Key Stage 4 science · Chemistry – Earth and atmospheric science

GCSE chemistry: the atmosphere and climate change

This GCSE chemistry topic tells the story of Earth’s atmosphere, from its early volcanic beginnings to the air we breathe now, and then asks what burning fossil fuels and farming are doing to it. Students look at the evidence for how the atmosphere has changed, why carbon dioxide and methane trap energy, what the effects of rising levels could be, and what can be done. They are also asked to be honest about where the evidence is uncertain.

The atmosphere and climate change, explained

Today’s dry air is about 78% nitrogen, 21% oxygen, just under 1% argon and roughly 0.04% carbon dioxide, with traces of other gases and a variable amount of water vapour. It was not always so. In its first billion years, intense volcanic activity released gases that formed an early atmosphere, thought to be mostly carbon dioxide with water vapour, nitrogen and small amounts of methane and ammonia. There was almost no oxygen. As the Earth cooled, water vapour condensed to form the oceans.

Over a long time, the proportion of carbon dioxide fell. Some dissolved in the oceans, and some was locked into sedimentary rocks such as limestone and into fossil fuels, which formed from the remains of living things. Algae and later plants evolved and photosynthesised, removing carbon dioxide and releasing oxygen, which built up to the level we have today. The evidence for all this is indirect, because nobody was there to measure it. Scientists use rocks, fossils and models, and there is a real uncertainty about the details of what happened billions of years ago.

The greenhouse effect is natural, and without it Earth would be too cold for most life. Energy from the Sun reaches the surface, which warms and radiates energy back as infrared radiation. Greenhouse gases, including carbon dioxide, methane and water vapour, absorb some of this infrared and re-emit it in all directions, including back to the surface, which keeps the lower atmosphere warmer than it would be. The concern is the enhanced effect. Burning fossil fuels and clearing forests add carbon dioxide, while cattle, rice growing, landfill and fossil fuel leaks add methane. Carbon dioxide has risen from around 280 parts per million before industrialisation to over 420 parts per million today.

Students are asked to evaluate evidence, not just state a conclusion. Measurements from ice cores, tree rings, thermometers and satellites support the picture, and the scientific consensus is that human activity is the main cause of the warming seen since the mid-1800s. Uncertainties lie in predicting how large future changes will be, how quickly, and how local weather will respond. Possible effects include rising sea levels, melting ice, more extreme weather and changes to where plants and animals can live. Mitigation includes cutting emissions through renewable energy and efficiency, protecting forests, capturing carbon dioxide, and reducing methane from farming and waste.

Words your child will hear

Greenhouse gas: A gas, such as carbon dioxide or methane, that absorbs infrared radiation and helps keep the atmosphere warm.
Anthropogenic: Caused by human activity.
Carbon footprint: The total carbon dioxide and other greenhouse gases released by an activity, product or person.
Fossil fuel: A fuel formed from the remains of living things over millions of years, such as coal, oil and natural gas.
Carbon neutral: Having no net release of carbon dioxide, because emissions are balanced by removal.
Mitigation: Action to reduce the causes or the severity of climate change.
Correlation: A link between two measurements that rise or fall together, which does not by itself prove one causes the other.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about evidence for composition and evolution of the Earth's atmosphere since its formation
Students should be taught about evidence, and uncertainties in evidence, for additional anthropogenic causes of climate change
Students should be taught about potential effects of, and mitigation of, increased levels of carbon dioxide and methane on the Earth's climate

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

Try it: evaluating a claim with evidence

A graph shows that global average temperature and the level of carbon dioxide in the air have both risen since 1900. Does this prove that carbon dioxide is causing the warming? Write a balanced answer.

Describe the pattern accurately: both quantities rise over the same period. Use the figures if you have them, and name the axes.
Say what a correlation can and cannot show. Two things rising together may be linked, but may both be caused by something else, so the graph alone does not prove cause.
Add a mechanism. Laboratory measurements show that carbon dioxide and methane absorb infrared radiation. Physics gives a reason why more of them would warm the lower atmosphere.
Rule out other explanations. Changes in the Sun’s output and volcanic eruptions are both measured, and they do not account for the size and pattern of the warming over this period.
State the uncertainties. The science is firm that the climate is warming and that human activity is the main cause. It is less certain how fast future warming will be and what the local effects will be, because the climate system is complex.

The graph shows a correlation and does not prove cause by itself. Together with the absorption of infrared by greenhouse gases, measured human emissions and evidence that natural causes cannot explain the warming, it supports the conclusion that human activity is the main driver, while the size of future effects is still uncertain.

Common mistakes and how to help

Confusing the greenhouse effect with the hole in the ozone layer

Both are problems with the atmosphere caused by human pollution, so they are easily merged into one story.

What helps: The greenhouse effect is about gases trapping energy near the surface. Ozone depletion is about a different gas, ozone, in the upper atmosphere that absorbs ultraviolet radiation. They have different causes and different solutions.

Treating a cold day as proof that climate change is not happening

Weather is what we experience, and one cold winter feels like strong evidence.

What helps: Weather is what is happening today. Climate is the average over decades. Ask your child to compare one day’s temperature with the average of many years, as a scientist would.

Saying 0.04% is too small to matter

Small percentages sound negligible, and carbon dioxide is a tiny part of the air.

What helps: Small amounts of some substances have big effects, just as a little food colouring tints a glass of water. Carbon dioxide absorbs infrared strongly, so a small change in a small amount matters.

Assuming the atmosphere has always been like it is today

Textbook pictures show the current mix of gases, and we take breathable air for granted.

What helps: The early atmosphere had very little oxygen. Oxygen built up as photosynthesis took place over a very long time. Ask “where did the oxygen come from?” and trace it back.

How to help at home

Check a graph together. Find a climate graph in a newspaper or a book and ask what is on each axis, over what period, and who produced it. Then ask what it shows and what it cannot show. This is the evaluation habit that exam questions reward.
Estimate a household carbon footprint. List the biggest sources in your home: heating, electricity, travel and food. Look at an energy bill and find the units of electricity used. You do not need exact numbers, just a sense of which choices matter most, which also helps with the mitigation part of the topic.
Keep the conversation calm. Some teenagers feel anxious about climate change. Talk about what is known, what is uncertain, and what people and governments are doing. Knowing the science and the practical options is generally more reassuring than avoiding the subject.
Make a simple model of the atmosphere’s history. Draw a timeline on a long strip of paper with the Earth forming, oceans appearing, algae producing oxygen and humans burning fossil fuels. Marking how late the human part comes is a memorable way to see why scientists use indirect evidence for the earliest stages.

Builds on

Key Stage 3: The Earth, the rock cycle and resources
Key Stage 3: The carbon cycle, atmosphere and climate