🌍 The Greenhouse Effect
Why Earth is a cosy glass house — and why we're cranking up the thermostat.
The Analogy
Imagine a garden greenhouse. Sunlight (shortwave) easily passes through the glass and heats everything inside. But when the warm plants try to release heat as infrared radiation (longwave), the glass bounces it back in. The temperature inside rises. CO₂ and CH₄ are Earth's glass roof.
Mechanism Sequencer — Click to Build
Step 1: Sunlight Enters
The Sun blasts shortwave radiation (visible light) towards Earth. These high-energy waves are SHORT enough to slip straight through the atmosphere — greenhouse gases are nearly invisible to them. The glass roof lets the light in.
🔬 Key Point
Shortwave radiation passes THROUGH the atmosphere freely.
Planetary Simulator
Shortwave sunlight
Infrared / thermal
Re-emitted heat
The Culprit Gases — Molecular Level
Formula
CO₂
Main Sources
Burning fossil fuels, deforestation
Why it's a greenhouse gas:
Its molecular structure allows it to absorb longwave infrared radiation from Earth, then re-emit thermal energy in all directions — including back downward.
⭐ Syllabus Keywords — Reveal & Practise
These exact phrases appear in Cambridge mark schemes. Learn them precisely.
⚖️ Good Greenhouse Effect vs Bad?
The greenhouse effect is natural and essential. Without it, Earth's average temperature would be around -18°C — far too cold for life. The PROBLEM is the enhanced greenhouse effect from burning fossil fuels.
⚠️ Examiner Traps
✗ Common mistake
Greenhouse gases 'trap' heat like a lid on a pot.
✓ Correct
They absorb, then re-emit thermal energy — some goes back to Earth. Not a physical 'lid'.
✗ Common mistake
The ozone hole causes global warming.
✓ Correct
Ozone depletion and global warming are SEPARATE issues. CO₂ causes warming. CFCs damage ozone.
✗ Common mistake
Sunlight is blocked by CO₂.
✓ Correct
CO₂ is transparent to shortwave (sunlight). It only absorbs longwave (infrared) from Earth.
✗ Common mistake
All greenhouse gases are harmful.
✓ Correct
Natural CO₂ and H₂O vapour are essential. The issue is EXCESS from human activity.
Cambridge IGCSE Chemistry 0620 · Topic 10.3 · Supplement
Greenhouse gases: CO₂ and CH₄ · Mechanism: absorption, reflection and emission · Effect: reducing thermal energy loss to space
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