce crop yields and disrupt the marine food chain.

The cause of depletion

Ozone molecules in the stratosphere are constantly being produced and destroyed by the sun's various types of UV radiation. Normally the production and destruction of ozone are balanced, so the total amount of ozone stays fairly stable. However, scientists discovered that certain man made chemicals in the stratosphere react with UV radiation, break apart and release chlorine or bromine atoms. These atoms, in turn, deplete the ozone layer.

Ozone depleting substances (ODS) such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) were once widely used in refrigerants, insulating foams, solvents and other products. All of these substances release chlorine atoms into the stratosphere, and a single chlorine atom can break apart more than 100,000 ozone molecules.

Other ozone depleting chemicals include methyl bromide (a pesticide), halons (used in fire extinguishers) and methyl chloroform (used as a solvent in industry). When methyl bromide and halons break down they release bromine atoms, which are about 60 times more destructive to ozone molecules than chlorine atoms.

Before the Montreal Protocol on Substances that Deplete the Ozone Layer and its later amendments, atmospheric levels of these substances rose rapidly. Over the last two decades, however, the atmospheric levels of nearly all of them have fallen significantly.

Polar stratospheric clouds

Reactions on the surface of liquid and solid polar stratospheric clouds (PSCs) can sharply increase the amount of the most reactive form of chlorine. These reactions change the reservoir forms of reactive chlorine gases, chlorine nitrate (ClONO2) and hydrogen chloride (HCl), into the most reactive form, ClO.

• Denitrification: Once PSC particles form, gravity causes them to fall to lower altitudes. During the long, low temperature winter and spring period over Antarctica, the largest particles can descend several kilometres in the stratosphere. Because PSCs often contain a significant fraction of the available nitric acid (HNO3), their descent removes HNO3 from the ozone layer region. This is known as stratospheric denitrification. With less HNO3 present, the highly reactive chlorine gas ClO stays chemically active for longer, which increases chemical ozone destruction.

Agreements so far

Vienna Convention

By 1985, the world had made significant progress in understanding ozone depletion and its effects on human health and the environment. In response, the Vienna Convention for the Protection of the Ozone Layer was established. It is a framework convention that lays out principles many countries have agreed upon, but it does not by itself require countries to take control measures. Those binding measures would come later, in the form of the Montreal Protocol. The Vienna Convention came into effect in 1988 and achieved universal ratification in 2009.

Montreal Protocol

The Montreal Protocol on Substances that Deplete the Ozone Layer is a global agreement that aims to protect the Earth's ozone layer by phasing out ozone depleting chemicals. The phase out plan covers both the production and the consumption of these substances. This landmark agreement was signed in 1987 and came into effect in 1989. It is one of the most successful environmental treaties ever agreed, and it is the only United Nations treaty to be ratified by every country in the world.

Kigali Amendment to the Montreal Protocol (phasing out HFCs)

The Montreal Protocol was never meant to be a tool for tackling climate change, but it became one with the Kigali Amendment, signed in the Rwandan capital in October 2016. Hydrofluorocarbons (HFCs) were brought in as replacements for ozone depleting chemicals. HFCs do not deplete the ozone layer, but they are powerful greenhouse gases, with a global warming potential (GWP) ranging from about 12 to 14,000. Global warming potential is a number that represents how much heat a gas traps compared with the same mass of carbon dioxide. Carbon dioxide, the main greenhouse gas from human activity, has a GWP of 1.

Like the Montreal Protocol, the Kigali Amendment sets different timelines for higher and lower income countries. Countries such as the United States must meet their target by 2036, while China has until 2045 and India has until 2047.

The Kigali Amendment matters even more when read alongside the Paris Agreement, another key climate treaty. The Paris Agreement, signed in 2015, is a legally binding treaty that requires countries to cut greenhouse gas emissions in order to slow global warming. It aims to keep the rise in global temperature "well below" 2 degrees Celsius above pre industrial levels, so as to avoid the most extreme effects of climate change.

Why it still matters

The ozone story is not just history. It is one of the few times the whole world spotted an environmental emergency, agreed on a fix and made it work, and scientists are still tracking the results every single year.

In 2025, NASA and the United States National Oceanic and Atmospheric Administration (NOAA) reported that the Antarctic ozone hole was the fifth smallest since 1992, the year the Montreal Protocol really began to take effect. The hole reached its largest extent for the year on 9 September 2025, covering about 22.86 million square kilometres, and it began breaking up almost three weeks earlier than the average of the past decade. That does not mean the job is done. As senior NASA scientist Paul Newman put it, the world still has a long way to go before the layer returns to its 1980s condition.

The bigger picture is encouraging. A United Nations backed scientific assessment in 2023 found that nearly 99 per cent of the banned ozone depleting substances have already been phased out. If countries stay on course, the ozone layer is expected to return to its 1980 values by about 2040 for most of the world, by 2045 over the Arctic, and by 2066 over the Antarctic, where the damage was worst. The same assessment pointed to a climate bonus: by cutting HFCs, the Kigali Amendment could avoid up to 0.3 to 0.5 degrees Celsius of extra global warming by 2100.

So the ozone layer is a rare good news story, and it is worth understanding why it worked: clear science, a treaty every country signed, and industries that switched to safer chemicals. You can connect this to other real world issues on the Learnacy Hub, browse our wider set of study notes, or dig into more environment and ecology notes for the full context.

Sources

1. NASA, NOAA 2025 Ozone Hole Update, NASA Science (2025)
2. 2025 Antarctic Ozone Hole Among Smallest in Decades, UNEP Ozone Secretariat (2025)
3. Ozone layer recovery is on track, due to success of Montreal Protocol, UN News (2023)

Key takeaways

• Stratospheric ozone shields Earth from harmful UV rays, while ground-level ozone is a harmful air pollutant that contributes to urban smog.
• Certain man-made chemicals like CFCs and HCFCs release chlorine and bromine atoms that destroy ozone, with a single chlorine atom capable of breaking apart over 100,000 ozone molecules.
• Polar stratospheric clouds cause denitrification, which removes nitric acid from the stratosphere and allows reactive chlorine to stay active longer, increasing ozone destruction.
• The Montreal Protocol (1987) successfully phased out ozone-depleting substances and is the only UN treaty ratified by every country in the world.
• The Kigali Amendment (2016) addresses HFCs, which are greenhouse gas replacements for ozone-depleting chemicals but have high global warming potential.

Test yourself

What makes stratospheric ozone 'good' and ground-level ozone 'bad'?

Stratospheric ozone filters ultraviolet radiation from the sun, protecting Earth from harmful UV rays. Ground-level ozone is an air pollutant that is hazardous to health and harms crops, trees, and vegetation, contributing to urban smog.

How do ozone-depleting substances destroy ozone molecules?

Man-made chemicals like CFCs release chlorine or bromine atoms into the stratosphere when they react with UV radiation. These atoms then break apart ozone molecules, with a single chlorine atom capable of destroying more than 100,000 ozone molecules.

What is the purpose of the Kigali Amendment to the Montreal Protocol?

The Kigali Amendment addresses hydrofluorocarbons (HFCs), which were introduced as replacements for ozone-depleting chemicals. While HFCs do not deplete the ozone layer, they are powerful greenhouse gases, so the amendment aims to phase them down to help combat climate change.

Frequently asked questions

What is the primary function of the stratospheric ozone layer?

The stratospheric ozone layer acts as a natural filter, shielding life on Earth from the sun's harmful ultraviolet (UV) radiation. It absorbs UV rays that can otherwise cause skin cancer, cataracts, and harm ecosystems.

How do chlorofluorocarbons (CFCs) contribute to ozone depletion?

CFCs release chlorine atoms when broken down by UV radiation in the stratosphere. A single chlorine atom can catalytically destroy over 100,000 ozone molecules, accelerating ozone layer thinning.

Why are bromine atoms more destructive to ozone than chlorine atoms?

Bromine atoms are approximately 60 times more destructive to ozone molecules than chlorine atoms. They are released from chemicals like halons and methyl bromide, which were used in fire extinguishers and pesticides.

What role do polar stratospheric clouds play in ozone depletion?

Polar stratospheric clouds (PSCs) provide surfaces for chemical reactions that convert stable chlorine compounds into highly reactive forms like ClO. This prolongs chlorine's destructive activity, especially during polar winters.