The Greenhouse Effect
Some gases in the atmosphere act like the glass of a greenhouse: they let the Sun’s energy in but slow its escape, keeping the surface warm enough for life. The examiner’s favourite phrase here is “short and long wavelength radiation” — get that sequence right and the marks follow.
Water vapour, carbon dioxide and methane. They keep the Earth’s temperature high enough to support life — without them the surface would be far colder.
Short-wavelength radiation comes in from the Sun (tight waves); the surface absorbs it and re-emits long-wavelength infrared (stretched waves — about double the wavelength). A greenhouse-gas molecule absorbs that infrared and re-radiates it in all directions — some back down, trapping heat.
- Short-wavelength radiation from the Sun passes through the atmosphere and is absorbed by the Earth’s surface, warming it.
- The surface re-emits energy as longer-wavelength (infrared) radiation.
- Greenhouse gases absorb this long-wavelength radiation and re-radiate it in all directions — some back to the surface, trapping heat and keeping the Earth warm.
- Mixing up the wavelengths. Short-wavelength radiation comes in; the Earth re-emits long-wavelength (infrared); greenhouse gases absorb the long-wavelength radiation.
- Saying greenhouse gases “reflect” radiation. They absorb and re-radiate it — not mirror-like reflection.
- Confusing this with the ozone layer or with pollution. The greenhouse effect is about trapping heat, not blocking UV or smog near the ground.
The Enhanced Greenhouse Effect
The natural greenhouse effect above is essential — it keeps the planet warm enough for life. The problem is the enhanced greenhouse effect: as human activity adds extra CO2 and methane, more infrared is trapped, the average global temperature rises, and the climate changes (section 5). In the animation below, drag the greenhouse-gas slider to add more molecules to the air — watch more infrared get sent back to the surface, and the surface thermometer climb.
Press play to follow the energy: short-wavelength radiation in from the Sun, long-wavelength infrared back out, and some of it trapped. Drag the slider to add greenhouse gas.
🧪 Exam-style questions
Which two of these are greenhouse gases? Tick (✓) two boxes, then press Check.
What do greenhouse gases do to keep the Earth warm? Tick (✓) one box.
Which statement describes the radiation in the greenhouse effect correctly? Tick (✓) one box.
Describe the greenhouse effect in terms of short- and long-wavelength radiation. Build the answer in your head (or on paper), then compare it with the model answer below.
Show a model answer
- Short-wavelength radiation from the Sun passes through the atmosphere and is absorbed by the Earth’s surface, warming it. 1 mark
- The surface re-emits energy as longer-wavelength (infrared) radiation. 1 mark
- Greenhouse gases (water vapour, carbon dioxide, methane) absorb this long-wavelength radiation. 1 mark
- They re-radiate it in all directions — some back down to the surface — trapping heat and keeping the Earth warm. 1 mark
Watch the wording: greenhouse gases absorb and re-radiate — never “reflect”.
Human Activity & Climate Change
A growing population, burning more fuel and clearing more land, is adding extra carbon dioxide and methane to the air. That enhances the greenhouse effect and, most scientists agree, is driving global climate change. You need the human activities, the evidence, and a fair view of its uncertainties.
Carbon dioxide — recall two:
- Burning fossil fuels (for electricity, heating and transport).
- Deforestation — fewer trees means less CO2 removed by photosynthesis.
Methane — recall two (each of these is a separate activity):
- Farming livestock (cattle and other animals produce methane in digestion).
- Rice paddy fields (flooded fields release methane).
- Landfill / decomposing waste (rotting rubbish gives off methane).
By comparing the percentage of CO2 in the atmosphere with global temperature over many years, scientists find a clear correlation: as CO2 rises, temperature rises. This evidence is peer-reviewed — checked by other scientists — which is why most scientists now agree human activity is warming the surface. A correlation this strong is powerful evidence, but a correlation on its own does not prove that CO2 causes the warming — the case rests on the physics of the greenhouse effect (section 4) and the correlation together.
CO2 and global temperature have risen together since the Industrial Revolution — a strong correlation. Be careful in exams: a correlation is powerful evidence, but on its own it does not prove cause.
- Peer review — results are checked by other scientists before they are trusted; this filters out errors and bias.
- Why it’s uncertain — the climate is hugely complex and hard to model, and historical data (from tree rings, ice cores) is less precise than modern measurements. This leads to simplified models and, in the media, speculation and opinions that may be based on only part of the evidence and may be biased.
- Communication matters — results should be shared with a wide range of audiences so decisions are based on the full picture, not the headlines.
- Rising sea levels (melting ice caps and glaciers) → coastal flooding and loss of land.
- More frequent and severe storms (warmer oceans give storms more energy).
- Changes to rainfall — droughts and floods → water shortages and reduced food production.
- Temperature and habitat changes → shifts in the distribution of species and loss of habitats.
You should be able to describe four effects and discuss the scale, risk and environmental implications.
🧪 Exam-style questions
Which human activity increases the amount of methane in the atmosphere? Tick (✓) one box.
Explain how deforestation increases the amount of carbon dioxide in the atmosphere.
Show answer
- Trees remove carbon dioxide from the air by photosynthesis. 1 mark
- With fewer trees, less CO2 is absorbed, so more stays in the atmosphere. 1 mark
Allow: burning or rotting the felled wood also releases CO2.
A graph shows atmospheric CO2 from 1850 to 2020. Which best describes the change? Tick (✓) one box.
Why is it important that evidence about climate change is peer-reviewed? Tick (✓) one box.
A newspaper article claims that human activity is causing climate change. Evaluate the evidence for this claim. This is a levels-of-response question — a good answer weighs evidence on both sides and reaches a justified conclusion. Plan, then compare.
Show a model answer
How it is marked (levels of response):
- Level 3 (5–6): a balanced answer using evidence for human-caused change and the uncertainties/limitations, ending with a justified conclusion.
- Level 2 (3–4): describes evidence for the claim and gives at least one limitation.
- Level 1 (1–2): one or two simple relevant points.
Evidence the claim is correct:
- CO2 and average global temperature have risen together since the Industrial Revolution — a strong correlation.
- There is a mechanism: extra CO2 and methane enhance the greenhouse effect, trapping more long-wavelength radiation.
- Human activities (burning fossil fuels, deforestation, farming) raise CO2 and methane, and the evidence is peer-reviewed.
Reasons to be cautious (uncertainty):
- A correlation does not prove cause on its own.
- The climate is very complex and hard to model, so models are simplified; older data is less precise.
- Media reports may be biased or based on only part of the evidence.
Conclusion (needed for Level 3): based on peer-reviewed evidence, most scientists agree human activity is the main cause — the mechanism and the correlation together make a strong case, even though some uncertainty remains. 6 marks
The Carbon Footprint
If climate change is driven by greenhouse-gas emissions, one useful way to measure the impact of anything we make or do is its carbon footprint.
The carbon footprint is the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, a service or an event.
“Full life cycle” means everything — getting the raw materials, manufacturing, transport and distribution, use, and disposal at the end.
The carbon footprint adds up the greenhouse gases emitted at every stage of a product’s life — not just while it is being used.
Emissions of CO2 and methane can be reduced by, for example:
- using renewable energy (solar, wind) instead of burning fossil fuels, and improving energy efficiency;
- using public transport or electric vehicles and cutting unnecessary journeys;
- carbon capture and storage — trapping CO2 and storing it underground;
- government taxes, targets and incentives to encourage low-carbon choices.
Exam questions love the “why is this hard?” side. Reasons actions are limited include:
- Cost — renewable technology and changes can be expensive.
- Incomplete or developing technology — some solutions (eg large-scale carbon capture) are still being developed.
- Economic and political concerns — governments fear harming their economies or jobs, and international agreement is hard.
- Lifestyle resistance — people are reluctant to change habits (travel, diet).
🧪 Exam-style questions
What is meant by the “carbon footprint” of a product? Tick (✓) one box.
Which action would reduce a person’s carbon footprint? Tick (✓) one box.
Give one reason why actions to reduce carbon emissions may be limited.
Show answer
Any one of: the changes are often expensive (cost); the technology is still developing; economic or political concerns (governments fear harming the economy or jobs); or people are reluctant to change their lifestyles. 1 mark
A company wants to reduce the carbon footprint of its products. Suggest actions it could take, and explain why such actions may be limited. Aim for a balance of actions and reasons, then compare with the model answer.
Show a model answer
How it is marked (levels of response):
- Level 3 (5–6): a balanced answer giving several actions and several reasons they may be limited, logically linked, ending with a justified conclusion.
- Level 2 (3–4): gives some actions and at least one reason they may be limited, with some linking.
- Level 1 (1–2): one or two simple relevant points (an action or a limitation).
Actions to reduce the carbon footprint (any three):
- Use renewable energy and improve energy efficiency in manufacturing.
- Cut transport emissions — source locally, use electric vehicles, reduce journeys.
- Use carbon capture and storage, or use less / recyclable materials.
Why actions may be limited (any three):
- Cost — renewable technology and changes can be expensive.
- Some technology is still developing (e.g. large-scale carbon capture).
- Economic/political concerns (higher prices, competitiveness, hard international agreement) and people’s reluctance to change habits.
Conclusion (needed for Level 3): the company could cut its carbon footprint in several ways, but in practice cost, developing technology and economic, political and lifestyle factors limit how far and how quickly these can be applied. 6 marks