Key Stage 3 science · Chemistry – Earth and atmosphere

KS3 carbon cycle, atmosphere and climate change: a guide for parents

The air around us is a mixture of gases, and carbon moves constantly between the air, living things, the oceans and rocks. In Years 7 to 9 children learn what the atmosphere is made of, how the carbon cycle works, and how burning fossil fuels and other human activity add carbon dioxide and change the climate.

The carbon cycle, atmosphere and climate, explained

Pupils learn the make-up of dry air: about 78% nitrogen, about 21% oxygen, a little under 1% argon, and carbon dioxide at around 0.04%, together with tiny amounts of other gases and a variable amount of water vapour. The surprise for many families is that nitrogen, not oxygen, is the biggest part, and that carbon dioxide is such a small fraction. Even a small share of a gas can matter a lot, which is the main idea in the rest of this topic.

The carbon cycle describes how carbon atoms move between stores. Plants take in carbon dioxide for photosynthesis and build it into sugars and other substances. Animals get carbon by eating plants or other animals. Carbon dioxide returns to the air by respiration in plants and animals, by decomposition as microbes break down dead material, and by combustion when wood or fuel is burned. The oceans absorb and release carbon dioxide, and some carbon ends up in shells and then limestone. Fossil fuels (coal, oil and gas) are the remains of ancient living things that were buried and slowly changed over millions of years, which locked their carbon away from the air.

Carbon dioxide is a greenhouse gas. The greenhouse effect is natural and necessary: sunlight warms the Earth’s surface, which gives out heat, and gases such as carbon dioxide, methane and water vapour absorb some of that heat and send part of it back, keeping the planet warm enough for life. The concern is the enhanced effect. Burning fossil fuels for electricity, heating and transport, and clearing forests, adds carbon dioxide faster than plants and the oceans can absorb it, so more stays in the atmosphere and more heat is trapped. The global average temperature has risen by a little over one degree Celsius since the late 1800s, and scientists have found that human activity is the main cause.

Pupils consider the likely effects: melting glaciers and ice sheets together with warmer, expanding oceans raise sea levels; heatwaves become more frequent and heavy rainfall more intense in some places; and habitats shift, so some species struggle. They also consider responses, from using energy more efficiently and switching to renewable sources to protecting forests. Teachers also stress that weather and climate are different. Weather is what is happening on one day, while climate is the average pattern over decades, so a single cold winter does not disprove global warming.

Words your child will hear

Atmosphere: The layer of gases surrounding the Earth.
Carbon cycle: The movement of carbon between the air, living things, oceans, rocks and fuels.
Fossil fuel: Coal, oil or gas formed over millions of years from the remains of living things.
Greenhouse gas: A gas such as carbon dioxide or methane that traps heat in the atmosphere.
Global warming: The rise in the Earth’s average surface temperature over many decades.
Climate: The usual pattern of weather in a region, measured over many years.
Renewable energy: Energy from sources such as wind, solar and waves, which are not used up.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about the carbon cycle
Pupils should be taught about the composition of the atmosphere
Pupils should be taught about the production of carbon dioxide by human activity and the impact on climate

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A model answer: why does burning fossil fuels raise carbon dioxide?

Trees take in carbon dioxide. Use the carbon cycle to explain why the amount of carbon dioxide in the air is still rising.

Say where the carbon was before. Fossil fuels contain carbon taken from the air by living things millions of years ago. This carbon had been locked underground for a very long time.
Describe what burning does. When coal, oil or gas is burned, the carbon combines with oxygen and is released into the air as carbon dioxide. In around two centuries, people have released carbon that took millions of years to lock away.
Include the natural removal. Photosynthesis in plants and absorption by the oceans take carbon dioxide out of the air.
Compare the rates. Human activity is adding carbon dioxide faster than these natural processes can remove it. Cutting down forests makes the problem worse because there is less photosynthesis, and burning or rotting the wood releases more.
Link to the climate. More carbon dioxide in the atmosphere traps more heat, which raises the average global temperature.

Burning fossil fuels releases carbon that was locked away for millions of years, faster than plants and oceans can absorb it, so carbon dioxide builds up and traps more heat.

Common mistakes and how to help

Confusing the greenhouse effect with the hole in the ozone layer

Both are described as environmental problems involving the atmosphere, and they are often discussed together in the news.

What helps: Keep them separate. The greenhouse effect is heat trapped by gases such as carbon dioxide. The ozone layer is a part of the upper atmosphere that absorbs harmful ultraviolet light, and it is a different issue with a different cause.

Saying a cold day or a snowy winter proves global warming is not happening

Weather is what children experience directly, while climate is an average that cannot be seen on one day.

What helps: Compare daily weather with long-term patterns, like the difference between one game score and a team’s record over a season. Keeping a weather diary shows how much a single day can vary.

Believing air is mostly oxygen

We talk about breathing in oxygen, so it seems natural to assume that air is mostly made of it.

What helps: Imagine air as a bag of 100 marbles: 78 would be nitrogen, 21 oxygen and about 1 argon. You would need a bag of about 2,500 marbles before one of them was carbon dioxide.

Seeing carbon dioxide only as a harmful pollutant

Climate stories focus on the problem, not on what carbon dioxide does naturally.

What helps: Remind your child that plants need it for photosynthesis, and the greenhouse effect keeps the Earth habitable. The problem is the extra amount added by burning fuels.

How to help at home

Keep a weather diary. Record the temperature, cloud cover and rainfall for a few weeks, then discuss how it varies day to day. Ask how many years of such records you would need before you could talk about the climate.
Follow the carbon. Choose a meal or a car journey and trace where its carbon came from and where it goes. A sandwich holds carbon taken from the air by plants, and petrol releases carbon that was underground for millions of years.
Energy detective. Look at the energy labels on appliances and talk about which ones use the most. Ask which activities at home use energy that probably comes from burning fuels, and what could be changed.
Draw the air. Make a bar of 100 small squares and colour in the gases in dry air: 78 nitrogen, 21 oxygen, and the rest as a thin sliver. Discuss why such a tiny amount of carbon dioxide can still affect the planet.
Read graphs critically. When a news story shows a graph about temperature or carbon dioxide, ask your child to say what is on each axis, over what time period, and where the data came from.

Leads on to

Key Stage 4: The atmosphere and climate change
Key Stage 4: Air pollutants and potable water