Atmosphere and climate. Attnom Academy

S4.14.1 Chapter 1

The air and its layers

Mei climbs a high mountain and finds it hard to breathe. What is the air made of, and why is there less of it up there?

Your guess

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

Your guess

A Air is mostly oxygen, and it is hard to breathe up there because it is colder.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Air is mostly nitrogen, and the air is thinner up there, so each breath holds less oxygen.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Air is mostly carbon dioxide, and it sinks, so there is less of it up high.
You picked C. Read on. At the end, the chapter comes back to your guess.
D Air is the same everywhere. Mei is only tired from the climb.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Give the rough share of each gas in dry air
Name the layers of the atmosphere in order
Explain how the ozone layer protects life from ultraviolet

The atmosphere is the layer of gases that wraps round the Earth. We live at the bottom of it. Gravity holds it in place, as it holds the sea. Air looks like nothing, but it is a mixture of several gases.

Scientists give the make-up of dry air. That means air with its water vapour left out, because the amount of water vapour changes from place to place. It can be almost 0 % over a desert and up to about 4 % in hot, damp air.

Here is dry air as a hundred squares. One square is one percent, so each gas gets as many squares as its percent.

Dry air. The one black square is about 0.9 % argon, 0.04 % carbon dioxide and traces of other gases.

Nitrogen is about 78 % of dry air. Oxygen is about 21 %. Argon, a gas that does not react with anything, is about 0.9 %. Carbon dioxide is only about 0.04 %. It is so small that it does not fill even one square.

Scientists often count a very small share in parts per million, or ppm. One percent is 10 000 ppm. In 2023 the air held about 420 ppm of carbon dioxide. That means 420 molecules in every million. A small share, but later chapters show that it matters a great deal.

Worked example

Out of 20 000 molecules of dry air, about how many are oxygen? Use 21 %.

A share of 21 % means 21 in every 100.
20 000 molecules is 200 lots of 100.
So 200 × 21 = 4 200 molecules.

Answer: 4 200 molecules

Worked example

Carbon dioxide is 420 ppm. Write that as a percent.

420 ppm means 420 parts in every 1 000 000.
A percent is parts in every 100. Count the hundreds in a million: 1 000 000 ÷ 100 = 10 000.
So divide by 10 000: 420 ÷ 10 000 = 0.042 %.

Answer: 0.042 %

The air is thickest at the ground, because the air above is pressing down. As you climb, there is less air above you, so the air is thinner and the air pressure is lower. The picture shows how fast it falls.

Air pressure falls with height. By about 5.5 km, half of the air at sea level is below you. The share of oxygen is still 21 %. There are just fewer molecules in each breath.

This is why Mei finds it hard to breathe. At the top of a high mountain each breath holds fewer oxygen molecules, even though the air is still 21 % oxygen.

The atmosphere has four main layers, one above the other. Look at how the temperature changes as you go up. It falls, then rises, then falls again, and then rises. The turns mark the edges of the layers. The lines are straightened, to show the pattern.

Draw it first

How the temperature changes with height

1 In the lowest layer the air gets colder as you go up. At about 11 km it is about −56 °C.
2 In the next layer the air gets warmer as you go up. A gas called ozone takes in ultraviolet from the Sun there. At about 50 km it is near 0 °C.
3 In the third layer the air gets colder again, to about −86 °C at 86 km. Above that it warms again.

Now we can name the layers. The troposphere is the lowest layer, from the ground to about 12 km. All weather happens here, and about three quarters of all the air is here. It is thinner over the poles, about 8 km, and thicker over the tropics, about 18 km.

Above it is the stratosphere, to about 50 km. It holds the ozone layer. Next comes the mesosphere, to about 85 km, the coldest layer. Most meteors burn up there. Above is the thermosphere, from about 85 km out to about 600 km. The air there is so thin that it hardly feels warm, even though its temperature is high. The edges between layers are not sharp, and the heights change a little with place and season.

The layers on one scale. The troposphere, where we live and where the weather is, is the thin bottom slice.

Things at different heights. A space station circles in the thermosphere.

The sunlight that reaches the Earth has more than light in it. It also has ultraviolet light, or UV. It is a kind of wave just beyond violet light, and we cannot see it. UV has more energy than visible light. It can burn skin, damage the eyes and harm the cells of plants and tiny sea creatures.

Experts split ultraviolet into three kinds. UV-A has the least energy. UV-B has more. UV-C has the most and is the most harmful. A little UV is useful, because skin uses it to make vitamin D. But too much is harmful.

In the stratosphere there is a thin band of ozone, a gas whose molecule is three oxygen atoms joined, O₃. It is spread through the air between about 15 and 35 km. The ozone layer takes in nearly all UV-C and most UV-B. Most UV-A and all of the visible light go through.

A solid arrow is most or all of it going on. A dashed arrow is a little. The ozone layer stops UV-C, stops most UV-B, and lets most UV-A and nearly all the visible light through.

How does the ozone layer form? Sunlight splits an oxygen molecule into two oxygen atoms. Each atom can join a whole oxygen molecule to make ozone. Ozone takes in UV, and in doing so it splits again, and the pieces can rejoin. This cycle goes on all the time, and each turn of it takes in UV.

See it three ways

As a picture

In words

UV splits an oxygen molecule into two atoms. An atom joins another oxygen molecule to make ozone. Ozone takes in UV and splits. Each time UV is taken in, so less reaches the ground.

In symbols

O₂ + UV → O + O. O + O₂ → O₃. O₃ + UV → O₂ + O.

In the 1970s scientists showed that some gases that people made, called CFCs, could destroy ozone. CFCs were used in fridges and spray cans. In 1985 a very thin patch, the ozone hole, was found over the south pole in the southern spring. It is a patch where there is much less ozone, not a real hole. In 1987 countries signed an agreement to cut CFCs, and a few years later they agreed to stop making them. The ozone layer is now slowly recovering. The United Nations reports that it should be back to its 1980 level about 2040 for most of the world.

Try it

A model of the atmosphere on a strip of string

You need: a long piece of string, about 7 m, a ruler or a tape measure, a pen or small bits of tape, a long room, a yard or a corridor.

Be safe: Keep the string off the floor of a room where people walk, so no one trips. Work in a room or yard with room to move.

Let 1 cm on the string stand for 10 km. Lay the string out straight.
Mark the string every 10 cm, starting at one end. The first mark is for 100 km, the edge of space.
Mark where the stratosphere starts, at 1.2 cm from the end. Mark where the mesosphere starts, at 5 cm.
Now measure how far you need to go to show the Earth’s radius, 6 371 km. Walk to the end of the string.

What you will see: The layer that holds the weather is only about 1 cm on your string. Even the first 100 km is only 10 cm. The Earth’s radius is over 6 metres on the same scale. Next to the Earth, the atmosphere is a very thin skin.

If you cannot try it, this is what happens.

A wrong idea many people have

Air is nearly all oxygen.

Why it feels right: Oxygen is the gas we need to breathe, and fire needs it, so it seems it must be the main gas.

The test that breaks it: Look at the hundred squares. Oxygen has 21 squares. Nitrogen has 78. If you breathe in 100 molecules of air, about 78 are nitrogen. Your body breathes most of that nitrogen straight out again.

What is true: Nitrogen is the main gas in air, about 78 %. Oxygen is about 21 %.

A wrong idea many people have

The higher you go, the closer you are to the Sun, so the warmer it gets.

Why it feels right: We think of the Sun as the source of heat, so being nearer to it seems to mean being warmer.

The test that breaks it: Look at the temperature graph. In the lowest layer, which is where mountains are, the air gets about 6.5 °C colder with every km. The ground absorbs sunlight and warms the air above it. The air higher up is far from that warm ground. The Sun is about 150 million km away, so 10 km makes no difference to the Sun’s distance.

What is true: In the troposphere the air gets colder as you go up. Higher, in the stratosphere, it warms again, but for a different reason: ozone takes in ultraviolet.

A wrong idea many people have

Ozone is good wherever it is.

Why it feels right: We hear that the ozone layer protects us, so ozone seems always to be helpful.

The test that breaks it: Ozone high in the stratosphere takes in UV. Ozone near the ground, made when sunlight acts on fumes from vehicles and factories, hurts the lungs and plants. It is a pollutant there.

What is true: Ozone high up is a shield. Ozone at ground level, where we breathe, is a pollutant. It is the same molecule, O₃, in a different place.

Key words

atmosphere: the layer of gases that surrounds the Earth.
air pressure: the push of the air on everything it touches. It is lower higher up.
troposphere: the lowest layer, from the ground to about 12 km. It holds the weather.
stratosphere: the layer above, to about 50 km. It holds the ozone layer.
mesosphere: the layer above that, to about 85 km. It is the coldest layer.
thermosphere: the top layer, from about 85 km out to about 600 km. The air there is very thin.
ultraviolet: (UV) light just beyond violet that we cannot see. Too much harms skin and eyes.
ozone: a gas with molecules of three oxygen atoms, O₃. The ozone layer takes in most UV.

Back to your guess

The answer is B: Air is mostly nitrogen, and the air is thinner up there, so each breath holds less oxygen.

About four fifths of the air is nitrogen, and about one fifth is oxygen. The air is thinner high up, so each breath holds fewer molecules of oxygen. The share of oxygen has not changed. The number in each breath has.

Keep it

Copy this into your exercise book.

Dry air: 78 % nitrogen, 21 % oxygen, 0.9 % argon, 0.04 % carbon dioxide. Layers: troposphere (weather), stratosphere (ozone), mesosphere, thermosphere. Ozone takes in UV.

Draw: A tall column with four layers and their heights: 12, 50, 85 km. Mark the ozone layer inside the second one.

S4.14.2 Chapter 2

Weather and air pressure

The radio says a low pressure area is coming. Why does that mean cloud and rain, and what does a high bring?

Your guess

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

Your guess

A A low brings cloud and rain, because air rises in it. A high brings clear, settled weather, because air sinks in it.
You picked A. Read on. At the end, the chapter comes back to your guess.
B A low brings sunshine, because there is less air to make cloud from.
You picked B. Read on. At the end, the chapter comes back to your guess.
C A high brings rain, because the heavy air presses the water out of the clouds.
You picked C. Read on. At the end, the chapter comes back to your guess.
D Pressure has nothing to do with weather. Only temperature does.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how high and low pressure bring different weather
Build a rain gauge and a wind vane and keep records

Weather is what the air is doing now and today: warm or cold, wet or dry, calm or windy. Weather changes from hour to hour. To forecast it, people measure the air, and one thing they measure is its pressure.

You already know that air has weight. The air pressure at the ground is the weight of all the air above, pressing on each patch of ground. On each square centimetre it is about the weight of 1 kg. At sea level it averages about 1 013 hPa. A hPa, or hectopascal, is 100 pascals.

Air pressure is not the same everywhere, and it changes with the weather. A barometer measures it. If you keep a record of the pressure, you can see the weather coming. Here is a trace over three days.

The pressure trace of three days. Falling pressure often brings cloud and rain. Rising pressure often brings clearer weather.

Why? Think of air as a river with slopes. Air flows from places of high pressure towards places of low pressure. Where the ground is warm, or where winds meet, air is pushed upward. A place where air rises has a lower pressure at the ground. We call it a low.

In a low, air flows in along the ground and rises. As it rises it meets lower pressure, spreads out and cools. Cool air holds less water vapour. The vapour condenses into droplets, which make cloud. If the droplets grow big, they fall as rain.

A low: air flows in, rises, cools, and makes cloud and rain.

A place where air sinks has a higher pressure at the ground. We call it a high. In a high, air sinks and then flows out along the ground. Sinking air is squeezed a little by the air above, so it warms, and cloud droplets dry up. The sky is clear, the winds are light, and the weather stays the same for days. We call that settled weather.

A high: air sinks and flows out. The sky is clear and the weather is settled.

See it three ways

As a picture

In words

In a low, air rises, and cloud and rain form. In a high, air sinks, and the sky clears. The winds in a low are usually stronger than in a high.

In symbols

L → air rises → cloud and rain. H → air sinks → clear sky.

Weather forecasters draw the pressure on a map. They join places that have the same pressure with a line called an isobar. Each isobar has a number in hPa. Round a low the numbers fall towards the middle. Round a high they rise towards the middle.

The isobars also tell how windy it is. Where isobars are close together, the pressure changes quickly over a short distance, and the wind is strong. Where they are far apart, the wind is light.

Wind blows from high pressure to low pressure. But the Earth is spinning, and this turns the wind as it goes. In the northern hemisphere the wind turns to the right. So it spirals in anticlockwise round a low, and out clockwise round a high. In the southern hemisphere it is the other way round.

A weather map for the northern hemisphere. The isobars round the low are closer together than those round the high, so the wind is stronger near the low.

People name a wind by the direction it comes from. A west wind blows from the west, towards the east. A wind vane shows this. It is a pointer that turns freely. Its head swings round to face into the wind, and its tail points the way the wind is going.

A west wind. The vane head points to the west, into the wind. The wind blows towards the east.

Rain is measured with a rain gauge. It is a container with straight sides and a flat bottom. The depth of water in it, in millimetres, is the depth of rain that fell. Because the sides are straight, 1 mm in the gauge is 1 mm of rain on the ground everywhere. The picture strip “Rain adds up in the gauge”, further on, shows one.

Worked example

Over one week a gauge was emptied each day. The readings in mm were 0, 4, 12, 0, 0, 7 and 3. How much rain fell in the week?

Add the seven readings: 0 + 4 + 12 + 0 + 0 + 7 + 3.
0 + 4 = 4, and 4 + 12 = 16.
16 + 7 = 23, and 23 + 3 = 26.
So 26 mm of rain fell in the week.

Answer: 26 mm

The week of rain from the worked example, one bar for each day.

There is a useful fact for roofs and farms. 1 mm of rain on 1 m² of ground is 1 litre of water. So the water from a roof is its area in m² times the rain in mm, in litres.

Worked example

25 mm of rain falls on a flat roof of 40 m². How many litres of water fall on the roof?

1 mm of rain on 1 m² is 1 litre.
The roof has 40 m², so each mm of rain gives 40 litres.
There are 25 mm of rain, so 25 × 40 = 1 000 litres.

Answer: 1 000 litres

Rain from the roof, run into a tank. The water stands at the number of litres that the sum gives.

Draw it first

Rain adds up in the gauge

1 Read the top of the water against the scale. After the first day it is just above the 5 mm tick, at 6 mm. That is the rain of one day.
2 After the second day, with the jar not emptied, it shows 14 mm. The second day gave 8 mm.
3 After the third day it shows 22 mm. The third day gave 8 mm again. Empty the gauge after each reading to get one day at a time.

Try it

Build a rain gauge and a wind vane, and keep a record

You need: a tin can or a jar with straight sides and a flat bottom, a ruler with mm marks, a drinking straw, a pencil with an eraser, and a pin, a piece of card, tape, a stick or a post to hold things in the open, paper and a pencil for a record.

Be safe: An adult should push the pin and cut the card. Push the point of the pin into the eraser at once, and keep the pin out of reach of small children. Do not use a can with a sharp rim: ask an adult to tape over the edge. Do not go out in a storm to read the gauge.

Gauge: stand the can or jar in the open, away from trees and walls, on a flat spot. Wedge it with stones so it cannot tip over.
Each day at the same time, stand the ruler upright on the bottom of the can. Read the depth of the water in mm. Many rulers have a gap before their 0 mark. If yours does, add that gap, or mark a strip of card from its very end. Write it down. Then empty the can.
Vane: cut an arrow head and a tail fin from card. Tape the head to one end of the straw and the fin to the other. Ask an adult to push the pin through the middle of the straw into the eraser of the pencil.
Fix the pencil upright in a post. Check that the straw turns freely. Mark north, east, south and west on the ground with stones. Find north with a compass. Without one, face where the Sun rises, which is roughly east, and north is on your left.
Each day, read the way the arrow head points. Write the wind as the direction it comes from. Add a note about the sky: clear, cloud or rain.

What you will see: After a week you have a record of the rain, the wind and the sky. Compare it with what the pressure was doing, if a barometer is near. Days of rain often come with winds from one direction and falling pressure.

If you cannot try it, this is what happens.

A wrong idea many people have

A west wind blows towards the west.

Why it feels right: Many words for direction mean where something is going: we go west, we head north.

The test that breaks it: Look at the vane. Its head points into the wind, to the west, and the wind arrows blow from there towards the east. Stand facing west on a day with a west wind. The wind hits your face.

What is true: A wind is named for where it comes from. A west wind comes from the west and blows towards the east.

A wrong idea many people have

Air does not weigh anything, so air pressure is not real.

Why it feels right: We cannot feel the weight of air on us, and air seems like nothing.

The test that breaks it: Air pressure at sea level is about 101 000 Pa. That is a push of about 10 N on each cm², the weight of about 1 kg. You do not feel it because the fluids in your body push back outwards just as hard. A tin can that has had its air pumped out is crushed flat.

What is true: Air has weight. Over each m² of ground there is about 10 000 kg of air, which is air pressure. We do not feel it because we are balanced from inside.

A wrong idea many people have

High pressure means stormy weather, because high sounds strong.

Why it feels right: The word high sounds like more of everything, so we expect more wind and rain.

The test that breaks it: Look at the two pictures of the air. In a high the air sinks, and sinking air clears cloud. In a low the air rises, and cloud and rain form. Strong wind needs close isobars, not a high number.

What is true: A high brings settled weather. A low brings cloud and rain. It is not the number that makes the storm. It is whether the air rises or sinks.

Key words

weather: what the air is doing now: warm or cold, wet or dry, calm or windy.
air pressure: the push of the air, from the weight of the air above. At sea level it is about 1 013 hPa.
low: a place of low air pressure, where air rises and cloud and rain form.
high: a place of high air pressure, where air sinks and the sky clears.
isobar: a line on a map that joins places with the same air pressure.
wind vane: a pointer that turns to face into the wind, to show where the wind comes from.
rain gauge: a straight sided container that measures the depth of rain in mm.

Back to your guess

The answer is A: A low brings cloud and rain, because air rises in it. A high brings clear, settled weather, because air sinks in it.

In a low, air flows in near the ground and rises. Rising air cools, its water vapour condenses, and cloud and rain form. In a high, air sinks and flows out. Sinking air warms a little, so cloud melts away and the sky clears.

Keep it

Copy this into your exercise book.

Low: air rises, cloud and rain. High: air sinks, clear sky. Close isobars, strong wind. A west wind comes from the west. 1 mm of rain on 1 m² is 1 litre.

Draw: A low and a high side by side, from the side, with arrows for the air and a cloud over the low.

S4.14.3 Chapter 3

Winds, monsoons and storms

Hana lives on a coast. In the afternoon a cool wind blows in from the sea, and at night the wind blows out to sea. Why?

Your guess

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

Your guess

A The sea breathes in and out with the tides, and pushes the air with it.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The wind always blows from the land to the sea, and in the afternoon it only feels stronger.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The Moon pulls the air towards the sea at night and releases it by day.
You picked C. Read on. At the end, the chapter comes back to your guess.
D By day the land warms more than the sea, so air rises over the land and cool air flows in. At night the pattern turns round.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain land and sea breezes using convection
Explain how monsoons and tropical storms form

Last chapter you saw that wind blows from high pressure to low pressure. Why does the pressure differ from place to place? A large part of the answer is that the Sun warms the Earth unevenly. Warm air and cool air behave in different ways.

Warm air expands, so it is less dense than the cool air round it. It floats upward. Cool air is denser and sinks. This movement is convection. Where air rises, the pressure at the ground is lower. Cool air flows in along the ground to take its place.

Land and sea warm at very different speeds. Water has a specific heat capacity of 4 200 J/kg °C. Dry soil has about 800 J/kg °C. The same sunshine warms the land about five times as much as the same mass of sea water. Water also mixes, so the heat is spread down into the sea, while on land it stays near the surface.

Worked example

The same 84 000 J of heat goes into 10 kg of water and into 10 kg of dry soil. By how many °C does each warm up? Use 4 200 J/kg °C for water and 800 J/kg °C for soil.

Use the rise in temperature = energy ÷ (mass × specific heat capacity).
Water: 84 000 ÷ (10 × 4 200) = 84 000 ÷ 42 000 = 2 °C.
Soil: 84 000 ÷ (10 × 800) = 84 000 ÷ 8 000 = 10.5 °C.

Answer: water 2 °C, soil 10.5 °C

The same heat warms the soil about five times as much as the water.

Now put land beside the sea on a sunny day. The land warms up faster than the sea. The air over the land warms and rises. The pressure near the ground falls over the land. The cooler, heavier air over the sea flows in to fill the gap. Higher up, the air that rose spreads out and flows back towards the sea, where it sinks. The air goes round a loop.

Draw it first

A sea breeze, step by step

1 In the sun, the land warms faster than the sea. Here it is 30 °C against 26 °C.
2 Air over the warm land warms, expands and rises.
3 Cooler air from over the sea flows in along the ground to take its place. This wind is named for where it comes from: a sea breeze.
4 The rising air flows back out over the sea at height, and sinks there. The air goes round a loop.

The sea breeze starts in the morning, as the land warms. It is often strongest in the afternoon, and it can carry cool air some tens of kilometres inland. It is why many coasts are cooler than the land behind them on hot days.

At night the Sun is gone and the land loses heat quickly. The sea cools slowly, so the sea becomes the warmer side. The loop turns round. Air rises over the sea, and cool air flows out from the land along the ground. This is the land breeze. It is usually gentler than the sea breeze.

The land breeze at night. The sea is warmer, so air rises over it and the wind blows out from the land.

See it three ways

As a picture

In words

The wind along the ground blows from the cooler side to the warmer side. A breeze is named for the place it comes from.

In symbols

Day: land warmer → sea breeze. Night: sea warmer → land breeze.

Now think much bigger. A monsoon is a wind that changes direction with the season. The word comes from an Arabic word for season. Monsoons are found in several parts of the world, such as South Asia, East Asia, West Africa and northern Australia.

A monsoon is a sea breeze and a land breeze on a huge scale, and over months, not hours. In the warm season, the Sun is high over a large land mass. The land heats and a low forms over it. Moist air flows in from the warm ocean, rises, cools and drops heavy rain. This is the wet season.

The wet season. The land is hot, with low pressure. Moist air flows in from the sea and rain falls on the land.

In the cool season the land loses heat and becomes cooler than the sea. A high forms over the land, and a low over the sea. The wind blows out from the land to the sea. It is dry, because it comes from the land. This is the dry season.

The dry season. The land is cooler than the sea, with high pressure. Dry air flows out to the sea.

The monsoon is helped by the tilt of the Earth. As the seasons change, the place where the Sun is overhead moves north or south of the equator. The belt of heaviest rain moves with it. Many farmers plant when the rains come. A late or weak monsoon can bring drought. A very strong one can bring floods.

Last, the most violent winds on Earth. A tropical storm is a huge spinning storm that forms over warm sea in the tropics. Its strongest kind is called a hurricane, a typhoon or a cyclone, according to the sea where it forms. They are the same kind of storm.

One kind of storm, three names. The south Pacific also has cyclones, and the east Pacific has hurricanes.

A tropical storm needs three things. First, a sea warmer than about 26 to 27 °C, down to about 50 m. Second, a place at least about 5° of latitude from the equator, so that the Earth’s spin can turn the winds. Third, winds that stay much the same from low to high, so the storm can build a tall tower.

Here is how it grows. Warm sea water evaporates, and warm, moist air rises. As it rises it cools, and the vapour condenses into cloud. Condensing gives out heat, which warms the air and makes it rise even faster. The pressure at the sea falls, more air flows in, and the whole thing spins.

A tropical storm from above, in the northern hemisphere. In the southern hemisphere it spins clockwise.

The middle of the storm is the eye. It is often 30 to 60 km wide, and it is calm and nearly clear, because air sinks there. Round it is the eyewall, a tower of cloud where the wind is strongest and the rain is heaviest.

A storm cut through the middle. Air spirals in over the warm sea, rises in the eyewall, spreads out at the top, and sinks in the eye.

The storm feeds on warm, moist air. Over land or cold water, it loses its fuel and weakens. A storm is called a hurricane, typhoon or cyclone when its steady wind passes about 120 km/h. It can be several hundred km across.

The greatest danger is often the storm surge. The wind and the low pressure raise the sea level, sometimes by several metres, and the sea floods the coast. Heavy rain also causes floods and landslides. When a warning is given, move to the shelter that the authorities name.

Try it

Watch the wind change through the day

You need: the wind vane from the last chapter, or a strip of light cloth tied to a stick, paper and a pencil, a place in the open.

Be safe: Do not go out in a storm to read the vane. Stay away from deep water and cliffs, and do not stand under a tree when there is lightning.

Put the vane or the cloth in the open, away from walls. Mark north.
On a calm sunny day, note the wind direction in the morning, at midday, in the late afternoon and after dark. Write the direction the wind comes from.
Repeat for several days. If you live by the sea or a large lake, note the direction against the water.
Look for a pattern: does the wind come from the water in the afternoon and from the land at night?

What you will see: Near a coast or a large lake, the wind often comes from the water in the afternoon. At night it often comes from the land. Inland, far from water, you may see little pattern, because the breeze does not reach so far.

If you cannot try it, this is what happens.

A wrong idea many people have

By day the sea is warmer than the land, so the air rises over the sea.

Why it feels right: Water holds a great deal of heat, and it can feel warm to swim in, so it seems warmer than the ground.

The test that breaks it: Dry soil has a specific heat capacity of 800 J/kg °C and water has 4 200 J/kg °C. In the worked example the same heat raised the soil by 10.5 °C and the water by 2 °C. In the sun the land gets warmer than the water.

What is true: By day the land is the warmer side, so air rises over the land and a sea breeze blows in. At night it turns round, because the sea holds its heat longer.

A wrong idea many people have

A monsoon is a heavy rainstorm.

Why it feels right: On the news, the word monsoon is used when there is heavy rain, so it seems to mean the rain.

The test that breaks it: The word means season. A monsoon is a seasonal wind that changes its direction. It brings a wet season, when the wind blows from the sea, and a dry season, when it blows from the land.

What is true: A monsoon is a seasonal change in wind. The heavy rain belongs to the wet part of the cycle, and the dry season belongs to it too.

A wrong idea many people have

The eye is the stormiest part of a tropical storm.

Why it feels right: It is the centre, so it seems it must hold the most violent weather.

The test that breaks it: Look at the cross section. In the eye the air sinks, and sinking air clears cloud. The air rises in the eyewall, round the eye, where the cloud is tallest and the wind is strongest.

What is true: The eye is calm and often nearly clear. The eyewall round it has the strongest winds and the heaviest rain. As the eye passes, the wind drops, and then it returns from the other direction.

Key words

convection: the movement of heat by a fluid: warm air rises and cool air sinks.
land breeze: a night wind that blows from the land out to the sea.
monsoon: a wind that changes direction with the season, with a wet season and a dry one.
tropical storm: a huge spinning storm that forms over warm sea in the tropics.
eye: the calm centre of a tropical storm.
eyewall: the ring of tall cloud round the eye, with the strongest wind and rain.
storm surge: a rise in sea level, driven by a storm, that floods the coast.

Back to your guess

The answer is D: By day the land warms more than the sea, so air rises over the land and cool air flows in. At night the pattern turns round.

The land warms and cools faster than the sea. By day air rises over the warm land and cooler air flows in from the sea. At night the land is cooler, so the air rises over the warmer sea and the wind blows out. Warm air rising and cool air sinking is convection.

Keep it

Copy this into your exercise book.

Day: land warmer, air rises over land, sea breeze. Night: sea warmer, land breeze. Monsoon: wind turns with the season. Storm: warm sea, spirals, eye calm, eyewall fierce.

Draw: Draw a coast with a loop of arrows. They go up over the warm side, back at height, down over the cool side, and in along the ground.

S4.14.4 Chapter 4

Climate zones

Kofi lives on the equator, near the coast, where it is warm and wet all year. His cousin lives on the equator too, high in the mountains, and wears a jumper. Why are their climates so different?

Your guess

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

Your guess

A The cousin lives closer to the Sun, so the air is thinner and colder.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Mountains are windier, and wind makes the whole place colder at the same height.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The air gets colder as you go up, about 6.5 °C for every km, so the mountain town is cooler.
You picked C. Read on. At the end, the chapter comes back to your guess.
D Both towns are on the equator, so they must have the same climate.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain why climate differs with latitude, height and distance from the sea
Read a climate graph of temperature and rainfall

Weather is what the air is doing today. Climate is the pattern of the weather in a place over many years. Scientists usually take the average of about 30 years. A cold day in a hot place does not change its climate.

Weather is today. Climate is the long pattern.

Three things above all decide a place’s climate: its latitude, its height, and its distance from the sea. Latitude is how far a place is north or south of the equator, measured in degrees.

Start with latitude. Near the equator the Sun climbs high in the sky, and its light comes down steeply. Near the poles the Sun stays low, and its light comes in at a slant. The same beam of light is then spread over a longer patch of ground.

The same beam of sunlight. A low Sun spreads it over a longer patch, so each part of the ground gets less heat.

When the Sun is 30° above the horizon, the patch is twice as long as the beam is wide. Each part of the ground gets half the heat. Low Sun light also has to pass through more air, which takes in and scatters some of it. That is why the poles are cold and the tropics are hot.

Air pressure adds to the pattern. Near the equator the warm ground heats the air, and it rises. That gives a belt of low pressure, cloud and heavy rain. The air spreads out high up, cools, and sinks again near 30° of latitude. Sinking air is dry, so there are high pressure belts there, and many of the world’s great deserts.

The air in three loops, from the equator to a pole. Where it rises it is wet, near 0° and 60°. Where it sinks it is dry, near 30° and 90°.

This is a rough picture. Land, sea and mountains bend it. But it explains the broad pattern. Climate scientists sort the world into five main kinds. Their edges are not sharp.

The five main climate zones. Tropical is nearest the equator, and polar is nearest the poles.

Now height. The air at the ground is warmed by the ground. The higher you climb, the further you are from that warm ground. In the lowest layer of the atmosphere the air gets about 6.5 °C colder for each km of height. This is why there are snow caps on mountains near the equator.

A mountain where the sea level is 30 °C. Each km up is 6.5 °C colder.

Worked example

A town by the sea has a mean temperature of 28 °C. Another town on the same latitude is 2 km above sea level. What is its mean temperature?

The air gets about 6.5 °C colder for each km.
Two km is 2 × 6.5 = 13 °C of cooling.
So the town is at 28 − 13 = 15 °C.

Answer: 15 °C

Last, the distance from the sea. Water holds heat well. In summer the sea is cooler than the land, and in winter it is warmer. Places near the sea are warmed and cooled by sea air, so their temperatures stay in a small range. Places far from the sea have a large range. A climate with a small range is called maritime, and one with a large range continental.

To see these differences, scientists draw a climate graph. It shows one place through a year. Each bar is the rainfall of a month, in mm, read on the right hand scale. The line is the mean temperature of each month, in °C, read on the left hand scale. The letters along the bottom are the months, January to December.

Here is Town A. It is hot all year and has a wet season. Which months are warm or wet depends on where in the world a place lies.

Town A: hot all year, with a wet season and a dry season.

Worked example

Use the graph of Town A. What is the range of the monthly temperature, and what is the total rainfall in the year? The rainfall of the months in mm is 250, 230, 260, 170, 90, 40, 30, 30, 50, 120, 200 and 240.

The range is the hottest month minus the coldest month. The line peaks at 28 °C and falls to 25 °C.
So the range is 28 − 25 = 3 °C.
Add the bars. 250 + 230 + 260 + 170 + 90 + 40 = 1 040, and 30 + 30 + 50 + 120 + 200 + 240 = 670.
The total is 1 040 + 670 = 1 710 mm.

Answer: 3 °C and 1 710 mm

Now compare two places at about the same latitude. Town B is on a coast. Town C is far inland. Both are in the northern hemisphere. They use the same scales, so you can compare them fairly.

Town B, on the coast. The temperature stays in a small range, and there is rain in every month.

Town C, far inland. The temperature swings widely, from well below 0 °C to about 20 °C.

Town B has a small range. Town C has a large one. The sea keeps Town B mild all year. The land of Town C heats up and cools down fast. Other things matter too. Ocean currents carry warm or cold water along coasts. Mountains force air up, so one side of a range is wet and the other dry. The prevailing winds decide which places get sea air.

See it three ways

As a picture

In words

Read the line on the left scale, in °C. Read the bars on the right scale, in mm. The range is the highest month minus the lowest. The total rain is all the bars added up.

In symbols

range = highest − lowest. total rain = month 1 + month 2 + … + month 12.

Try it

Draw a climate graph for a place you know

You need: a table of the mean temperature and rainfall of each month for a place, squared paper, a ruler and a pencil, two colours of pencil, or one pencil and a different kind of line.

Be safe: Use a blunt pencil and keep scissors, if you cut paper, away from small children. Take care crossing roads if you collect figures from a weather station or an office.

Get the figures from a weather office, a school, a radio programme or an atlas. Or keep your own record for a year with the gauge from the earlier chapter.
Draw two scales on squared paper: temperature in °C on the left, and rainfall in mm on the right. Make them line up at the 0 line.
Draw a bar for each month for the rain. Mark a dot for each month for the temperature, and join the dots with a line.
Work out the range and the total rainfall. Then say whether the place is wet or dry, and hot, mild or cold.

What you will see: You have turned a table of 24 numbers into one picture. It shows the climate of a place at a glance.

If you cannot try it, this is what happens.

A wrong idea many people have

A cold day means that the climate of a place is cold.

Why it feels right: Weather and climate are both about how warm, wet or windy a place is. So the two words seem to mean the same thing.

The test that breaks it: Look at the climate graph of Town A. Its coolest month is 25 °C. A single cold spell of 15 °C in a month would not change the average of 30 years of readings.

What is true: Weather is today. Climate is the pattern over many years, usually an average of 30. One cold day, or one hot summer, does not change the climate.

A wrong idea many people have

Mountains and the poles are cold because they are nearer to, or further from, the Sun.

Why it feels right: We think of the Sun as a fire: the closer we are, the warmer we are.

The test that breaks it: The Sun is about 150 million km away. A mountain 5 km high changes that distance by a tiny share. So does the whole width of the Earth, about 12 700 km. The real causes are the slant of the light, and the height above the warm ground.

What is true: The poles are cold because low Sun light is spread over more ground. High places are cold because the air gets about 6.5 °C colder for each km.

A wrong idea many people have

A desert is always hot.

Why it feels right: The deserts people see in pictures are hot, with sand and Sun.

The test that breaks it: A desert is a place with very little rain. Some deserts are cold, such as high deserts and the ice of the polar regions, which get very little snow. Desert air is dry, so heat escapes fast at night, and many hot deserts are cold after dark.

What is true: A desert is a place that is dry. It can be hot or cold.

Key words

climate: the pattern of weather in a place over many years, often the average of 30 years.
latitude: how far a place is north or south of the equator, in degrees.
climate graph: a graph of one place through the year: bars for the rain, a line for the temperature.
range: the highest value minus the lowest. For temperature, the hottest month minus the coldest.

Back to your guess

The answer is C: The air gets colder as you go up, about 6.5 °C for every km, so the mountain town is cooler.

In the lowest layer of the atmosphere the air gets about 6.5 °C colder for each km you climb. A town 2 km up is about 13 °C cooler than a town by the sea at the same latitude. Height is one of three big things that shape a climate. The others are latitude and the distance from the sea.

Keep it

Copy this into your exercise book.

Climate: the pattern over about 30 years. It depends on latitude, height (6.5 °C colder for each km) and distance from the sea. Bars are rain, the line is temperature.

Draw: A climate graph: bars for rain, a line for temperature, a scale on each side, and the months along the bottom.

S4.14.5 Chapter 5

Air pollution

On some days a brown haze hangs over Priya’s town and her eyes sting. Where does it come from?

Your guess

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

Your guess

A It is cloud that has sunk to the ground.
You picked A. Read on. At the end, the chapter comes back to your guess.
B It is smoke and gases from burning fuel, held near the ground.
You picked B. Read on. At the end, the chapter comes back to your guess.
C It is dust that blows in only from deserts.
You picked C. Read on. At the end, the chapter comes back to your guess.
D It is water vapour that has come in from the sea.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Explain how burning fuels causes smog and acid rain
Explain why smoke from indoor cooking fires harms health

Air pollution is a harmful substance in the air. Some pollution is natural: smoke from wildfires, ash from volcanoes and dust from deserts. Most of the pollution in towns comes from burning fuels, such as petrol, diesel, coal, oil, wood and dung.

When a fuel burns in plenty of air, it makes carbon dioxide and water. For carbon: C + O₂ → CO₂. But real burning is not so tidy. Fuels have other things in them. Air has more than oxygen. A fire or engine seldom gets just the right amount of air.

Four harmful things come from burning. With too little oxygen, fuel makes carbon monoxide and soot instead of carbon dioxide. Carbon monoxide is a poison that you cannot see or smell. It sticks to the red blood cells and stops them from carrying oxygen.

See it three ways

As a picture

In words

Carbon burns to carbon dioxide when there is plenty of oxygen. With too little oxygen it makes carbon monoxide and soot.

In symbols

C + O₂ → CO₂. 2C + O₂ → 2CO.

Third, coal and oil have a little sulfur in them. It burns too, to make sulfur dioxide, S + O₂ → SO₂. It has a sharp smell and makes people cough. Fourth, the flame of an engine is so hot that the nitrogen and oxygen of the air itself join. They make nitrogen oxides, N₂ + O₂ → 2NO. NO then turns into NO₂ in the air.

Four pollutants that come from burning fuel, and the harm that each does.

Soot is made of tiny solid particles. Particles less than 2.5 µm across, from soot and from other sources, are called PM2.5. They are many times thinner than a hair. They go deep into the lungs and some pass into the blood. The World Health Organization sets a guide for a day. Averaged over a day, the air should hold no more than 15 µg of PM2.5 in each m³. Only 3 or 4 days a year should pass that. Averaged over a year, it should hold less than 5 µg in each m³. We call 15 the guide for a day.

Worked example

On one day a town’s air has 45 µg of PM2.5 in each m³. The guide for a day is 15 µg per m³. The reading is how many times the guide?

Divide the reading by the guide.
45 ÷ 15 = 3.
So the air has 3 times the guide.

Answer: 3 times

The reading of the day is three times the guide.

Now think about how these gases and soot make smog. The word is smoke and fog put together. Smog is a haze of pollution that hangs near the ground. The air in a high is still, and nothing blows the haze away.

Smog is worst under a warm layer of air. Usually the air gets colder with height, so warm polluted air rises and spreads out. On some days a layer of warm air lies over colder air. This is called a temperature inversion. The warm layer works as a lid. The smoke cannot rise through it, so it spreads out and gathers under it.

On a normal day smoke escapes upward. Under a warm lid it is trapped, and smog builds up.

There are two common kinds of smog. In cold weather, coal smoke and sulfur dioxide mix with fog. In 1952 a smog like this stayed over one big city for five days, and thousands of people died of it. In hot, sunny weather, sunlight acts on nitrogen oxides and petrol vapour. This makes ozone and other irritants near the ground. It is the brown haze of a busy city.

The ozone high in the sky, in chapter 1, was a shield. Ozone down here, where we breathe, is a pollutant. It hurts the lungs and damages crops. It is the same gas in a different place.

Now acid rain. Sulfur dioxide and nitrogen oxides rise into the air and dissolve in the water droplets of clouds. There they turn into acids: sulfuric acid, H₂SO₄, and nitric acid, HNO₃. They fall in rain, snow, fog and dry dust, sometimes hundreds of km from where the gases were made.

From the chimney or the exhaust to acid rain, and what it harms.

Clean rain is slightly acid already, because carbon dioxide dissolves in it. Its pH is about 5.6. Acid rain has a pH below that, often between 4 and 5. Each step of 1 on the pH scale is a factor of 10 in acid.

Acid rain has a pH of about 4 to 5. It is far weaker than vinegar, but it adds up over years.

Worked example

Natural rain has a pH of 5.6. Acid rain has a pH of 4.6. How many times more acid is the acid rain? Each step of 1 on the pH scale is a factor of 10.

Find the difference in pH: 5.6 − 4.6 = 1.
One step on the pH scale is a factor of 10.
So the acid rain is 10 times more acid.

Answer: 10 times

Acid rain is too weak to hurt your skin. But over years it harms lakes, where it frees metals that poison fish and their young. It leaches nutrients from soil and damages leaves, so forests suffer. It wears away stone and corrodes metal. Marble and limestone are calcium carbonate, and an acid reacts with it: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂.

There are ways to cut these gases. Low sulfur fuel makes less sulfur dioxide. Power stations can pass their smoke through scrubbers, which use limestone to catch the sulfur dioxide. A catalytic converter in a car cleans the exhaust. It turns carbon monoxide, nitrogen oxides and unburnt fuel into carbon dioxide, nitrogen and water. One reaction is 2CO + 2NO → 2CO₂ + N₂. In many places the rules have cut sulfur dioxide sharply, and acid rain has eased.

Try it

What an acid does to shell

You need: two small clear cups or jars, vinegar, water, two small pieces of eggshell, seashell or limestone, about the same size.

Be safe: Do not drink or taste anything in the cups. Keep vinegar away from your eyes, and wash your hands afterwards. Throw the shell and vinegar away down a sink with plenty of water.

Pour vinegar into one cup and the same amount of water into the other. Vinegar is a weak acid, but it is about 100 times more acid than acid rain. So it shows in a day what acid rain does over many years.
Put a piece of eggshell into each cup.
Watch the pieces for several minutes. Then look again after a few hours, and after a day.

What you will see: Bubbles of carbon dioxide gather on the shell in the vinegar. None gather in the water. After a day the shell in the vinegar is softer and thinner. Acid reacts with calcium carbonate and slowly eats it away. This is what acid rain does to marble statues, only far more slowly.

If you cannot try it, this is what happens.

Pollution is not only outdoors. Think of a home where people cook on an open fire or a plain stove with no chimney. They burn wood, crop waste, dung, charcoal or kerosene. The smoke fills the room. The World Health Organization says that more than 2 billion people cook with polluting fuels like these.

A room with a fire and no chimney. The smoke fills the room, and the child breathes it.

The smoke has soot particles, carbon monoxide and other gases in it, at levels far higher than outdoors. Breathing it day after day causes lung infections, especially in young children, and long lung diseases, lung cancer, heart disease and strokes. The World Health Organization links smoke from cooking indoors to about 3 million deaths each year. Women and young children are in the smoke the most.

What helps? A chimney or hood that takes the smoke out of the room. A stove that burns cleaner. A cleaner fuel, such as gas, biogas, electricity or sunshine in a solar cooker. Dry fuel, which makes less smoke than damp. And keeping young children away from the fire. A window that lets in fresh air helps too.

The same room with a chimney. The smoke goes out, fresh air comes in, and the room is clear.

Try it

A smoke walk round a kitchen

You need: paper and a pencil, a kitchen or cooking place, with an adult.

Be safe: Do not light a fire for this activity, and stay away from a hot stove. Never stay in a closed room with a charcoal or wood fire, because carbon monoxide can kill without any smell.

Do not light a fire for this. Look at where food is cooked in your home or a home you know.
Draw a plan. Mark the fire or stove, the doors and windows, and any chimney, vent or hood.
Show where the smoke goes when cooking. Mark where children sit or sleep.
List three changes that would make the air cleaner. Talk about them with the people who cook.

What you will see: Often the smoke goes where people sit, because there is nowhere else for it to go. Changes such as a vent, a window or a stove nearer the door can make a big difference.

If you cannot try it, this is what happens.

A wrong idea many people have

Smog is just fog.

Why it feels right: Smog looks like a grey mist, and we see it on foggy days.

The test that breaks it: Fog is only tiny droplets of water. Smog is also made of soot, smoke and gases such as sulfur dioxide, nitrogen oxides and ozone. Fog clears and does no harm to the lungs. Smog stings the eyes and makes people cough.

What is true: Smog is smoke and fog together. The harm comes from the pollutants in it.

A wrong idea many people have

Acid rain is a strong acid that burns your skin.

Why it feels right: The word acid sounds dangerous, like the acid in a battery.

The test that breaks it: Look at the pH scale. Acid rain has a pH of about 4 to 5. Vinegar, which we put on food, has a pH of about 2.5, which is far more acid. You can stand in acid rain without being hurt.

What is true: Acid rain is only a weak acid. It does its harm by falling on the same lakes, forests and stone over years.

A wrong idea many people have

Smoke from a cooking fire is natural, so it cannot be harmful.

Why it feels right: People have cooked on fires for thousands of years, so it seems safe.

The test that breaks it: The World Health Organization links the smoke from cooking indoors to about 3 million deaths a year. Long ago, most cooking was outdoors or in open spaces. Carbon monoxide has no colour and no smell, so people cannot tell when it is in the air.

What is true: Woodsmoke and other fuel smoke harm the lungs and heart, however old the habit is. A chimney, a cleaner stove or a cleaner fuel makes cooking safer.

Key words

air pollution: harmful substances in the air.
carbon monoxide: (CO) a poisonous gas with no colour or smell, made when fuel burns with too little oxygen.
sulfur dioxide: (SO₂) a gas that comes from sulfur in coal and oil. It makes acid rain.
nitrogen oxides: gases (NO and NO₂) made when air is heated in an engine or flame.
smog: smoke and fog together: a haze of pollution near the ground.
temperature inversion: a layer of warm air over colder air, which traps pollution near the ground.

Back to your guess

The answer is B: It is smoke and gases from burning fuel, held near the ground.

A brown haze over a town is mostly made of smoke, soot and gases from burning fuel in vehicles, factories and fires. Sunlight turns some of the gases into more harmful things. On still days nothing blows the haze away, so it builds up.

Keep it

Copy this into your exercise book.

Burning fuel makes CO, soot, SO₂ and nitrogen oxides. Smog is trapped under a warm lid. SO₂ and nitrogen oxides make acid rain. Indoor smoke harms lungs: use a chimney and clean fuel.

Draw: A chain of boxes: fuel burns, gases rise, dissolve in cloud, acid rain falls on lakes, forests and stone.

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

A The air keeps some of the heat the ground gives out, so the Earth is warmer than it would be without air.
You picked A. Read on. At the end, the chapter comes back to your guess.
B The Earth is bigger than the Moon, so it holds much more heat.
You picked B. Read on. At the end, the chapter comes back to your guess.
C The Earth’s own heat from deep inside warms the surface.
You picked C. Read on. At the end, the chapter comes back to your guess.
D The Earth is closer to the Sun than the Moon is.
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.

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 Sunlight goes through the air to the ground. The molecules of greenhouse gas do not stop it.
2 The warm ground gives out infrared. Some goes straight out to space. Some meets a molecule of greenhouse gas and is absorbed.
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.

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.

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?

From C + O₂ → CO₂, 12 kg of carbon makes 44 kg of carbon dioxide.
6 kg is half of 12 kg, so it makes half of 44 kg.
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.

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?

Find the rise: 420 − 280 = 140 ppm.
Divide by the starting value: 140 ÷ 280 = 0.5.
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

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.

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.
Cover one jar with the clear plastic or cling film. Leave the other open.
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 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.

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.

S4.14.7 Chapter 7

Climate change

Joseph’s grandmother says the seasons have shifted and the sea comes further up the shore than it did. How do scientists know that the Earth is warming?

Your guess

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

Your guess

A From a few hot summers that people remember.
You picked A. Read on. At the end, the chapter comes back to your guess.
B From one thermometer at one place, read for many years.
You picked B. Read on. At the end, the chapter comes back to your guess.
C From computer models alone, with no measurements.
You picked C. Read on. At the end, the chapter comes back to your guess.
D From records at thousands of places on land and sea, and from satellites, which agree with each other.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Use data to describe how global temperature has changed
Explain effects such as rising seas, droughts and floods

Climate change is a lasting change in the pattern of weather in a place, or in the whole Earth. The climate has changed before. In the past, ice ages came and went over thousands of years. The causes were slow changes in the Earth’s orbit. Changes in ice and in the carbon dioxide of the air made the swings larger. What is new is the speed of today’s change, and its cause.

Start with the data. The air temperature near the Earth’s surface is measured at thousands of places. They are on land, on ships and on buoys at sea. Several groups of scientists, in different countries, work out the average for the whole Earth in different ways. They include space agencies, weather offices and universities. Their results agree to within about 0.1 °C.

Scientists give the answer as how far the global average is above the average of 1850 to 1900. That is the late 1800s, before much fossil fuel was burned. Because it is an average of many readings, it hardly moves in a single hot or cold season.

Worked example

Over 2001 to 2010 the global average was 0.9 °C above 1850 to 1900. Over 2011 to 2020 it was 1.1 °C above. By how many °C did it rise from one decade to the next?

The rise is the later value minus the earlier value.
The later value is 1.1 °C and the earlier is 0.9 °C.
1.1 − 0.9 = 0.2 °C.

Answer: 0.2 °C

The global average temperature, as a ten year average, above 1850 to 1900. The values are rounded to 0.1 °C. The major records differ by about 0.1 °C.

The ten years from 2015 to 2024 averaged about 1.2 °C above the late 1800s level. The eleven years from 2015 to 2025 were the eleven warmest in the records. The year 2024 was the warmest, at about 1.55 °C above. 2023 and 2025 came next, at about 1.4 °C. With the ups and downs of single years smoothed out, the long-term warming is now about 1.3 °C. One year above 1.5 °C is not the same as a long-term warming of 1.5 °C. Each of the last four decades was warmer than every decade before it.

The air is not the only thing that changed. Many signs point the same way.

Five signs of a warming world. Each is measured on its own, and all point the same way.

What is the cause? Scientists studied the Sun, volcanoes, the Earth’s orbit and the gases in the air. The result is clear. Recent warming is caused mainly by human activity, mostly by burning coal, oil and gas. This is the conclusion of the Intergovernmental Panel on Climate Change. It collects the work of thousands of scientists. The national science academies of countries all over the world agree.

There are several separate lines of evidence. Carbon dioxide takes in infrared. Laboratories showed this in the 1800s, and calculations soon showed that more of it would warm the Earth. The extra carbon dioxide in the air matches the amount we burn. About half of what we put into the air stays there. The sea and plants take the rest. Carbon from fossil fuels has no carbon 14 left, and the extra carbon dioxide has the same mark.

The pattern of the warming also fits. The lowest layer of air is warming while the stratosphere above it is cooling, which is what extra greenhouse gas does. If the Sun were the cause, every layer would warm. The Sun’s output has not risen since the 1980s, while temperatures have kept rising. Volcanoes make the air cooler for a while, not warmer.

Possible causes of the recent warming, and what the evidence says of each.

Now the effects. The sea is rising. The global average sea level has risen about 20 cm since 1900. Roughly two fifths of this is because water expands when it warms. This is thermal expansion. Most of the rest is melting ice on land. So far it has come mostly from glaciers, and more and more from the great ice sheets of Greenland and Antarctica. Since about 1970, expansion has been about half. The melt water runs into the sea.

The rate of sea level rise, in mm each year. The rise is speeding up. Over 2015 to 2024 it was about 4.7 mm a year (WMO).

Worked example

From 2006 to 2018 the sea level rose by about 3.7 mm each year. If this rate held, how many mm would it rise in 25 years?

Multiply the rate by the number of years.
Split 3.7 into 3 and 0.7. Then 25 × 3 = 75, and 25 × 0.7 = 17.5.
So the rise is 75 + 17.5 = 92.5 mm.

Answer: 92.5 mm

Not all melting ice raises the sea. Ice that already floats on the sea, such as Arctic sea ice, takes up the space of its own melt water. When it melts, the level hardly changes. Ice on land is different. Its melt water is new water for the sea.

When floating ice melts, the level stays the same. When ice on land melts, the level rises.

Rising seas flood low coasts, river deltas and small islands. They also push salt water into farmland and drinking water, and they make storm surges reach further. People living on low ground have the most to lose.

Next, rain. Warmer air can hold more water vapour. For each °C of warming it can hold about 6 to 7 % more. When the vapour condenses, more of it can fall at once. So the heaviest downpours become heavier, and floods are worse.

The most water vapour that 1 m³ of air can hold. Warmer air holds a good deal more.

Worked example

Air at 20 °C can hold up to 17.3 g of water vapour in each m³. Air at 30 °C can hold up to 30.4 g. By what percent does the amount rise? Give a whole number.

The rise is 30.4 − 17.3 = 13.1 g.
Divide by the starting value: 13.1 ÷ 17.3 is about 0.76.
As a percent, 0.76 × 100 = 76 %.
So the amount of vapour rises by about 76 %.

Answer: 76 %

That is for each 10 °C. For each 1 °C it is about 6 to 7 %. Warmer air also dries the land faster, because water evaporates more quickly from soil, lakes and plants. So the same warming that makes the heaviest rain heavier can also dry soils between rains. Some places get wetter and others drier. A drought is a long spell with too little water. Droughts and floods can both become worse, in different places and at different times.

There are other effects too. Heat waves are hotter and more frequent. The sea takes in carbon dioxide and becomes a little more acid. So far the pH has fallen by about 0.1. This harms shell making animals and corals. Corals bleach when the sea is too warm. The effects differ from place to place, and the poorest people, who have least to protect themselves, often face the most risk.

Scientists are careful about single events. A single storm, flood or drought has many causes. But the warming makes some of them more likely, or stronger. It loads the dice, and over many events the change in the odds can be measured.

Try it

Ice on water and ice on land

You need: two glasses of the same size, water, two ice cubes, if you have ice, a small stone that sticks out of the water, a marker pen or a strip of tape.

Be safe: If you do not have ice, read the result and picture instead. Wipe up any spilled water so no one slips. Do not drink the water.

Put the stone in one glass. Pour water into both glasses to the same height, so that the stone sticks out above the water in the first glass.
In the second glass, float an ice cube on the water. In the first glass, rest the second ice cube on the top of the stone.
Mark the water level on the outside of each glass with the pen or tape. Be careful not to move the glasses.
Wait until the ice has melted. Look at the marks. In which glass has the level changed?

What you will see: In the glass with the floating ice the level has not changed. In the glass with the ice on the stone the level has gone up. Floating ice takes up the room of its own melt water. Ice that sits on land adds new water.

If you cannot try it, this is what happens.

A wrong idea many people have

A very cold winter shows that the Earth is not warming.

Why it feels right: If the Earth were warming, it seems every day and every season would be warmer.

The test that breaks it: The average of the whole Earth over a whole year, with thousands of readings, is different from the weather outside one window. Cold spells and hot spells come and go, and a warming climate has both. The 10 year average keeps rising.

What is true: Weather varies from day to day and from place to place. Climate change shows in the long average of the whole Earth.

A wrong idea many people have

The melting of the Arctic sea ice is what will raise the sea level most.

Why it feels right: Melting ice sounds like it must add water to the sea.

The test that breaks it: Sea ice floats, like the ice cube in the glass. It already pushes aside its own weight of water, so when it melts the level hardly changes. Ice on land, in glaciers and the ice sheets, adds new water.

What is true: Sea level rises mainly because of ice melting on land, and because warming water expands.

A wrong idea many people have

Climate has always changed naturally, so people cannot be the cause of this change.

Why it feels right: It is true that the climate changed before people burned fuel, so the argument sounds fair.

The test that breaks it: Forests have always burned by lightning too. But if a fire is lit by a match, we can still tell that the match lit it. In the same way, scientists have measured the natural causes. They are the Sun, volcanoes and the orbit. They are too small, or too slow, to explain the warming. The carbon in the extra carbon dioxide carries the mark of fossil fuel.

What is true: Natural causes changed the climate in the past. This time, the evidence shows that the cause is human activity, mainly burning fossil fuels.

Key words

climate change: a lasting change in the pattern of weather, in a place or in the whole Earth.
thermal expansion: the growth in volume of a substance when it is warmed. Warm sea water takes up more room.
sea level: the height of the surface of the sea. The average of the whole Earth has been rising.
drought: a long spell with too little water, when the soil and rivers run dry.

Back to your guess

The answer is D: From records at thousands of places on land and sea, and from satellites, which agree with each other.

Temperatures are measured at thousands of places on land, on ships and buoys, and from satellites. Several groups of scientists work out the average in different ways, and they agree. Memory and a single thermometer can mislead. Many records together are a strong proof.

Keep it

Copy this into your exercise book.

The Earth is about 1.3 °C warmer than in the late 1800s. People cause most of it. Seas rise: warm water expands, land ice melts. Warm air holds more vapour: heavier rain, drier soil.

Draw: Two glasses with ice: floating ice leaves the level the same, ice on a rock raises it.

S4.14.8 Chapter 8

Responding to climate change

Leila’s village gets less reliable rain, and her town needs more power. What can people do about the causes of climate change, and about its effects?

Your guess

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

Your guess

A Nothing: the change has already started, so nothing we do now can matter.
You picked A. Read on. At the end, the chapter comes back to your guess.
B Only the countries that burn the most fuel can do anything, so villages can do nothing.
You picked B. Read on. At the end, the chapter comes back to your guess.
C Two things: cut greenhouse gas emissions to slow the change, and adapt to the changes that will come anyway.
You picked C. Read on. At the end, the chapter comes back to your guess.
D Planting a few trees would be enough to stop it.
You picked D. Read on. At the end, the chapter comes back to your guess.

By the end you can

Compare ways to cut greenhouse gas emissions
Explain how people adapt, such as by storing water and growing new crops

The last chapter showed that the Earth has warmed and that people are the cause. What can be done? There are two kinds of response. Mitigation means cutting the greenhouse gases that people put into the air, to slow the warming. Adaptation means changing how we live, so that we are safer from the changes that are coming.

Both are needed. Cutting emissions makes the changes smaller. Adapting protects people from the changes that cannot now be avoided. The more we cut, the less we need to adapt to.

Start with the causes. Carbon dioxide stays in the air for centuries, so the warming depends on the total that people add. Scientists say that the Earth stops warming roughly when the net emissions of carbon dioxide fall to zero. Net means what is put into the air, less what is taken out again, by plants for example. This goal is called net zero.

To cut emissions, we first need to know where they come from. The world’s greenhouse gas emissions are shared across several kinds of activity.

Where the world’s greenhouse gas emissions come from, in 2019, rounded. Energy supply means making electricity and heat, and getting coal, oil and gas out of the ground, with their leaks. Buildings counts only the fuel burned in them. The shares add up to a little over 100 % because of rounding.

The biggest share is from energy supply, and most of that is making electricity and heat. So cleaner electricity is the biggest single change. Burning coal, oil and gas gives out carbon dioxide. Wind, sun, water and nuclear power give out almost none while they run. Making the equipment does use energy, so the whole life of each source gives out a little.

Greenhouse gas for each kWh of electricity, over the whole life of the source. Water power gives about 24 and nuclear power about 12. The values are rounded, from the IPCC.

Worked example

A home uses 3 000 kWh of electricity in a year. How many kg of carbon dioxide are released if the electricity comes from coal, at 820 g for each kWh? How many if it comes from wind, at 11 g for each kWh?

There are 1 000 g in 1 kg. So kg = kWh × g for each kWh ÷ 1 000.
Coal: 3 000 × 820 ÷ 1 000 = 2 460 kg.
Wind: 3 000 × 11 ÷ 1 000 = 33 kg.

Answer: coal 2 460 kg, wind 33 kg

Each way of making power has limits. The sun and wind come and go, so they need a way to store energy or a back up. Dams for water power flood land and move people. Nuclear power leaves waste that must be kept safe for a very long time, and its plants are costly. The best mix depends on the place, whether it is sunny, windy or has rivers.

The second way is to use less energy for the same job. This is energy efficiency. A 10 W LED lamp gives about as much light as an old 60 W filament lamp. Insulation keeps houses warm or cool with less energy. A good stove burns less fuel to cook the same food.

Worked example

An old lamp of 60 W is on for 5 hours each day through a year of 365 days. How much energy does it use in kWh? An LED of 10 W gives the same light. How many kWh does the LED save?

Convert watts to kilowatts: 60 W is 0.06 kW. The energy is power × hours.
The old lamp: 0.06 kW × 5 h × 365 = 109.5 kWh.
The LED: 0.01 kW × 5 h × 365 = 18.25 kWh.
The saving is 109.5 − 18.25 = 91.25 kWh.

Answer: 109.5 kWh, saving 91.25 kWh

Two lamps that give about the same light. The LED uses about one sixth of the power.

The other ways are more varied. In transport, walking and cycling give out nothing. Buses, trains and electric vehicles charged from clean power give out much less carbon dioxide for each person. In farming and on land, stopping the clearing of forests keeps carbon out of the air. Planting new forests takes it in. Better feeding of cattle, and draining rice fields now and then, cut methane. Wasting less food also helps, because about a tenth of emissions come from food that is thrown away.

Methane deserves special mention. It lasts only about a dozen years in the air. So cutting it slows warming fast. People can mend leaks from gas pipes and mines. They can catch the gas from rubbish tips. Some factories can also catch carbon dioxide from their chimneys and store it deep underground. This is called carbon capture. It is still done on a small scale.

Cleaner cooking links back to the chapter on air pollution. A stove with a chimney takes the smoke out of homes. So does a cleaner fuel, such as biogas, gas or electricity. They also use less wood. This is better for health, and it cuts emissions as well.

Now adaptation. Even if emissions fall fast, some warming and some changes are already coming. People can get ready. The right choices depend on the risk in each place.

Some risks, and some responses to them. The best response depends on the place.

Storing water is an old and sound idea. Rain from a roof can run into a tank. Ponds and small dams can hold rain for the dry months, and wells can be recharged by letting rain soak in. A tank turns a short wet season into a store for a long dry one. Remember that 1 mm of rain on 1 m² is 1 litre.

Worked example

A flat roof of 30 m² collects 30 mm of rain into a tank. A family of 5 uses 20 litres each every day, for drinking, cooking and washing. For how many days does the water last?

1 mm on 1 m² is 1 litre, so the roof gives 30 × 30 = 900 litres.
The family uses 5 × 20 = 100 litres a day.
The water lasts 900 ÷ 100 = 9 days.
So the tank lasts for 9 days.

Answer: 9 days

The water from the roof of the worked example, in a tank.

Farmers can grow other crops. Millet, sorghum and cassava need less water than many crops. Seeds that ripen in a shorter season, or that stand floods, are another choice. Farmers can also change the planting dates. They can spread the risk with more than one crop. A covering of straw or leaves keeps water in the soil.

On the coast, mangrove trees and reefs break the waves. Sea walls and raised houses keep the water out, and a family can move to higher ground. Storm warnings by radio and phone, with shelters that people know, have saved many lives in tropical storms. In hot places, shade trees, light coloured roofs and rest in the heat of the day protect health.

Governments, towns and families all have a part. In 2015 almost all the world’s countries made an agreement. They would try to hold the warming well below 2 °C above the late 1800s level, and aim for 1.5 °C. The places that have added least to the problem often face the most risk, so help in paying for adaptation matters.

Try it

How much water could your roof collect?

You need: a tape measure, or a stick of known length, or your own steps, a rain gauge reading from the earlier chapter, or a figure from the radio, paper and a pencil.

Be safe: Never climb on a roof to measure it. Measure from the ground. Keep stored water covered, so that mosquitoes cannot breed in it, and a child cannot fall in.

From the ground, measure the length and width of a roof. Walk along the wall below it. Do not walk on the roof. Do not climb on a roof.
Work out the area in m²: length × width, for the part that rain falls on, seen from above.
Take a rain figure in mm, from your rain gauge for one rainy day, or from a local figure for a month.
Multiply the area by the rain in mm. That is the water in litres that falls on the roof.

What you will see: A modest roof gives a lot of water from even one good rain. Rain tanks make this water available when the rain has stopped. You can compare the litres with what the family uses in a day.

If you cannot try it, this is what happens.

A wrong idea many people have

The Earth has already started to warm, so nothing we do now can matter.

Why it feels right: If some change cannot be undone, it seems there is no point in trying to change what is left.

The test that breaks it: Warming depends on the total amount of carbon dioxide put into the air. Each tonne less means a little less warming. The difference between a warming of 1.5 °C and one of 3 °C is a large difference in harm.

What is true: Every cut helps, and the sooner the better. Mitigation limits how much worse things get, and adaptation protects people from what is coming.

A wrong idea many people have

Planting trees is enough to solve climate change.

Why it feels right: Trees take in carbon dioxide, so it seems that planting enough of them would clear the air.

The test that breaks it: Trees do take in carbon dioxide, and it is very useful to protect forests and plant new ones. But burning fossil fuels puts far more into the air each year than new trees could take in. And a forest that burns or is cut down gives its carbon back.

What is true: Forests help, but cutting the burning of coal, oil and gas is needed as well. No single action is enough.

A wrong idea many people have

Wind and solar power give out no greenhouse gas at all.

Why it feels right: Nothing burns while the wind turns the blades or the sun shines on a panel.

The test that breaks it: Making steel, glass and panels, and building and moving the equipment, uses energy. Over its whole life, wind gives out about 11 g of carbon dioxide for each kWh and solar about 48 g. Coal gives out about 820 g.

What is true: Wind and solar give out very little compared with coal and gas, but not nothing.

Key words

mitigation: cutting greenhouse gas emissions, to slow the warming.
adaptation: changing how we live, to be safer from the changes that are coming.
net zero: the point where the carbon dioxide put into the air equals what is taken out.
energy efficiency: doing the same job with less energy.

Back to your guess

The answer is C: Two things: cut greenhouse gas emissions to slow the change, and adapt to the changes that will come anyway.

There are two kinds of response, and both are needed. Cutting emissions slows the warming. Adapting protects people from the changes that cannot now be avoided. Every tenth of a degree of warming that is avoided makes things easier to live with.

Keep it

Copy this into your exercise book.

Mitigation cuts emissions: clean power, less waste, better farming. Adaptation protects people: store water, new crops, coastal defences, warnings. Both are needed.

Draw: Two boxes side by side. One is titled mitigation, with three ways to cut emissions. One is titled adaptation, with three ways to cope.

How the temperature changes with height

A model of the atmosphere on a strip of string

Rain adds up in the gauge

Build a rain gauge and a wind vane, and keep a record

A sea breeze, step by step

Watch the wind change through the day

Draw a climate graph for a place you know

What an acid does to shell

A smoke walk round a kitchen

The greenhouse effect in three steps

A jar model of slowing the heat

Ice on water and ice on land

How much water could your roof collect?

Key words, A to Z