What If? Lab
What if Earth had no ozone layer?
High in the sky there is a thin layer of a gas called ozone. It soaks up most of the Sun's harmful light. What if it was not there? Let's imagine it with Olly!
High above the clouds, a gas called ozone soaks up most of the Sun's harmful ultraviolet light before it reaches us. Chemicals made by people thinned it, so in 1987 countries agreed to phase them out. Imagine Earth with no ozone layer at all, and follow what might change.
About 90 percent of Earth's ozone sits in the stratosphere, where it absorbs all of the Sun's UV-C and most of its UV-B radiation. Ozone-depleting substances thinned it from the 1970s, and the 1987 Montreal Protocol is now letting it recover. This thought experiment removes the layer and reasons about what might follow.
The big question
Sunlight helps us see and keeps us warm.
But some sunlight can hurt our skin and eyes. A gas high in the sky blocks most of it. What if that gas was gone?
The Sun sends out light we can see, and also ultraviolet light, or UV, which we cannot see. Too much UV can harm skin, eyes and plants.
High above us, a gas called ozone soaks up most of the harmful UV. What if Earth had no ozone layer at all? In this thought experiment you can follow three paths: what the Sun's UV light does, how people protect themselves, and how the real ozone layer is healing.
Stratospheric ozone is a thin shield: brought down to the surface as pure gas, all of Earth's ozone makes a layer only about 3 millimeters thick, which NOAA's ozone report gives as 300 Dobson units.
This thought experiment deletes that shield and asks what might change for surface ultraviolet radiation, for living things and for the stratosphere itself. It cannot say how Earth might lose its ozone; it reasons from what ozone does today, using the WMO/UNEP Scientific Assessment of Ozone Depletion: 2022 and NASA and NOAA monitoring.
Explore the idea
Make a prediction
What does the ozone layer do for us?
What does the ozone layer high in the sky do for life on Earth?
Which job does stratospheric ozone do for life at the surface?
- Blocks most of the Sun's harmful UV light
- Makes the rain fall
- Keeps the Moon close to Earth
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Established fact
The ozone layer blocks most of the Sun's harmful light.
The ozone layer soaks up all of the Sun's UV-C and most of its UV-B, and the Montreal Protocol is helping it recover (NOAA, NASA, UNEP).
Stratospheric ozone absorbs all UV-C and most UV-B; controls under the Montreal Protocol are driving its gradual recovery (NOAA, NASA, UNEP).
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What the model shows
Scientists use computers to see when the ozone layer might heal.
Recovery dates such as 2066 for Antarctica come from computer models that assume countries keep following the Montreal Protocol.
Recovery years are chemistry-climate model projections assuming continued compliance; they depend on future greenhouse gas emissions.
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An idea scientists discuss
A world with no ozone layer is pretend. We imagine what might happen.
Earth's ozone layer is not going away. What might happen without it is an idea built from what ozone does today.
Every consequence of removing the ozone layer here is reasoned from present-day processes and might differ greatly in reality.
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Established fact
Most of Earth's ozone is high up in the sky.
About 90 percent of Earth's ozone is in the stratosphere, and the part with the most ozone, about 15 to 35 kilometers up, is called the ozone layer.
About 90 percent of atmospheric ozone is stratospheric; the peak concentration, about 15–35 km up, is the ozone layer (NOAA, Twenty Questions 2022).
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Established fact
All the ozone squeezed together is only as thick as two coins stacked up.
Brought down to the ground as pure ozone, all of Earth's ozone makes a layer only about 3 millimeters thick, about the height of a stack of two coins.
Brought to the surface as pure gas, the total ozone column averages about 3 mm, or 300 Dobson units (NOAA).
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Established fact
The ozone layer blocks most of the Sun's harmful UV light.
UV-C is all soaked up in the ozone layer, most UV-B is soaked up, and UV-A mostly passes through.
UV-C (100–280 nm) is entirely absorbed within the ozone layer, most UV-B (280–315 nm) is absorbed, and UV-A (315–400 nm) is only weakly absorbed (NOAA).
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Established fact
Too much UV can hurt skin, eyes and plants.
Too much UV-B raises the risk of skin cancer and eye cataracts, and it can damage crops, other plants and tiny living things in the sea.
Increased UV-B exposure raises the risks of skin cancer and cataracts and suppresses the immune system; excessive UV-B damages crops, single-celled organisms and aquatic ecosystems (NOAA).
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An idea scientists discuss
Without ozone, much more strong sunlight might reach the ground.
Without ozone, much more UV-B might reach the ground, and plants, animals and people might all be harmed more by sunlight.
Removing the ozone column might raise surface UV-B far beyond measured levels and let more UV-C penetrate; EPA notes ordinary oxygen also absorbs UV-C, so the amount is beyond this experiment.
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An idea scientists discuss
The air high above the clouds might change too.
Ozone warms the stratosphere by soaking up sunlight. Without it, that high layer of air might stay colder.
Ozone's absorption of sunlight is a natural stratospheric heat source that makes temperature rise with altitude (NOAA); without it, the stratosphere's warm upper part might disappear.
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Established fact
When your shadow is shorter than you, the Sun's UV is stronger.
EPA's shadow rule: when your shadow is shorter than you, around midday, UV is stronger, so seek shade and protect your skin and eyes.
EPA's UV Index scale: 1–2 low, 3–7 protection needed, 8 and above extra protection; a shadow shorter than you signals higher UV exposure.
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Established fact
Less ozone overhead means stronger UV.
Measurements around the world show that when the ozone overhead drops by half, the UV Index doubles.
Observations at six sites show a 50 percent decline in total ozone is associated with a factor-of-two increase in the UV Index (NOAA).
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What the model shows
Scientists used computers to imagine a world without the 1987 agreement.
Computer models of a world without the Montreal Protocol show the springtime UV Index in Antarctica doubling between 1996 and 2020.
Simulations without the Montreal Protocol show springtime UVI in Antarctica doubling and mid-southern summer UVI rising about 20 percent over 1996–2020 (NOAA).
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An idea scientists discuss
Without ozone, people might need shade and hats much more often.
With no ozone layer at all, people might need sun protection on many more days, and for longer.
With no ozone column, the UV Index might sit in EPA's extreme range far more often; how high it might go is beyond this experiment.
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Established fact
In 1987 countries agreed to stop using chemicals that hurt the ozone layer.
The Montreal Protocol was signed in 1987 and came into force in 1989. It phases out the chemicals that destroy ozone.
The Montreal Protocol, signed in 1987 and in force since 1989, phases out production and consumption of ozone-depleting substances; the 2016 Kigali Amendment phases down HFCs (UNEP Ozone Secretariat).
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Established fact
The ozone hole in 2025 was smaller than in many years before.
NASA and NOAA ranked the 2025 Antarctic ozone hole the fifth smallest since 1992. It averaged about 18.71 million square kilometers at the height of the season.
The 2025 hole averaged 18.71 million km² from Sept. 7 to Oct. 13 and was the fifth smallest since 1992 (14th smallest in the 46-year satellite record); NOAA's Stephen Montzka reports Antarctic ozone-depleting substances down about a third since about 2000, relative to pre-ozone-hole levels (NASA).
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What the model shows
Scientists expect the ozone layer to heal in your lifetime.
If current policies stay in place, scientists expect the ozone layer to return to 1980 levels around 2066 over Antarctica, 2045 over the Arctic and 2040 over the rest of the world.
The WMO/UNEP Scientific Assessment of Ozone Depletion: 2022 projects a return to 1980 values around 2066 (Antarctic), 2045 (Arctic) and 2040 (rest of the world) if current policies remain (reported by UNEP); future greenhouse gas emissions add uncertainty to the model projections (NOAA).
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An idea scientists discuss
Our world with no ozone is pretend. The real ozone layer is healing.
The real ozone layer was never lost. Our no-ozone world is pretend, and its changes are ideas built from what ozone does today.
Every consequence of a missing ozone layer here is reasoned from present-day absorption and effects; the real layer thinned but was never removed, and it is recovering.
In our world
Ozone is made of oxygen, like the air we breathe. Most of it is high in the sky.
It works like sunscreen for the whole planet.
An ozone molecule is made of three oxygen atoms. The oxygen we breathe has two. NOAA's ozone report says about 90 percent of Earth's ozone is in the stratosphere, the layer of air above the clouds, and the part with the most ozone, about 15 to 35 kilometers up, is called the ozone layer.
Ozone is made up there by sunlight: UV light breaks oxygen molecules apart, and the loose oxygen atoms join other oxygen molecules to make ozone. NASA calls the layer a planetary sunscreen.
Ozone (O₃) is formed in the stratosphere when solar ultraviolet radiation splits O₂ and each free oxygen atom binds to another O₂ molecule. Production is largest in the tropical stratosphere and is balanced by destruction reactions involving hydrogen and nitrogen oxides, chlorine and bromine.
NOAA's report places about 90 percent of ozone in the stratosphere (from 10–15 km up to about 50 km) and about 10 percent in the troposphere. Even at the peak of the layer there are only a few thousand ozone molecules for every billion air molecules.
Imagine no ozone layer
Let's pretend the ozone layer is gone.
More of the Sun's strong light might reach the ground. Let's find out why that matters.
Now imagine the whole ozone layer gone. Today it soaks up all of the Sun's most dangerous UV, called UV-C, and most of the next kind, UV-B. Without it, much more UV-B might reach the ground, and some UV-C might get farther down too, though ordinary oxygen also soaks up UV-C.
People, animals, crops and tiny living things in the sea might all be exposed to more harmful light. The air high above the clouds might change as well. These are ideas: we reason from what ozone does today.
With no ozone column, surface UV-B might rise far beyond anything measured, and UV-C, now entirely absorbed within the ozone layer, might reach lower altitudes; EPA notes that ordinary oxygen also absorbs UV-C, so how much might arrive is beyond this experiment.
The stratosphere's structure might change too. NOAA's report explains that ozone's absorption of sunlight is a natural heat source there, which is why temperature rises with altitude in the stratosphere. Without that heating, the layer might lose its warm top. These are reasoned ideas, not predictions.
Why the ozone layer matters
Too much UV light can give you a sunburn.
It can hurt plants and tiny sea creatures too. The ozone layer blocks most of it.
Scientists sort the Sun's UV into three kinds. NOAA's ozone report says UV-C is all soaked up in the ozone layer, most UV-B is soaked up and the rest reaches the ground, and UV-A mostly passes through.
UV-B causes sunburn, and too much of it raises the risk of skin cancer and eye cataracts. It can also damage crops, other plants and tiny living things in the sea. Measurements show that when the ozone overhead drops by half, the UV Index doubles.
UV-C (100–280 nm) is entirely absorbed within the ozone layer; UV-B (280–315 nm) is mostly absorbed; UV-A (315–400 nm) is only weakly absorbed. Increased UV-B exposure raises the risks of skin cancer and cataracts and suppresses the immune system, and excessive UV-B damages crops, single-celled organisms and aquatic ecosystems.
Observations at six sites show a 50 percent decline in total ozone goes with a doubling of the UV Index. Near the ground, by contrast, extra ozone from air pollution is harmful to breathe: NOAA's report calls it "bad" ozone.
A layer that is healing
Long ago, some sprays and fridges used chemicals that hurt the ozone layer.
In 1987 countries agreed to stop. Now the ozone layer is slowly getting better!
Chemicals called CFCs were once used in aerosol sprays, foams, air conditioners and refrigerators. High in the sky, they release chlorine and bromine, which destroy ozone. A deep thinning, called the ozone hole, appears over Antarctica every spring.
In 1987 countries agreed the Montreal Protocol to phase these chemicals out. NASA and NOAA say the layer is gradually recovering: the 2025 Antarctic ozone hole was the fifth smallest since 1992. The 2022 WMO/UNEP Scientific Assessment of Ozone Depletion expects it to return to 1980 levels over Antarctica around 2066 if current policies stay in place.
The 1987 Montreal Protocol, built on the framework of the 1985 Vienna Convention, entered into force in 1989 and has been amended since, most recently by the 2016 Kigali Amendment on HFCs. The WMO/UNEP Scientific Assessment of Ozone Depletion: 2022 confirms the phase-out of nearly 99 percent of banned ozone-depleting substances (UNEP).
NOAA's Stephen Montzka reports that these substances in the Antarctic stratosphere have fallen by about a third, relative to pre-ozone-hole levels, since peaking around 2000. If current policies remain, the 2022 assessment expects the layer to return to 1980 values around 2066 over the Antarctic, 2045 over the Arctic and 2040 for the rest of the world.
Amazing facts
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One tiny bit of chlorine can break apart lots and lots of ozone.
EPA says a single chlorine atom can destroy more than 100,000 ozone molecules before it is removed from the stratosphere.
One chlorine atom can destroy over 100,000 ozone molecules before it is removed from the stratosphere (EPA).
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There is a special day for the ozone layer: 16 September.
The day the Montreal Protocol was agreed, 16 September, is now the International Day for the Preservation of the Ozone Layer.
16 September, the date the Montreal Protocol was agreed, is celebrated as the International Day for the Preservation of the Ozone Layer (NOAA, Twenty Questions 2022).
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The name ozone comes from a word that means to smell.
The word ozone comes from the Greek word ozein, meaning to smell. Ozone has a sharp smell that can be noticed even in tiny amounts.
Ozone was discovered in laboratory experiments in the mid-1800s; its name comes from the Greek ozein, to smell, and its pungent odor can be detected at very low amounts (NOAA).
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Scientists send balloons up high to measure ozone.
NOAA scientists measure ozone above the South Pole with instruments carried on weather balloons. The lowest value ever recorded there was 92 Dobson units, in October 2006.
Ozonesonde balloon data over the South Pole showed the 2025 season's lowest value, 147 DU, on Oct. 6; the lowest ever recorded there was 92 DU in October 2006 (NASA).
Words to know
- ozone
- a gas whose molecules are made of three oxygen atoms
- ozone layer
- the part of the stratosphere, about 15 to 35 km up, with the most ozone
- stratosphere
- the layer of air above the one we live in, high above the clouds
- ultraviolet (UV) light
- light from the Sun that our eyes cannot see; too much can harm skin, eyes and plants
- CFCs
- chlorofluorocarbons, chemicals once used in sprays and fridges that destroy ozone high in the sky
- Montreal Protocol
- the 1987 agreement between countries to phase out chemicals that destroy ozone
- UV Index
- a number that tells how strong the Sun's UV light is at a place and time
Discovery challenge
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How many oxygen atoms make one ozone molecule?
How many oxygen atoms are in one molecule of ozone?
An ozone molecule (O₃) contains how many oxygen atoms?
- Three
- Two
- One
Show the answer
Three
Three! The oxygen we breathe has two.
Three. The oxygen we breathe has two oxygen atoms in each molecule.
Three; atmospheric oxygen (O₂), about 21 percent of the air, has two (NOAA).
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In which year did countries agree to stop using chemicals that hurt the ozone layer?
In which year was the Montreal Protocol, the agreement to protect the ozone layer, signed?
The Montreal Protocol on Substances that Deplete the Ozone Layer was signed in which year?
- 1987
- 1957
- 2016
Show the answer
1987
1987!
1987. It came into force in 1989.
1987, in force from 1989; 2016 is the year of the Kigali Amendment (UNEP Ozone Secretariat).
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If we squeezed all the ozone into one layer at the ground, how thick is it?
If all of Earth's ozone were brought down to the ground as pure ozone, about how thick a layer does NOAA say it makes?
Total column ozone of about 300 Dobson units corresponds to a layer of pure ozone at the surface about how thick?
- About 3 millimeters
- About 3 meters
- About 3 kilometers
Show the answer
About 3 millimeters
Only about 3 millimeters. Very thin!
About 3 millimeters, which scientists call 300 Dobson units.
About 3 mm (0.12 in); 100 DU corresponds to 1 mm (NOAA, NASA).
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Which shadow tells you to find some shade?
By EPA's shadow rule, which shadow means the Sun's UV is stronger?
Under EPA's shadow rule, which observation signals higher UV exposure?
- A shadow shorter than you
- A shadow taller than you
- No shadow because of clouds
Show the answer
A shadow shorter than you
A short shadow means strong sun. Find shade!
A shadow shorter than you, around midday, means stronger UV: seek shade and protect your skin and eyes.
A shadow shorter than you (around midday) indicates higher UV exposure (EPA).
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When might the ozone hole over Antarctica be healed?
If current policies stay in place, when do scientists expect the ozone layer over Antarctica to return to 1980 levels?
Under current policies, the 2022 assessment projects Antarctic ozone returning to 1980 values around…
- Around the 2060s
- Next year
- Never
Show the answer
Around the 2060s
Around the 2060s. It is slowly getting better.
Around 2066, according to the 2022 WMO/UNEP scientific assessment. NASA says the late 2060s.
Around 2066 over the Antarctic, 2045 over the Arctic and 2040 elsewhere (WMO/UNEP Scientific Assessment of Ozone Depletion: 2022, reported by UNEP).
Experiments to try
- Watch Try it What If? Lab What if Earth had no atmosphere? Earth's air works like a jacket: it keeps heat in and carries it around. Take the air away in our model of a sandy patch of ground on the equator, and the coldest part of the night drops from about 13 °C to about 2 °C.
- Watch What If? Lab What if the air were full of dust? Every year, winds lift millions of tons of dust from the Sahara Desert and carry it across the Atlantic Ocean. Dust and other tiny particles in the air can color sunsets red, make the sky hazy and get into our lungs. Imagine air that was always full of dust, and find out why clean air matters.
- Watch What If? Lab What if hurricanes could be steered? Hurricanes are huge spinning storms that grow over warm ocean water. Nobody can steer or stop one today: a hurricane holds far more energy than people could ever push around. What people can do is forecast where it may go and get ready early. Imagine steering one, then see why forecasts are the real answer.
Explore next
- Watch Try it What If? Lab What if Earth had no atmosphere? Earth's air works like a jacket: it keeps heat in and carries it around. Take the air away in our model of a sandy patch of ground on the equator, and the coldest part of the night drops from about 13 °C to about 2 °C.
- Watch What If? Lab What if the air were full of dust? Every year, winds lift millions of tons of dust from the Sahara Desert and carry it across the Atlantic Ocean. Dust and other tiny particles in the air can color sunsets red, make the sky hazy and get into our lungs. Imagine air that was always full of dust, and find out why clean air matters.
- Watch What If? Lab What if hurricanes could be steered? Hurricanes are huge spinning storms that grow over warm ocean water. Nobody can steer or stop one today: a hurricane holds far more energy than people could ever push around. What people can do is forecast where it may go and get ready early. Imagine steering one, then see why forecasts are the real answer.
- Watch Try it What If? Lab What if there were no greenhouse gases? Greenhouse gases such as carbon dioxide and water vapor hold some of Earth's heat in, like a blanket. Take them all away in our energy-balance model and Earth's average temperature falls from about 15 °C to about −19 °C, well below freezing.
- Watch Try it What If? Lab What if there were more greenhouse gases? Adding greenhouse gases makes it harder for Earth's heat to escape, so the planet warms until it balances again. In our model a small change in how freely heat escapes warms Earth by about 2.6 °C, and the oceans make the warming take years.
- Watch Try it What If? Lab What if Earth were covered in ice? White ice bounces sunlight back to space, so an icy planet soaks up less of the Sun's warmth. In our energy-balance model, making Earth reflect 60 percent of its sunlight instead of 30 percent drops its average temperature from about 15 °C to about −23 °C.
- Watch Try it What If? Lab What if glaciers melted? Glaciers are huge, slow-moving masses of ice on land. When land ice melts, its water runs into the ocean and raises the sea. In our model, melting every mountain glacier on Earth would lift the sea by about 40 centimeters; scientists' best estimate is a little less, about 32.
- Watch Try it What If? Lab What if sea levels rose? When the sea rises, the water's edge moves inland. How far depends on the land: on a steep cliff it barely moves, but on a flat, gentle coast a rise of 76 centimeters in our model moves the shore about 760 meters.
For parents & teachers
What children will learn
- The ozone layer in the stratosphere absorbs all UV-C and most UV-B radiation from the Sun.
- Too much UV-B harms skin, eyes, plants and sea life, which is why people use shade, hats, sunglasses and sunscreen.
- Chemicals such as CFCs thinned the ozone layer, and the 1987 Montreal Protocol is helping it recover.
- Thought experiments separate known facts, model projections and reasoned ideas.
Recommended ages: 11 to 17 · About 15 minutes
Talk about it
- Why did the ozone problem need almost every country in the world to agree?
- The ozone layer is slowly healing. What does that tell us about solving other environmental problems?
- Why is it important to call the no-ozone outcomes ideas rather than facts?
Try at home or in class
- Try EPA's shadow rule on a sunny day: compare your shadow with your height in the morning, at midday and in the late afternoon. When your shadow is shorter than you, UV is stronger, so move into the shade.
- Look up today's UV Index and match it to EPA's scale (1–2 low, 3–7 protection needed, 8 and above extra protection), then pack the right sun protection for an outing.
Key vocabulary: ozone, ozone layer, stratosphere, ultraviolet (UV) light, CFCs, Montreal Protocol, UV Index
Where these facts come from
- Twenty Questions and Answers About the Ozone Layer: 2022 Update , NOAA Chemical Sciences Laboratory (WMO/UNEP Scientific Assessment of Ozone Depletion: 2022)
- NASA, NOAA Rank 2025 Ozone Hole as 5th Smallest Since 1992 , NASA
- Basic Ozone Layer Science , U.S. Environmental Protection Agency
- UV Index Scale , U.S. Environmental Protection Agency
- Ozone layer recovery is on track, helping avoid global warming by 0.5°C , UN Environment Programme
- The Montreal Protocol on Substances that Deplete the Ozone Layer , UNEP Ozone Secretariat
Checked, not yet human-reviewed. Last updated October 24, 2026. Spotted a mistake? Tell us.