Key Idea: Topic 6.1 carries few marks, and almost all of them are on the natural greenhouse effect. Learn how it works, then the gases, the layers and the three circulation cells.
1. The greenhouse effect (most marks)
- Sunlight (short-wave) passes through the air and warms the surface.
- The warm surface gives off infrared (long-wave) radiation.
- Greenhouse gases absorb and re-emit it, so Earth averages about 15 °C, not about -18 °C.
2. Greenhouse gases and aerosols
- Main GHGs: water vapour, carbon dioxide, methane, nitrous oxide, ozone, CFCs.
- Cattle and rice paddies give off methane; burning fuels gives carbon dioxide.
- Most aerosols, such as sulfate, reflect sunlight and cool; soot absorbs it and warms.
3. Layers and circulation
- Air is about 78% nitrogen, 21% oxygen, under 1% argon and about 0.04% carbon dioxide.
- Weather happens in the troposphere; the ozone layer is in the stratosphere.
- Uneven heating drives three cells in each hemisphere: Hadley, Ferrel and polar.
How it is examined:
| Question | How to score |
|---|---|
| [1] Name a greenhouse gas | Name it exactly: methane, not 'gas from cows'. |
| [1] Give evidence a gas is a greenhouse gas | It absorbs infrared, so the temperature rises with it. |
| [4] Outline the greenhouse effect | Short-wave in, surface warms, long-wave out, GHGs absorb and re-emit. |
| [7] Explain how the atmosphere keeps Earth habitable | Greenhouse effect, ozone absorbing UV, heat moved by circulation. |
Key terms, one line each:
| Term | In one line |
|---|---|
| Greenhouse gas | a gas that absorbs and re-emits infrared radiation |
| Aerosol | tiny solid or liquid particles in the air, such as sulfate or soot |
| Troposphere | the lowest layer, where weather happens |
| Stratosphere | the layer above, which holds the ozone layer |
| Tricellular model | three circulation cells in each hemisphere move heat to the poles |
Last-minute check: answer, then tap to see.
What kind of radiation do GHGs absorb? Infrared, the long-wave radiation from the warm surface.
Without the greenhouse effect, how cold would Earth be? About -18 °C on average, not 15 °C.
Name a greenhouse gas from farming. Methane from cattle and rice, or nitrous oxide from fertiliser.
Which layer holds the ozone layer? The stratosphere.
The Moon is about the same distance from the Sun as Earth, but it has no atmosphere and its surface swings from very hot to very cold. Explain how Earth's atmosphere keeps its surface warm.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Exam tips
- For the greenhouse effect, say short-wave in and long-wave (infrared) out.
- Say GHGs absorb and re-emit infrared, not that they 'trap' or 'block' sunlight.
- Do not mix up the greenhouse effect with the ozone hole.
- Name the gases exactly: carbon dioxide, methane, nitrous oxide, water vapour.