# How the Ozone Layer Works

> In September 2026 the Copernicus Atmosphere Monitoring Service reported that the Antarctic ozone hole grew faster than usual, reaching about 25 million square kilometres on 12 September, although other measures stayed close to average. This film explains what sits behind that number: what ozone is and where the ozone layer lies; how little of it there is, measured in Dobson units; how it blocks the Sun's harmful ultraviolet light; how sunlight makes and breaks ozone in the Chapman cycle; how chlorofluorocarbons carry chlorine into the stratosphere, where one chlorine atom can destroy tens of thousands of ozone molecules; how British scientists found the hole over Antarctica in 1985; why it forms there, in the polar vortex and its icy clouds; and how the Montreal Protocol of 1987 cut the chemicals so that the layer is expected to recover by about 2066.

Interactive film: https://www.planetpast.com/stories/how-the-ozone-layer-works/  
Publisher: PlanetPast (https://www.planetpast.com/) · d => T('readPublished', l, { d: dateIn(d, l) }) 2026-10-29 · Chemistry, Earth science, History

## Prologue

### How the ozone layer works
*Chemistry · Earth · History*

A thin shield, a hole, and a repair.

In late August 2026, a hole over Antarctica grew larger than Antarctica itself: fifteen million square kilometres. It is not a hole in the air, but in a thin layer of a gas called ozone, high above the ice. By 12 September it had reached about twenty-five million square kilometres, some five million above the average for that date, according to the European Union's Copernicus service. Other measures, such as how low the ozone fell, stayed close to average. So this year's hole was unusually wide, but not unusually deep. The hole comes back every southern spring. To see why, and why scientists still expect it to heal, we start with what ozone is, and what it does for us.

- **15 million km²**: the hole
- **14.2 million km²**: Antarctica itself

![The hole opens when sunlight returns to air that has been dark and bitterly cold all winter. AI-generated still.](https://assets.planetpast.com/a/e8/e87993c94706fea9.jpg)

- 2026: ~25
- Average for the date: ~20
- Record, 9 Sep 2006: 29.6

> …can vary considerably from year to year. (Laurence Rouil, Director of the Copernicus Atmosphere Monitoring Service, on the hole’s development)

- A shield three millimetres thick
- How sunlight makes ozone
- How chlorine destroys it
- The hole, and the treaty that is closing it

Prologue

#### The 2026 ozone hole

Copernicus Atmosphere Monitoring Service (CAMS), as reported by Down To Earth and MercoPress: the 2026 Antarctic ozone hole reached 15 million km² by the end of August, slightly earlier than average and slightly larger than Antarctica’s 14.2 million km². It grew sharply in early September to an estimated 25 million km² on 12 September, about 5 million km² above the average for that date. The ozone column minimum and the ozone mass deficit stayed close to historical averages. The record remains 29.6 million km², on 9 September 2006.

The average bar is the 2026 figure minus the reported 5 million km² difference.

The 3D scene is a teaching model: the atmosphere is drawn far thicker than life, molecules are enlarged and colours stand for wavelengths of light that the eye cannot see.

Sources: [Down To Earth (16 Sep 2026): ozone hole reaches notable threshold early again as area increases above average in 2026, Copernicus](https://www.downtoearth.org.in/climate-change/ozone-hole-reaches-notable-threshold-early-again-as-area-increases-above-average-in-2026-copernicus); [MercoPress (21 Oct 2026): the area of the Antarctic ozone hole during September reached 15 million km², says Copernicus](https://en.mercopress.com/2026/10/21/the-area-of-antarctic-ozone-hole-during-september-reached-15-million-km2-says-copernicus)

## Chapter I · A thin shield

### 

Ozone is a molecule of three oxygen atoms, O three, instead of the two in the oxygen we breathe. Most of it sits in the stratosphere, roughly fifteen to thirty-five kilometres above the ground. Even there it is rare. At its peak, ozone makes up only about eight to fifteen parts per million of the air: eight to fifteen molecules in every million. The French physicists Charles Fabry and Henri Buisson discovered the layer in 1913.

$$\mathrm{O_2}\;\text{(oxygen)}$$
$$\mathrm{O_3}\;\text{(ozone)}$$

- **15–35 km**: above the ground, lower stratosphere
- **thinner**: near the equator than near the poles

- **8–15**: parts per million at the peak
- **1913**: found by Fabry and Buisson

![The whole air is about a hundred kilometres deep on a planet 12,700 km across — the ozone layer is one band inside it. AI-generated still.](https://assets.planetpast.com/a/a2/a24fe21eb1f390ed.jpg)

- O₂: what we breathe, about a fifth of the air
- O₃: a trace gas, mostly high up

Chapter I · A thin shield

#### What and where

Wikipedia, Ozone layer: the ozone layer is mainly found in the lower portion of the stratosphere, from approximately 15 to 35 km above Earth; it peaks at 8 to 15 parts per million of ozone; it is generally thinner near the equator and thicker near the poles. It was discovered in 1913 by the French physicists Charles Fabry and Henri Buisson.

The 3D scene is a teaching model: the atmosphere is drawn far thicker than life, molecules are enlarged and colours stand for wavelengths of light that the eye cannot see.

Sources: [Wikipedia: Ozone layer](https://en.wikipedia.org/wiki/Ozone_layer)

### 

How much ozone is there? Scientists answer with an odd unit. Imagine taking all the ozone in the air above one spot, a column reaching up to the sky, and squeezing it to the pressure at sea level. Each hundredth of a millimetre of that pure layer counts as one Dobson unit. A typical column is about three hundred Dobson units: a layer just three millimetres thick. The unit is named after Gordon Dobson of Oxford, who built the first instrument on the ground to measure ozone in the 1920s, and set up a worldwide network of stations. Squeeze the layer and see.

$$1\ \mathrm{DU} = 0.01\ \mathrm{mm}$$
- $\mathrm{DU}$: Dobson unit: pure ozone at sea-level pressure and 0 °C

![Dobson's instrument compares two ultraviolet wavelengths in the same beam of skylight; the ratio gives the thickness. AI-generated still.](https://assets.planetpast.com/a/9f/9ff57c18aab6587f.jpg)

- **300 DU**: above one spot
- **3 mm**: if squeezed into one layer

- 1920s: Dobson builds a ground instrument
- 1928–1958: a worldwide network of stations

![Dobson set up a network of these around the world, so the same quantity was read the same way everywhere. AI-generated still.](https://assets.planetpast.com/a/c8/c8ae4c5db49a7c88.jpg)

Chapter I · A thin shield

#### The Dobson unit

Wikipedia, Dobson unit: one Dobson unit is the thickness, in units of 10 μm (0.01 mm), that the ozone in a column would have as a layer of pure gas at standard temperature and pressure; a typical column amount of 300 DU would form a 3 mm layer. It is named after Gordon Dobson of the University of Oxford, who built the first instrument to measure total ozone from the ground in the 1920s. Wikipedia, Ozone layer: between 1928 and 1958 Dobson established a worldwide network of ozone monitoring stations.

Try it: the column of blue dots collapses into the 3 mm layer it would make at the surface (drawn larger than life).

The 3D scene is a teaching model: the atmosphere is drawn far thicker than life, molecules are enlarged and colours stand for wavelengths of light that the eye cannot see.

Sources: [Wikipedia: Dobson unit](https://en.wikipedia.org/wiki/Dobson_unit); [Wikipedia: Ozone layer](https://en.wikipedia.org/wiki/Ozone_layer)

### 

That thin layer matters because of ultraviolet light. Sunlight carries three bands of it. UV-A, with the longest waves, mostly passes through ozone and reaches the ground. UV-C, the shortest and most energetic, is stopped completely, by oxygen and ozone together, high in the atmosphere. In between lies UV-B. The ozone layer absorbs ninety-seven to ninety-nine percent of this middle band, and ozone is the only gas in the air that absorbs it. That is fortunate, because UV-B damages DNA. It causes sunburn and is linked to skin cancer and cataracts. Choose a band and see how far it gets.

- UV-A, 315–400 nm: longest waves, least energy
- UV-B, 280–315 nm: the middle band
- UV-C, 100–280 nm: shortest waves, most energy

![UV-C is shorter than 280 nm, UV-B runs 280 to 315, UV-A 315 to 400. AI-generated still.](https://assets.planetpast.com/a/1f/1f6d76ae74de3b25.jpg)

- **most**: of UV-A
- **1–3%**: of UV-B
- **none**: of UV-C, gone by about 35 km

- **1 gas**: in the air absorbs UV-B: ozone

- Damages DNA in skin cells
- Sunburn
- Linked to skin cancer and cataracts

Chapter I · A thin shield

#### The ultraviolet filter

Wikipedia, Ozone layer: UV-A (400–315 nm), UV-B (315–280 nm) and UV-C (280–100 nm). Ozone is transparent to most UV-A; the layer absorbs 97 to 99 percent of the Sun’s medium-frequency ultraviolet light (UV-B); UV-C is entirely screened out by a combination of oxygen and ozone by about 35 km altitude. Wikipedia, Ozone depletion: ozone is the only atmospheric gas that absorbs UV-B; UV-B damages DNA, and these wavelengths cause sunburn and are linked to skin cancer and cataracts.

This film is education, not medical advice. The “1–3%” is 100% minus the 97–99% absorbed.

The 3D scene is a teaching model: the atmosphere is drawn far thicker than life, molecules are enlarged and colours stand for wavelengths of light that the eye cannot see.

Sources: [Wikipedia: Ozone layer](https://en.wikipedia.org/wiki/Ozone_layer); [Wikipedia: Ozone depletion](https://en.wikipedia.org/wiki/Ozone_depletion)

## Chapter II · Made by sunlight

### 

Ozone is made and unmade by sunlight itself. In 1930 the British scientist Sydney Chapman worked out how. First, hard ultraviolet light, with wavelengths shorter than two hundred and forty-two nanometres, splits an oxygen molecule into two free atoms. Each free atom joins an oxygen molecule to make ozone. Then ozone absorbs ultraviolet in turn and breaks apart again, and the energy it took in becomes heat. So ozone keeps being broken and rebuilt, and each time it soaks up ultraviolet and turns it into warmth. A slower reaction, between ozone and a free atom, removes some for good, so the amount settles at a balance. Most ozone is made over the tropics, high up. Fire a ray of sunlight into the gas.

$$\mathrm{O_2} + h\nu\;(\lambda < 242\,\mathrm{nm}) \rightarrow 2\,\mathrm{O}$$
- $h\nu$: a photon of ultraviolet light

$$\mathrm{O} + \mathrm{O_2} \rightarrow \mathrm{O_3}$$
- $\mathrm{O_3}$: ozone is made

$$\mathrm{O_3} + h\nu\;(240\text{–}310\,\mathrm{nm}) \rightarrow \mathrm{O_2} + \mathrm{O}$$
- $h\nu$: the ultraviolet ozone absorbs, then released as heat

$$\mathrm{O_3} + \mathrm{O} \rightarrow 2\,\mathrm{O_2}$$

- **~400 million t**: made every day
- **~3 billion t**: in the whole atmosphere

Chapter II · Made by sunlight

#### Made and unmade by sunlight

Wikipedia, Ozone–oxygen cycle: Sydney Chapman worked out the mechanism in 1930. Oxygen photodissociation (O₂ + hν → 2O, below 242 nm); ozone creation (O + O₂ + M → O₃ + M, where M is any third molecule that carries off energy); ozone photodissociation (O₃ + hν → O₂ + O, 240–310 nm); ozone conversion (O₃ + O → 2O₂). The net result is the conversion of penetrating UV-B light into heat without any net loss of ozone. Most ozone production occurs in the tropical upper stratosphere and mesosphere; about 400 million tonnes are produced per day, and the global total is about 3 billion tonnes.

Try it: each ray splits an O₂ (blue pair) and the free atoms join other O₂ to make O₃ (three balls). The third molecule M is left out of the picture.

The 3D scene is a teaching model: the atmosphere is drawn far thicker than life, molecules are enlarged and colours stand for wavelengths of light that the eye cannot see.

Sources: [Wikipedia: Ozone–oxygen cycle (Chapman cycle)](https://en.wikipedia.org/wiki/Ozone%E2%80%93oxygen_cycle)

## Chapter III · The chlorine problem

### 

In the twentieth century, people added something new. Chlorofluorocarbons, or CFCs, were used to cool refrigerators, to push spray from cans and to blow plastic foams. They were prized because they hardly react with anything. But that same stability means nothing breaks them down in the lower air. A CFC molecule takes about five to seven years to drift up to the upper atmosphere, and can stay there for about a century. In 1974 the chemists Mario Molina and Sherwood Rowland warned, in the journal Nature, that up there intense ultraviolet light would break CFCs apart and set chlorine atoms free.

![CFCs once circulated in pipes like these. AI-generated still.](https://assets.planetpast.com/a/40/403fd974b1dedb5a.jpg)

![Cooling, propellant and foam — all three wanted a gas that would not burn, corrode or poison anyone. AI-generated still.](https://assets.planetpast.com/a/7b/7baeddb0a6e259f0.jpg)

- **5–7 years**: to rise to the upper atmosphere
- **~100 years**: that it can stay there

$$\mathrm{CFCl_3} + h\nu \rightarrow \mathrm{Cl} + \mathrm{CFCl_2}$$
- $\mathrm{CFCl_3}$: CFC-11, one common CFC

Chapter III · The chlorine problem

#### The CFC warning

MIT News (1995): in 1974 Mario Molina and F. Sherwood Rowland published in Nature a warning that chlorofluorocarbons, used as propellants, refrigerants and in plastic foams, threatened the ozone layer: being chemically inert, CFCs could drift up to the ozone layer, where intense ultraviolet light would break them down and release chlorine atoms. Wikipedia, Ozone depletion: a CFC molecule takes an average of about five to seven years to go from ground level up to the upper atmosphere, and it can stay there for about a century; once in the stratosphere, Cl atoms are released by ultraviolet light, for example CFCl₃ + light → Cl + CFCl₂.

The 3D scene is a teaching model: the atmosphere is drawn far thicker than life, molecules are enlarged and colours stand for wavelengths of light that the eye cannot see.

Sources: [MIT News (1995): Mario Molina shares the Nobel Prize in Chemistry](https://news.mit.edu/1995/molina-1018); [Wikipedia: Ozone depletion](https://en.wikipedia.org/wiki/Ozone_depletion)

### 

A free chlorine atom attacks ozone. It takes one oxygen atom, leaving ordinary oxygen and a molecule of chlorine monoxide. Then the chlorine monoxide meets another ozone molecule and gives up its oxygen. The chlorine atom is free again, ready to repeat the attack. Because the chlorine comes out unchanged, it is a catalyst. Each chlorine atom can break down tens of thousands of ozone molecules before it is finally locked away in a harmless compound. Release one chlorine atom and count.

$$\mathrm{Cl} + \mathrm{O_3} \rightarrow \mathrm{ClO} + \mathrm{O_2}$$

$$\mathrm{ClO} + \mathrm{O_3} \rightarrow \mathrm{Cl} + 2\,\mathrm{O_2}$$
- $\mathrm{Cl}$: comes back out: a catalyst

![The cycle runs fastest on the surfaces of polar stratospheric clouds, which form only below about minus 78 Celsius. AI-generated still.](https://assets.planetpast.com/a/c5/c5892de79325ffed.jpg)

- **tens of thousands**: ozone molecules broken down before it is removed

- Hydrogen chloride, HCl
- Chlorine nitrate, ClONO₂
- Reservoirs: harmless until something sets the chlorine free again

Chapter III · The chlorine problem

#### A catalytic cycle

Wikipedia, Ozone depletion: Cl + O₃ → ClO + O₂, then ClO + O₃ → Cl + 2 O₂. The chlorine atom is regenerated, so it can repeat the cycle; each Cl or Br atom can break down tens of thousands of ozone molecules before it is removed (an average of about 100,000 is quoted for chlorine). Most stratospheric chlorine sits in reservoir compounds, mainly hydrogen chloride (HCl) and chlorine nitrate (ClONO₂), which keep it out of the cycle. Other chlorine cycles exist; this is one the source gives.

Try it: the counter in the scene is sped up and capped for display; it shows the idea, not a measured rate.

The 3D scene is a teaching model: the atmosphere is drawn far thicker than life, molecules are enlarged and colours stand for wavelengths of light that the eye cannot see.

Sources: [Wikipedia: Ozone depletion](https://en.wikipedia.org/wiki/Ozone_depletion)

## Chapter IV · The hole

### 

For years the damage was predicted but not seen. Then came the measurements from Antarctica. British scientists had measured ozone at Halley station since 1956, with one of Dobson's instruments. Jonathan Shanklin, Joseph Farman and Brian Gardiner of the British Antarctic Survey found that ozone in spring had been falling since the late 1970s. By 1984, the layer over Halley was only about two-thirds as thick as in earlier decades. They published in Nature in May 1985. Satellites then showed how big it was: a region of some twenty million square kilometres. The ozone hole had been found.

![A ground instrument reads how much ultraviolet light gets through. AI-generated still.](https://assets.planetpast.com/a/dc/dcbef5e67b3dd966.jpg)

![The continent is 14 million square kilometres; the hole measured larger than that. AI-generated still.](https://assets.planetpast.com/a/23/23bd7778bda98fcd.jpg)

- 1956: ozone records begin at Halley
- late 1970s: spring ozone starts to fall
- May 1985: the paper in Nature

- Earlier decades: 1
- 1984: ~⅔

- **~20 million km²**: the region satellites then mapped

Chapter IV · The hole

#### The discovery

UKRI / NERC: the British Antarctic Survey had continuous ozone data from Halley station going back to 1956, from a Dobson ozone spectrophotometer, which measures the ultraviolet light reaching the ground. Jonathan Shanklin found, and Joe Farman pushed to publish, that since the late 1970s there had been a systematic decline in spring ozone; by 1984 the layer over Halley was only about two-thirds as thick as it had been in earlier decades. The paper, by Farman, Gardiner and Shanklin, appeared in Nature in May 1985. Satellites showed that the hole extended over a region of 20 million km².

The 3D scene is a teaching model: the atmosphere is drawn far thicker than life, molecules are enlarged and colours stand for wavelengths of light that the eye cannot see.

Sources: [UKRI / NERC: the story behind the discovery of the ozone hole](https://www.ukri.org/who-we-are/how-we-are-doing/research-outcomes-and-impact/nerc/the-story-behind-the-discovery-of-the-ozone-hole/)

### 

Why Antarctica? In the southern winter, strong winds circle the pole. This polar vortex traps the air inside it and lets it grow extremely cold. At temperatures around minus eighty degrees, thin clouds form in the stratosphere. On the surfaces of their particles, chlorine that was locked in harmless compounds is turned back into forms that destroy ozone. When the Sun returns in spring, from August to November, its light splits that chlorine free to attack ozone, and ozone falls fast. The Arctic vortex is less cold and less steady, so its losses are smaller and vary more. This September's sharp growth came with a sudden drop in temperature about twenty kilometres above the South Pole. Make the stratosphere colder, and watch the clouds form.

- A ring of strong winds round the pole
- Traps the air inside and chills it
- Tight and very cold over Antarctica

- **~ −80 °C**: or colder, when they form

![Iridescent clouds high in a polar twilight sky. AI-generated still.](https://assets.planetpast.com/a/68/684a6b3950b9976a.jpg)

- **up to 30%**: ozone losses there in its coldest years

- **~20 km**: above the South Pole: height of the sudden cold

Chapter IV · The hole

#### Why the hole forms over Antarctica

Wikipedia, Ozone depletion: the polar vortex traps and chills the air; the Antarctic vortex is very tight and very cold, so polar stratospheric clouds form readily at temperatures around or below −80 °C, which is why ozone holes first formed, and are deeper, over Antarctica. On the clouds’ surfaces, chlorine from reservoir compounds is converted into reactive forms; sunlight in spring then drives rapid ozone loss. Arctic depletion is more variable, reaching up to 30 percent when the stratosphere is coldest. Copernicus, via Down To Earth: depletion runs in the austral spring (August to November); the rapid growth in early September 2026 coincided with a sudden drop in minimum temperature at about 20 km above the South Pole (50 hPa).

Try it: below about −80 °C in the model, clouds appear inside the vortex and the ozone over the pole thins when the Sun comes up.

The 3D scene is a teaching model: the atmosphere is drawn far thicker than life, molecules are enlarged and colours stand for wavelengths of light that the eye cannot see.

Sources: [Wikipedia: Ozone depletion](https://en.wikipedia.org/wiki/Ozone_depletion); [Down To Earth (16 Sep 2026): ozone hole reaches notable threshold early again as area increases above average in 2026, Copernicus](https://www.downtoearth.org.in/climate-change/ozone-hole-reaches-notable-threshold-early-again-as-area-increases-above-average-in-2026-copernicus)

## Chapter V · The repair

### 

The world acted quickly. In 1985, twenty nations signed the Vienna Convention, a framework for protecting the ozone layer. Two years later, on 16 September 1987, came the Montreal Protocol, which set out to phase out the chemicals that destroy ozone. Today it has one hundred and ninety-eight parties: one hundred and ninety-seven states and the European Union. According to the World Meteorological Organization, it has eliminated more than ninety-nine percent of the controlled ozone-depleting substances. A 2016 amendment, agreed in Kigali, also cuts HFCs, chemicals that are powerful greenhouse gases. In 1995, Molina and Rowland shared the Nobel Prize in Chemistry with Paul Crutzen, who had shown in 1970 that nitrogen oxides also speed up the loss of ozone.

- 1985: Vienna Convention, 20 nations
- 16 Sep 1987: Montreal Protocol agreed
- 1 Jan 1989: in force
- 2016: Kigali Amendment: phasing down HFCs

![The United Nations later made 16 September the International Day for the Preservation of the Ozone Layer. AI-generated still.](https://assets.planetpast.com/a/26/2657cd71cc11ad50.jpg)

- **198**: parties: 197 states and the EU
- **>99%**: of controlled ozone-depleting substances eliminated, says the WMO

> Perhaps the single most successful international agreement to date has been the Montreal Protocol. (Kofi Annan, then UN Secretary-General)

- **3**: laureates: Molina, Rowland, Crutzen

Chapter V · The repair

#### The treaty

Wikipedia, Montreal Protocol: the Vienna Convention of 1985 was signed by 20 nations and set a framework for negotiating rules on ozone-depleting substances. The Montreal Protocol was agreed on 16 September 1987 and entered into force on 1 January 1989; it has 198 parties (197 states and the European Union), the last state to ratify being the State of Palestine. The Kigali Amendment, adopted in October 2016, phases down hydrofluorocarbons (HFCs), potent greenhouse gases. Kofi Annan called it “perhaps the single most successful international agreement to date”. WMO Ozone Bulletin 2025, via Down To Earth: the protocol has eliminated more than 99% of controlled ozone-depleting substances. MIT News: Molina, Rowland and Crutzen shared the 1995 Nobel Prize in Chemistry; Crutzen had shown in 1970 that nitrogen oxides speed up ozone loss.

The 3D scene is a teaching model: the atmosphere is drawn far thicker than life, molecules are enlarged and colours stand for wavelengths of light that the eye cannot see.

Sources: [Wikipedia: Montreal Protocol](https://en.wikipedia.org/wiki/Montreal_Protocol); [Down To Earth (16 Sep 2025): WMO Ozone Bulletin, the ozone layer on track to recover](https://www.downtoearth.org.in/amp/story/environment/world-ozone-day-earths-protective-layer-on-track-to-return-to-1980s-levels-by-mid-century-says-wmo); [MIT News (1995): Mario Molina shares the Nobel Prize in Chemistry](https://news.mit.edu/1995/molina-1018)

### 

The chemicals have mostly stopped being made, but those already released last for decades, so the chlorine is leaving only slowly. By the 2022 count, chlorine in the atmosphere was down eleven and a half percent from its 1993 peak, and bromine, another ozone destroyer, fourteen and a half percent from its 1999 peak. The 2022 scientific assessment, co-chaired by NASA and NOAA scientists, expects ozone to return to its 1980 levels, from before the hole appeared, around 2040 over most of the world, around 2045 over the Arctic, and around 2066 over Antarctica, if today's policies hold. Recovery is not smooth. The largest hole on record came in 2006. In 2025 the World Meteorological Organization called the 2024 hole, which opened late, a robust sign of initial recovery, and in 2026 the hole grew fast again. A new assessment is due this year. Compare the years.

- Chlorine, since 1993: 11.5%
- Bromine, since 1999: 14.5%

- ~2040: most of the world
- ~2045: the Arctic
- ~2066: Antarctica

- **1980**: the yardstick: before the hole over Antarctica was reported, in 1985

- **29.6 million km²**: the record, 9 September 2006
- **46.1 million t**: the most ozone missing at once in 2024, below the 1990–2020 average

> This work is not yet finished. (Matt Tully, chair of the WMO’s scientific advisory group on ozone, 2025)

Chapter V · The repair

#### Recovery

NBC News, on the 2022 assessment co-chaired by NASA’s Paul Newman and NOAA’s David Fahey (January 2023): chlorine is down 11.5% since its 1993 peak and bromine 14.5% since its 1999 peak; ozone is expected to return to 1980 levels around 2040 for most of the world, around 2045 over the Arctic and in about 43 years over Antarctica. WMO Ozone Bulletin 2025, via Down To Earth: the 2022 assessment projects recovery to 1980 levels by 2066 over Antarctica, 2045 over the Arctic and 2040 elsewhere if current policies stay in place. The 2024 hole was smaller than those of 2020–2023, with a maximum ozone mass deficit of 46.1 million tonnes on 29 September 2024, below the 1990–2020 average; its depletion started slowly and stayed delayed through September, which the WMO called “a robust indication of initial recovery”. The next assessment is due in 2026. Copernicus: the record hole was 29.6 million km² on 9 September 2006.

Try it: the hole sizes shown for 2006 and 2026 are the reported areas; 2066 shows the projected return to 1980 levels as a hole that no longer opens, which is a simplification.

The 3D scene is a teaching model: the atmosphere is drawn far thicker than life, molecules are enlarged and colours stand for wavelengths of light that the eye cannot see.

Sources: [NBC News (Jan 2023): UN says ozone layer slowly healing, hole to mend by 2066](https://www.nbcnews.com/science/environment/un-says-ozone-layer-slowly-healing-hole-mend-2066-rcna64927); [Down To Earth (16 Sep 2025): WMO Ozone Bulletin, the ozone layer on track to recover](https://www.downtoearth.org.in/amp/story/environment/world-ozone-day-earths-protective-layer-on-track-to-return-to-1980s-levels-by-mid-century-says-wmo); [Down To Earth (16 Sep 2026): ozone hole reaches notable threshold early again as area increases above average in 2026, Copernicus](https://www.downtoearth.org.in/climate-change/ozone-hole-reaches-notable-threshold-early-again-as-area-increases-above-average-in-2026-copernicus)

## Epilogue

### The thin shield
*Epilogue*

Made by sunlight, torn by chlorine, mended by a treaty.

The ozone layer is a shield a few millimetres thick, made and remade by sunlight. A family of useful chemicals tore a hole in it, and scientists followed the chlorine from refrigerators to the sky. The repair is slow, because the molecules last for decades. But the measurements say it is working. Each southern spring the hole over Antarctica still opens, and each year scientists measure how it is changing.

- 1913: Fabry and Buisson find the layer
- 1930: Chapman explains how sunlight makes it
- 1974: Molina and Rowland warn about CFCs
- 1985: the hole over Antarctica is reported
- 1987: the Montreal Protocol
- ~2066: expected recovery over Antarctica

![Ground stations, weather balloons and satellites all watch the layer; the long record comes from putting the three together. AI-generated still.](https://assets.planetpast.com/a/5a/5a7462a52628c069.jpg)

- **~40 years**: from 2026 to the expected Antarctic recovery

- The 2026 scientific assessment
- Each spring’s Copernicus and WMO reports
- 16 September: World Ozone Day

Epilogue

#### Sources and notes

Every figure in this film comes from the sources below, which were read for it. Recovery dates are projections from the 2022 assessment and depend on current policies. The 3D scenes are teaching models and not to scale. All stills are AI-generated illustrations, not photographs of real places, instruments or people.

Sources: [Down To Earth (16 Sep 2026): ozone hole reaches notable threshold early again as area increases above average in 2026, Copernicus](https://www.downtoearth.org.in/climate-change/ozone-hole-reaches-notable-threshold-early-again-as-area-increases-above-average-in-2026-copernicus); [MercoPress (21 Oct 2026): the area of the Antarctic ozone hole during September reached 15 million km², says Copernicus](https://en.mercopress.com/2026/10/21/the-area-of-antarctic-ozone-hole-during-september-reached-15-million-km2-says-copernicus); [Wikipedia: Ozone layer](https://en.wikipedia.org/wiki/Ozone_layer); [Wikipedia: Dobson unit](https://en.wikipedia.org/wiki/Dobson_unit); [Wikipedia: Ozone–oxygen cycle (Chapman cycle)](https://en.wikipedia.org/wiki/Ozone%E2%80%93oxygen_cycle); [Wikipedia: Ozone depletion](https://en.wikipedia.org/wiki/Ozone_depletion); [MIT News (1995): Mario Molina shares the Nobel Prize in Chemistry](https://news.mit.edu/1995/molina-1018); [UKRI / NERC: the story behind the discovery of the ozone hole](https://www.ukri.org/who-we-are/how-we-are-doing/research-outcomes-and-impact/nerc/the-story-behind-the-discovery-of-the-ozone-hole/); [Wikipedia: Montreal Protocol](https://en.wikipedia.org/wiki/Montreal_Protocol); [Down To Earth (16 Sep 2025): WMO Ozone Bulletin, the ozone layer on track to recover](https://www.downtoearth.org.in/amp/story/environment/world-ozone-day-earths-protective-layer-on-track-to-return-to-1980s-levels-by-mid-century-says-wmo); [NBC News (Jan 2023): UN says ozone layer slowly healing, hole to mend by 2066](https://www.nbcnews.com/science/environment/un-says-ozone-layer-slowly-healing-hole-mend-2066-rcna64927)