Chapter 6 of 8

All 8 chapters
  1. 1The air and its layers, readunderstood
  2. 2Weather and air pressure, readunderstood
  3. 3Winds, monsoons and storms, readunderstood
  4. 4Climate zones, readunderstood
  5. 5Air pollution, readunderstood
  6. 6The greenhouse effect, readunderstood
  7. 7Climate change, readunderstood
  8. 8Responding to climate change, readunderstood

S4.14.6 Chapter 6

The greenhouse effect

The Moon is the same distance from the Sun as the Earth, but it is bitterly cold at night. Why is the Earth warm enough for life?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

  • You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain the greenhouse effect with a labelled diagram
  • Name the main greenhouse gases and where they come from

The Sun gives the Earth energy as light. About 30 % of it is reflected straight back to space by clouds, ice and bright ground. The rest is taken in by the air, the sea and the land. The ground warms, and anything warm gives out infrared, a kind of wave we feel as heat.

Over time the Earth must send out as much energy as it takes in. If it sent out less, it would warm up. If it sent out more, it would cool down. The gases of the air decide how fast the infrared gets out.

Scientists have worked out what the average temperature at the Earth’s surface would be with no greenhouse gases in the air. It would be about −18 °C. The real average is about 15 °C.

Two thermometers side by side, each with a scale from −30 °C to 30 °C. The left one, labelled without, shows −18 °C, for the Earth without greenhouse gases. The right one, labelled with, shows 15 °C, for the Earth with them. Labels: without, with, the scale numbers and °C.−30−20−100102030°Cwithout−30−20−100102030°Cwith
The average surface temperature of the Earth without greenhouse gases, and with them, as it is now.

How does it work? Follow it in three steps. Sunlight is mostly visible light. It goes through the air almost without being stopped, and it warms the ground.

Draw it first

The greenhouse effect in three steps

  1. A picture of the Sun at the top left, the air as a dotted band across the middle with four round greenhouse gas molecules in it, and the ground as a hatched strip at the bottom. Step 1: two solid arrows of sunlight go from the Sun slantwise through the air, past the molecules, to the ground. Step 2: four dashed arrows of infrared go up from the ground. Two go straight up out of the air to space. Two stop at a molecule. Step 3: from each of those two molecules, two dashed arrows go out, one up and one down. The ones going down reach the ground. A key at the bottom shows solid arrows for sunlight, dashed arrows for infrared and a hatched disc for greenhouse gas. Labels: Sun, space, ground, sunlight, greenhouse gas, infrared.Sunspacegroundsunlightgreenhouse gas

    1 Sunlight goes through the air to the ground. The molecules of greenhouse gas do not stop it.

  2. A picture of the Sun at the top left, the air as a dotted band across the middle with four round greenhouse gas molecules in it, and the ground as a hatched strip at the bottom. Step 1: two solid arrows of sunlight go from the Sun slantwise through the air, past the molecules, to the ground. Step 2: four dashed arrows of infrared go up from the ground. Two go straight up out of the air to space. Two stop at a molecule. Step 3: from each of those two molecules, two dashed arrows go out, one up and one down. The ones going down reach the ground. A key at the bottom shows solid arrows for sunlight, dashed arrows for infrared and a hatched disc for greenhouse gas. Labels: Sun, space, ground, sunlight, greenhouse gas, infrared.Sunspacegroundsunlightinfraredgreenhouse gas

    2 The warm ground gives out infrared. Some goes straight out to space. Some meets a molecule of greenhouse gas and is absorbed.

  3. A picture of the Sun at the top left, the air as a dotted band across the middle with four round greenhouse gas molecules in it, and the ground as a hatched strip at the bottom. Step 1: two solid arrows of sunlight go from the Sun slantwise through the air, past the molecules, to the ground. Step 2: four dashed arrows of infrared go up from the ground. Two go straight up out of the air to space. Two stop at a molecule. Step 3: from each of those two molecules, two dashed arrows go out, one up and one down. The ones going down reach the ground. A key at the bottom shows solid arrows for sunlight, dashed arrows for infrared and a hatched disc for greenhouse gas. Labels: Sun, space, ground, sunlight, greenhouse gas, infrared.Sunspacegroundsunlightinfraredgreenhouse gas

    3 The molecule gives the energy out again as infrared, in every direction. Some goes up and out. Some goes back down and warms the ground again.

The gases that do this are the greenhouse gases. The main ones have molecules of three or more atoms, and these take in infrared. Nitrogen and oxygen make up about 99 % of dry air. Their molecules have only two atoms of the same kind, and they take in almost none. So nearly all the air lets infrared pass, and only a small share of the air does the work.

The picture shows an important point. A greenhouse gas does not trap sunlight. Sunlight passes through. A greenhouse gas absorbs the infrared that the ground gives out, and sends part of it back. It slows the escape of heat.

The name comes from the glass greenhouse where plants grow. A glass greenhouse works in a different way, mainly by keeping warm air from blowing away. But the name stuck, and the effect of the air is similar: it lets energy in and slows it getting out.

There are four main greenhouse gases. Water vapour is the biggest part of the natural effect. Its amount depends on the temperature: warmer air holds more. Carbon dioxide, CO₂, is about 0.042 % of the air. Methane, CH₄, and nitrous oxide, N₂O, are present in much smaller amounts but trap more heat for each molecule.

A table with the columns Gas and Main source. Row 1: CO₂, burning fuels. Row 2: CH₄, cattle, rice. Row 3: N₂O, fertiliser. Row 4: H₂O, sea, land.GasMain sourceCO₂burning fuelsCH₄cattle, riceN₂OfertiliserH₂Osea, land
The main greenhouse gases and where most of the extra amount comes from. Water vapour comes from the sea and the land, and its amount follows the temperature.

Of the gases that people add, carbon dioxide has the biggest effect. There is so much of it, and it stays in the air for centuries. People add it when they burn coal, oil and gas, make cement and clear forests. Burning fossil fuels and making cement puts about 37 billion tonnes of carbon dioxide into the air each year. About half of that stays in the air. The sea and the plants take in the rest.

Methane comes from cattle, rice grown in flooded fields and rubbish tips. It leaks from natural gas wells and coal mines. It also comes from wetlands. Over 100 years it is about 28 times as strong as carbon dioxide for each kg. But it lasts only about a dozen years. Nitrous oxide comes mainly from fertiliser put on farmland. It is about 270 times as strong as carbon dioxide for each kg, and it lasts over a hundred years.

Carbon dioxide is made when carbon burns: C + O₂ → CO₂. A carbon atom has a mass of 12 and a CO₂ molecule has a mass of 44. So 12 kg of carbon makes 44 kg of carbon dioxide. The extra mass is the oxygen taken from the air.

A table with the columns Part and Mass. Row 1: C, 12. Row 2: O₂, 32. Row 3: CO₂, 44. The mass is in the same unit, for each atom or molecule.PartMassC12O₂32CO₂44
Masses of an atom of carbon, a molecule of oxygen and a molecule of carbon dioxide, in the same units. 12 + 32 = 44.

Worked example

Coal is mostly carbon. A power station burns 6 kg of carbon. How many kg of carbon dioxide does it make?

  1. From C + O₂ → CO₂, 12 kg of carbon makes 44 kg of carbon dioxide.
  2. 6 kg is half of 12 kg, so it makes half of 44 kg.
  3. Half of 44 is 22. So 6 kg of carbon makes 22 kg of carbon dioxide.

Answer: 22 kg

How much carbon dioxide is in the air? The amount has been measured at a mountain observatory far from cities since 1958, and other stations agree. Before about 1750 it was about 280 ppm. Look at the rise in recent decades.

A line graph of carbon dioxide in the air, in ppm, from 240 to 440 in steps of 40, against the years 1960 to 2020 in steps of 20. Seven dots are joined by a line that rises steadily, from 317 ppm in 1960 to 414 ppm in 2020, and the line bends upward. A dashed line across the graph at 280 ppm is labelled before 1750. Labels: CO₂ in ppm, before 1750, the scale numbers and years.CO₂ in ppm2402803203604004401960198020002020before 1750
Carbon dioxide in the air, in ppm, rounded. It was about 280 ppm before 1750, and in 2023 it was about 420 ppm.

Worked example

Before 1750 the air had about 280 ppm of carbon dioxide. In 2023 it had about 420 ppm. By what percent has it risen?

  1. Find the rise: 420 − 280 = 140 ppm.
  2. Divide by the starting value: 140 ÷ 280 = 0.5.
  3. As a percent, 0.5 × 100 = 50 %. The air has about 50 % more carbon dioxide.

Answer: 50 %

Do not mix this up with the hole in the ozone layer. They are two different problems. The ozone hole is a thin patch in the stratosphere, caused by CFCs, that lets more ultraviolet reach the ground. The greenhouse effect is about infrared and the lowest layer of the air.

See it three ways

As a picture

A table with the columns About, Ozone and Warming. Ozone means the ozone hole, and Warming means the greenhouse effect. Row 1, where: high up, low down. Row 2, gas: CFCs, CO₂. Row 3, harm: UV, warming. Row 4, fix: ban CFCs, cut CO₂.AboutOzoneWarmingwherehigh uplow downgasCFCsCO₂harmUVwarmingfixban CFCscut CO₂

In words

The ozone hole is high in the air and lets in more ultraviolet. The greenhouse effect is lower down and is about heat. The gases and the fixes are different.

In symbols

Ozone hole: CFCs → less O₃ → more UV. Greenhouse: more CO₂ → more infrared held → warming.

Try it

A jar model of slowing the heat

You need: two clear jars or glasses, the same size, two thermometers, if you have them, a clear plastic bag or cling film, a sunny place, and a clock.

Be safe: Never look at the Sun. Do not leave jars in a closed car or room in strong sun, because they can get very hot. Keep the plastic bag away from children, who must never put it over their heads. Do not touch the glass if it feels hot.

  1. Stand the two jars side by side in a sunny place, on a mat. Put a thermometer in each, and read them. They should read the same.
  2. Cover one jar with the clear plastic or cling film. Leave the other open.
  3. After 30 minutes, read both thermometers again. With no thermometer, put the back of a finger on the outside of each jar. Do not touch anything hot.

What you will see: The covered jar is warmer than the open one. This is only a model. The cover mainly stops the warm air from leaving, while the real atmosphere absorbs infrared. But it shows the idea: if something slows the escape of heat, a place gets warmer.

If you cannot try it, this is what happens.
A table with the columns Jar and After 30 minutes. Row 1: open, 31 °C. Row 2: covered, 38 °C. These are example readings.JarAfter 30 minopen31 °Ccovered38 °C

A wrong idea many people have

Greenhouse gases trap sunlight.

Why it feels right: The word greenhouse suggests a glass box that lets the Sun in and keeps it there. So we think the gases trap the light.

The test that breaks it: Look at the three steps. The sunlight arrows go right through the air to the ground. It is the infrared arrows, going up from the warm ground, that the molecules take in.

What is true: Sunlight passes through the air. Greenhouse gases absorb the infrared that the warm ground gives out, and send part of it back down.

A wrong idea many people have

The hole in the ozone layer causes global warming.

Why it feels right: Both are about gases in the air that people changed. Both are in the news. So they seem to be one problem.

The test that breaks it: The ozone hole lets in more ultraviolet light, not more heat. It forms high up, in the stratosphere. The greenhouse effect is about infrared and the lowest layer of the air. The table shows how different they are.

What is true: They are separate problems with separate gases and fixes. Some of the gases that damaged ozone, the CFCs, are greenhouse gases too. But the hole in the ozone layer is not what warms the Earth.

A wrong idea many people have

The greenhouse effect is bad in itself, and it would be better without it.

Why it feels right: We hear the greenhouse effect mentioned only as a problem, so it seems always to be harmful.

The test that breaks it: Look at the two thermometers. With no greenhouse gases the Earth would be at about −18 °C. Oceans would freeze, and most life would not survive.

What is true: The natural greenhouse effect keeps the Earth warm enough for life. The problem is that people are making it stronger by adding greenhouse gases.

Key words

infrared
a kind of wave that warm things give out. We feel it as heat.
greenhouse gas
a gas that absorbs infrared and sends some of it back down.
water vapour
water as a gas in the air. It is the biggest part of the natural greenhouse effect.
carbon dioxide
(CO₂) the greenhouse gas that people add most of, by burning fuels.
methane
(CH₄) a greenhouse gas from cattle, rice fields, rubbish tips and gas leaks.
nitrous oxide
(N₂O) a greenhouse gas that comes mainly from fertiliser.

Back to your guess

The answer is A: The air keeps some of the heat the ground gives out, so the Earth is warmer than it would be without air.

The Earth and the Moon are the same distance from the Sun. The main difference is the air. The Moon’s nights also last about two weeks, so it has a long time to cool. Some gases in the air absorb the infrared that the warm ground gives out and send some of it back down. This is the greenhouse effect. Without it the Earth would be well below freezing.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which kind of wave do greenhouse gases absorb most, as it leaves the ground?

    Tap your answer.

    1. Not this one. Sunlight passes through the air. The gases absorb the infrared that goes up from the warm ground. Go to S4.10.7. Try another.

    2. Not this one. Ultraviolet is taken in by ozone high in the air. The greenhouse gases work mostly on infrared. Go to S4.14.1. Try another.

    3. Yes, that is right. The ground, warmed by sunlight, gives out infrared. Greenhouse gases absorb it and send some of it back down.

    4. Not this one. Sound is not a wave of the same kind as light or infrared. It cannot travel through empty space. Try another.

    See the answer

    Infrared given out by the warm ground

    The ground, warmed by sunlight, gives out infrared. Greenhouse gases absorb it and send some of it back down.

    • If you chose A: Sunlight passes through the air. The gases absorb the infrared that goes up from the warm ground. Go to S4.10.7.
    • If you chose B: Ultraviolet is taken in by ozone high in the air. The greenhouse gases work mostly on infrared. Go to S4.14.1.
    • If you chose D: Sound is not a wave of the same kind as light or infrared. It cannot travel through empty space.

  2. Without greenhouse gases the average surface temperature of the Earth would be about −18 °C. With them it is about 15 °C. By how many °C does the greenhouse effect warm the Earth?

    Tap your answer.

    1. Not this one. You took the difference of 18 and 15, and lost the minus sign. A temperature of −18 °C is below zero, so the gap to 15 °C is bigger. Try another.

    2. Yes, that is right. From −18 °C up to 0 °C is 18 °C. From 0 °C up to 15 °C is 15 °C. So the greenhouse effect warms the Earth by 18 + 15 = 33 °C. Another way is 15 − (−18) = 33.

    3. Not this one. That is the temperature with the greenhouse effect, not the amount it adds. Work out how far it is above −18 °C. Try another.

    4. Not this one. That is how far below zero the cold temperature is. The warming is the gap from −18 °C up to 15 °C, which goes through zero. Try another.

    See the answer

    33 °C

    From −18 °C up to 0 °C is 18 °C. From 0 °C up to 15 °C is 15 °C. So the greenhouse effect warms the Earth by 18 + 15 = 33 °C. Another way is 15 − (−18) = 33.

    • If you chose A: You took the difference of 18 and 15, and lost the minus sign. A temperature of −18 °C is below zero, so the gap to 15 °C is bigger.
    • If you chose C: That is the temperature with the greenhouse effect, not the amount it adds. Work out how far it is above −18 °C.
    • If you chose D: That is how far below zero the cold temperature is. The warming is the gap from −18 °C up to 15 °C, which goes through zero.

  3. Which list names the main greenhouse gases?

    Tap your answer.

    1. Not this one. These make up nearly all the air, but their molecules take in almost no infrared. Go to S4.14.1. Try another.

    2. Not this one. These are pollutants, but they are not the main greenhouse gases. Go to S4.14.5. Try another.

    3. Not this one. Ozone and CFCs are greenhouse gases too, but they are not the main ones. Try another.

    4. Yes, that is right. The four main greenhouse gases are water vapour, carbon dioxide, methane and nitrous oxide.

    See the answer

    Water vapour, carbon dioxide, methane and nitrous oxide

    The four main greenhouse gases are water vapour, carbon dioxide, methane and nitrous oxide.

    • If you chose A: These make up nearly all the air, but their molecules take in almost no infrared. Go to S4.14.1.
    • If you chose B: These are pollutants, but they are not the main greenhouse gases. Go to S4.14.5.
    • If you chose C: Ozone and CFCs are greenhouse gases too, but they are not the main ones.

  4. Which of these is a main source of methane?

    Tap your answer.

    1. Yes, that is right. Methane comes from cattle, rice in flooded fields, rubbish tips, wetlands and leaks of natural gas and from coal mines.

    2. Not this one. Fertiliser is the main source of nitrous oxide, a different greenhouse gas. Try another.

    3. Not this one. Forests take in carbon dioxide. Wetlands, not forests, give out natural methane. Try another.

    4. Not this one. Burning fuel makes mainly carbon dioxide. Methane comes mainly from farming, rubbish tips and gas leaks. Try another.

    See the answer

    Rice grown in flooded fields, and cattle

    Methane comes from cattle, rice in flooded fields, rubbish tips, wetlands and leaks of natural gas and from coal mines.

    • If you chose B: Fertiliser is the main source of nitrous oxide, a different greenhouse gas.
    • If you chose C: Forests take in carbon dioxide. Wetlands, not forests, give out natural methane.
    • If you chose D: Burning fuel makes mainly carbon dioxide. Methane comes mainly from farming, rubbish tips and gas leaks.

  5. Which statement about the ozone hole and the greenhouse effect is true?

    Tap your answer.

    1. Not this one. The hole lets in more ultraviolet. It does not make the Earth warmer. The warming comes from greenhouse gases. Go to S4.14.1. Try another.

    2. Not this one. They are separate. The ozone hole is high in the air and is about UV. The greenhouse effect is about infrared and the lowest layer. Try another.

    3. Yes, that is right. The ozone hole is caused by CFCs and lets in more UV. The greenhouse effect is caused by gases that absorb infrared. Cutting CFCs does not stop global warming.

    4. Not this one. The ozone hole was made by CFCs, not by the greenhouse effect. Try another.

    See the answer

    They are two different problems, with different causes and effects.

    The ozone hole is caused by CFCs and lets in more UV. The greenhouse effect is caused by gases that absorb infrared. Cutting CFCs does not stop global warming.

    • If you chose A: The hole lets in more ultraviolet. It does not make the Earth warmer. The warming comes from greenhouse gases. Go to S4.14.1.
    • If you chose B: They are separate. The ozone hole is high in the air and is about UV. The greenhouse effect is about infrared and the lowest layer.
    • If you chose D: The ozone hole was made by CFCs, not by the greenhouse effect.

  6. How is the Earth’s air like a greenhouse?

    Tap your answer.

    1. Not this one. The air is not glass. And greenhouse gases do not trap sunlight. They take in infrared. Go to S3.8.5. Try another.

    2. Not this one. Most sunlight reaches the ground. That is why the ground warms. Try another.

    3. Not this one. The air does not make heat. It slows the escape of heat that the ground has taken from the Sun. Try another.

    4. Yes, that is right. Sunlight passes through the air to the ground. Greenhouse gases take in the infrared from the ground and send some of it back, so heat leaves more slowly.

    See the answer

    It lets sunlight in and slows the heat getting out.

    Sunlight passes through the air to the ground. Greenhouse gases take in the infrared from the ground and send some of it back, so heat leaves more slowly.

    • If you chose A: The air is not glass. And greenhouse gases do not trap sunlight. They take in infrared. Go to S3.8.5.
    • If you chose B: Most sunlight reaches the ground. That is why the ground warms.
    • If you chose C: The air does not make heat. It slows the escape of heat that the ground has taken from the Sun.

  7. From an earlier chapter

    Which gas from burning fuel makes acid rain?

    Tap your answer.

    1. Not this one. Carbon dioxide makes natural rain only slightly acid. Acid rain comes mostly from other gases. Go to S4.14.5. Try another.

    2. Yes, that is right. Sulfur in coal and oil burns to sulfur dioxide, which dissolves in cloud water to make sulfuric acid.

    3. Not this one. Nitrogen is harmless in the air. Only when it is heated in an engine does it make nitrogen oxides. Go to S4.14.5. Try another.

    See the answer

    Sulfur dioxide

    Sulfur in coal and oil burns to sulfur dioxide, which dissolves in cloud water to make sulfuric acid.

    • If you chose A: Carbon dioxide makes natural rain only slightly acid. Acid rain comes mostly from other gases. Go to S4.14.5.
    • If you chose C: Nitrogen is harmless in the air. Only when it is heated in an engine does it make nitrogen oxides. Go to S4.14.5.

  8. A stove burns 9 kg of carbon. Each 12 kg of carbon makes 44 kg of carbon dioxide. How many kg of carbon dioxide does the stove make?

    See the answer

    33 kg

    9 kg is three quarters of 12 kg. So the carbon dioxide is three quarters of 44 kg, which is 33 kg. Or work out 9 × 44 ÷ 12 = 33 kg.

  9. With the windows shut, the carbon dioxide in a classroom rose from 440 ppm to 550 ppm. By what percent did it rise?

    See the answer

    25 %

    The rise is 550 − 440 = 110 ppm. Divide by the start: 110 ÷ 440 = 0.25, which is 25 %.

Keep it

Copy this into your exercise book.

Sunlight passes in. The warm ground gives out infrared. Greenhouse gases absorb it and send some back down. Without this the Earth would be about −18 °C. CO₂, CH₄, N₂O.

Draw: Draw the Sun, solid arrows to the ground, and dashed arrows up. Add a molecule in the air that sends one arrow up and one down.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

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