Key Idea: Topic 6.1 carries few marks, and almost all of them are on the natural greenhouse effect. At HL, also learn the dynamic atmosphere, Milankovitch cycles and how life changed the air.
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. The atmosphere and HL ideas
- 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.
- At HL: the atmosphere as a dynamic system, gravity and air pressure, Milankovitch cycles and how life changed the air.
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 |
| Milankovitch cycles | changes in Earth's orbit and tilt that cause ice ages |
| Great Oxidation Event | oxygen from early photosynthesis building up in the air |
Last-minute check: answer, then tap to see.
Name the three Milankovitch cycles. Eccentricity, obliquity (tilt) and precession (wobble).
Why is most air in the lowest few kilometres? Gravity pulls air down, so pressure and density fall with height.
Where did the oxygen in the air come from? Photosynthesis, first by cyanobacteria.
Is the greenhouse effect natural? Yes; people have enhanced it.
Ice cores show that Earth has swung between glacials and interglacials about every 100,000 years. Explain how changes in Earth's orbit can start these cycles.
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.
- Learn the three Milankovitch cycles with their time spans.