Knowledge Organiser: The Greenhouse Effect
Greenhouse Gases
- Carbon dioxide (CO₂)
- Methane (CH₄)
- Water vapour (H₂O)
- N₂O (nitrous oxide)
- NOT N₂ or O₂
The Mechanism
- Short-wave solar radiation passes through atmosphere
- Earth's surface absorbs and warms
- Long-wave infrared emitted by surface
- Greenhouse gases absorb and re-emit IR
- Some IR returns to surface → warming
Natural vs Enhanced
- Natural: keeps Earth at +15°C (essential)
- Without it: -18°C (frozen)
- Enhanced: extra warming from human activities
- CO₂: 280 ppm → 420+ ppm (human era)
Sources of Greenhouse Gases
- CO₂: fossil fuels, deforestation, cement
- CH₄: cattle, rice paddies, landfill, gas leaks
- N₂O: fertilisers, agriculture
- H₂O: natural (feedback mechanism)
Key Equations
- No calculation equations — descriptive/mechanism topic
- CH₄ + 2O₂ → CO₂ + 2H₂O (combustion of methane — adds CO₂)
- Greenhouse gases absorb infrared radiation and re-emit it back to Earth's surface
Common Mistakes
- Saying the greenhouse effect is all bad: The NATURAL greenhouse effect is essential — without it Earth would be ~−18°C; it is the ENHANCED greenhouse effect (extra warming) that causes problems
- Saying O₂ and N₂ are greenhouse gases: Only polyatomic molecules (CO₂, CH₄, H₂O, N₂O) absorb infrared — diatomic N₂ and O₂ do NOT act as greenhouse gases
- Confusing the mechanism — saying greenhouse gases block sunlight: Greenhouse gases absorb and re-emit INFRARED radiation from Earth's surface — they do not block incoming solar radiation
- Forgetting methane is a significant greenhouse gas: Methane is ~25× more potent a greenhouse gas than CO₂ per molecule — cattle and rice paddies are major sources
Memory Aid
Greenhouse gases ABSORB infrared radiation and RE-EMIT it back to Earth — these are the two key words for mark schemes For greenhouse gases: "CO₂, CH₄, H₂O — the 3 C's-M-W" (Carbon dioxide, Methane, Water) For the mechanism: "Short-wave IN, long-wave ABSORBED and RE-EMITTED back — remember both words for full marks" Temperature without greenhouse effect: "-18°C = freezing death. With natural effect: +15°C = life. With enhanced effect: above +15°C = problem." Quick Check: Explain why nitrogen is NOT a greenhouse gas even though it makes up 78% of the atmosphere. Show Answer Nitrogen (N₂) does not absorb infrared radiation because of its symmetrical molecular structure. Only molecules like CO₂, CH₄, and H₂O have a structure that allows them to absorb infrared radiation. Greenhouse gases must be able to absorb and re-emit infrared radiation — N₂ cannot do this.