AQA GCSE Combined Science (8464), Higher tier · Chemistry › Chemistry of the atmosphere › Carbon dioxide and methane as greenhouse gases
Practise Greenhouse gases. 15 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Why the Earth is warm enough for life: greenhouse gases absorb long wavelength radiation given out by the Earth's surface. Expect 'name' questions and 2 to 4 mark 'describe' or 'explain' questions on the greenhouse effect in terms of short and long wavelength radiation.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Name three greenhouse gasesWater vapour, carbon dioxide and methane are greenhouse gases.
4
State why greenhouse gases are neededThey keep the Earth's temperature high enough to support life.
5
Describe what happens to the Sun's radiationShort wavelength radiation from the Sun passes through the atmosphere and is absorbed by the Earth's surface, warming it.
6
Describe the greenhouse effect using wavelengthsThe warm surface emits long wavelength radiation, which greenhouse gases absorb and re-emit in all directions.
7
Explain why more greenhouse gases cause warmingMore long wavelength radiation is absorbed and re-emitted towards the Earth, so less escapes to space and the average temperature rises.
8
Evaluate a model of the greenhouse effectCompare an experiment or model with the real atmosphere and judge what it does and does not show.
Notes
What greenhouse gases do
Greenhouse gases in the atmosphere keep the Earth's temperature high enough to support life. Without them the Earth would be far too cold.
The greenhouse gases you need to know are water vapour (H2O), carbon dioxide (CO2) and methane (CH4).
Nitrogen and oxygen, the main gases in air, are not greenhouse gases.
The greenhouse effect is natural and needed. The concern is that human activities are adding more greenhouse gases and making it stronger.
The greenhouse effect step by step
1. The Sun gives out mainly short wavelength radiation (for example visible light).
2. Most of this short wavelength radiation passes through the atmosphere. Much of it is absorbed by the Earth's surface, which warms up.
3. The warm Earth's surface gives out long wavelength radiation (infrared).
4. Greenhouse gas molecules absorb some of this long wavelength radiation.
5. They then re-emit it in all directions, including back towards the Earth's surface. This keeps the surface and the lower atmosphere warmer.
More greenhouse gas, more warming grade 7+
Greenhouse gases let most of the incoming short wavelength radiation through, but absorb some of the outgoing long wavelength radiation. The difference in wavelength is the key idea.
If the amount of greenhouse gases increases, more long wavelength radiation is absorbed and re-emitted back towards the Earth, so less escapes into space.
Energy still arrives from the Sun at the same rate but escapes more slowly, so the Earth's average temperature rises.
Cheatsheet
Greenhouse gases: water vapour, carbon dioxide, methane
They keep the Earth warm enough to support life
Sun → short wavelength radiation → passes through the atmosphere → absorbed by the Earth's surface
Warm Earth's surface → gives out long wavelength (infrared) radiation
Greenhouse gases absorb long wavelength radiation and re-emit it in all directions
More greenhouse gases → more radiation absorbed → higher average temperature
Nitrogen and oxygen are not greenhouse gases
How to answer each type of question
Name or identify greenhouse gases
1 to 2 marks3
Learn the three: water vapour, carbon dioxide, methane.
Tick only the number of boxes the question asks for.
Example. Which two of these gases are greenhouse gases? argon carbon dioxide methane nitrogen oxygen
Show the model answer
carbon dioxide (1) methane (1)
Explain the greenhouse effect in terms of radiation
3 to 4 marks6
Start with the Sun: short wavelength radiation passes through the atmosphere.
The Earth's surface absorbs it, warms up and emits long wavelength (infrared) radiation.
Greenhouse gases absorb some of the long wavelength radiation.
They re-emit it in all directions, including back towards the Earth.
Example. Explain how greenhouse gases keep the Earth's surface warm. Refer to the wavelengths of radiation in your answer.
Show the model answer
Short wavelength radiation from the Sun passes through the atmosphere (1) and is absorbed by the Earth's surface, which warms up (1). The Earth's surface emits long wavelength (infrared) radiation (1). Greenhouse gases absorb some of this long wavelength radiation and re-emit it in all directions, including back towards the Earth (1).
Explain the effect of increasing a greenhouse gas
2 marks7
Say that more of the long wavelength radiation emitted by the Earth is absorbed.
Say that less escapes into space (more is re-emitted towards the Earth), so the average temperature rises.
Example. The concentration of methane in the atmosphere is increasing. Explain why this could cause the Earth's average temperature to rise.
Show the model answer
More methane molecules absorb more of the long wavelength radiation emitted by the Earth's surface (1). More radiation is re-emitted back towards the Earth and less escapes into space, so the average temperature rises (1).
Evaluate an experiment or model
2 to 3 marks8
Compare the model with the real atmosphere: amount of gas, how the gas is heated, size and complexity.
Comment on the method: repeats, control variables, only one result.
Say what the model does show, as well as its limits.
Example. A student placed a thermometer in each of two sealed flasks. One contained air and the other contained air with extra carbon dioxide. Both flasks were placed under the same lamp. The flask with extra carbon dioxide warmed up more. The student said: 'This proves that more carbon dioxide in the atmosphere will cause global warming.' Give two reasons why the student's conclusion is not fully justified.
Show the model answer
Any two from: it was done only once, so the result may not be repeatable (1) the flask contains much more carbon dioxide than the atmosphere does (1) the lamp heats the gas directly, whereas in the atmosphere the gases absorb long wavelength radiation emitted by the Earth's surface (1) the real atmosphere is much more complex, e.g. clouds, oceans and other gases also affect the temperature (1)
Shortcuts and memory tricks
Short in, long out: short wavelength comes in from the Sun, long wavelength goes out from the Earth and is absorbed by greenhouse gases.
The three gases: 'Warm Climate Makers': Water vapour, Carbon dioxide, Methane.
Full-marks chain: Sun → short wavelength → surface absorbs → long wavelength emitted → gases absorb → re-emit in all directions.
Where marks are lost
Saying greenhouse gases 'trap heat' or 'reflect heat'. Say they absorb long wavelength (infrared) radiation and re-emit it.
Saying the radiation from the Sun is long wavelength. The incoming radiation is mainly short wavelength; the Earth emits long wavelength.
Mixing up the greenhouse effect with the ozone layer. They are not the same thing, so don't mention the ozone layer.
Forgetting that water vapour is a greenhouse gas, or naming nitrogen or oxygen as greenhouse gases.
Saying greenhouse gases are harmful. Without them the Earth would be too cold for life; the problem is increasing amounts of them.
Exam technique
If the question says 'in terms of radiation' or 'refer to wavelength', you must use the words short wavelength and long wavelength.
Write the steps in order: each step is usually one marking point.
'Absorb' and 're-emit in all directions' are the key phrases examiners look for.
You can write 'infrared' as well as 'long wavelength', but not 'heat rays'.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Name one other greenhouse gas.
water vapour
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) Which two gases are greenhouse gases? Tick (✓) two boxes.[2]
Argon
Carbon dioxide
Methane
Nitrogen
Oxygen
(b) Name one other greenhouse gas.[1]
(c) Why are greenhouse gases important for life on Earth?[1]
Show the answer and mark scheme
(a)Answer: Carbon dioxide; Methane
(b)Answer: water vapour
water vapour
(c)
they keep the Earth's temperature high enough to support life
Question 2Medium5 marks
Greenhouse gases in the atmosphere keep the Earth warm.
(a) What type of radiation is mainly given out by the Earth's surface? Tick (✓) one box.[1]
Long wavelength (infrared) radiation
Short wavelength visible light
Ultraviolet radiation
Gamma radiation
(b) Describe how greenhouse gases keep the Earth warm. Use ideas about short wavelength and long wavelength radiation in your answer.[4]
Show the answer and mark scheme
(a)Answer: Long wavelength (infrared) radiation
(b)
short wavelength radiation from the Sun passes through the atmosphere
the Earth's surface absorbs this radiation and emits long wavelength (infrared) radiation
greenhouse gases absorb (some of) the long wavelength radiation
and re-emit it in all directions, including back towards the Earth, so the Earth's surface and lower atmosphere are warmer
Question 3Hard4 marks
Large amounts of methane are frozen within permafrost (permanently frozen ground) in polar regions.
(a) Suggest why scientists are concerned that rising global temperatures could cause a 'positive feedback' involving permafrost.[2]
(b) Explain why this makes predicting future global temperature rise more difficult for scientists.[2]
Show the answer and mark scheme
(a)
as temperatures rise, permafrost thaws, releasing the methane trapped within it into the atmosphere
since methane is a greenhouse gas, this causes further warming, which thaws more permafrost, releasing yet more methane, and so on
(b)
the size and speed of this feedback effect is hard to measure and predict accurately
so computer models may underestimate future warming if this feedback is not fully accounted for