The big idea: Some gases in the air let sunlight in but trap the heat Earth sends back out.
Infrared (IR) is the invisible 'heat' radiation a warm surface gives off — longer-wavelength than visible light.
These greenhouse gases absorb that infrared and send some of it back down, keeping the surface warmer. That warming is the greenhouse effect.
| Greenhouse gas | Where it comes from | Why it traps heat |
|---|---|---|
| Carbon dioxide, CO_{2} | Burning fossil fuels (coal, oil, gas), deforestation | Its bonds vibrate at infrared frequencies, so it absorbs Earth's infrared |
| Methane, CH_{4} | Farming (cattle), rice fields, landfill, gas leaks | Strong infrared absorber per molecule — even small amounts matter |
| Water vapour, H_{2}O | Evaporation from oceans, lakes and plants | The biggest natural contributor; absorbs over a wide infrared range |
| Nitrous oxide, N_{2}O | Fertilisers, some industry | Absorbs infrared the other gases miss; long-lived in the air |
Spot it: Sunlight in (visible) → ground warms → Earth radiates infrared out → greenhouse gases catch some and send it back.
Without them Earth would be about 33 °C colder — far too cold for life. The greenhouse effect is natural and necessary; the problem is making it stronger.
Follow one packet of energy from the Sun to the ground and back. The key step is the middle one — what the gas molecules do to infrared:
| Step | What happens to the energy |
|---|---|
| 1 · Sunlight in | Short-wave sunlight (mostly visible) passes through the atmosphere and warms the ground |
| 2 · Earth radiates | The warm surface re-emits this energy as longer-wavelength infrared radiation |
| 3 · Gases absorb | Greenhouse-gas molecules (CO2, CH4, H2O, N2O) absorb that infrared — their bonds resonate at infrared frequencies |
| 4 · Re-emit (some back down) | The molecules re-emit infrared in all directions — so some is sent back down to the surface |
| 5 · Trapped → warmer | That returned energy keeps the surface warmer than it would be with no atmosphere |
Why these gases — and not nitrogen or oxygen: A molecule absorbs infrared only if its bonds can vibrate at infrared frequencies — this is called resonance (the radiation's frequency matches the bond's natural vibration).
CO_{2}, CH_{4}, H_{2}O and N_{2}O have bonds that resonate at infrared frequencies, so they absorb Earth's infrared.
The main air gases N_{2} and O_{2} do not — their simple two-atom bonds don't vibrate that way, so they let infrared straight through.
The two-way street: When a greenhouse-gas molecule re-emits, it radiates in all directions — not just upward.
So a fraction goes back down to the surface. That returned infrared is the extra warming the greenhouse effect provides.
Get feedback like a real examiner
Submit your answers and get instant feedback — what you did well, what's missing, and exactly what to write to score full marks.
Try AI Tutor Free7-day free trial • No card required
How this is tested: This micro is mostly words, tested two ways.
- Paper 1A: a quick 'identify' MCQ — e.g. the main human cause of the enhanced greenhouse effect (burning fossil fuels → more CO2). - Paper 2: a short 'outline' — describe the mechanism: greenhouse gases absorb the infrared the surface emits and re-emit some of it back down.
Classic trap: saying the gases stop sunlight coming in. They don't — sunlight passes through; the gases trap the infrared Earth radiates back out.
Natural vs enhanced: Natural greenhouse effect = the warming from gases that have always been there.
Enhanced greenhouse effect = the extra warming from gases we add — mainly CO_{2} from burning fossil fuels. That extra warming is global warming.
| Natural greenhouse effect | Enhanced greenhouse effect | |
|---|---|---|
| Cause | Naturally present H2O, CO2, CH4, N2O | Extra CO2 and CH4 from human activity |
| Driven by | Always been there | Burning fossil fuels, farming, deforestation |
| Effect | Keeps Earth ~33 °C warmer — needed for life | Extra warming on top → global warming |
IB-style question — outline the mechanism (2 marks)
Outline the mechanism by which greenhouse gases such as carbon dioxide cause the surface of the Earth to be warmer than it would be with no atmosphere.
How to answer
- Mark 1 — absorb the infrared. The warm surface emits infrared radiation; greenhouse-gas molecules absorb it (their bonds resonate at infrared frequencies).
- Mark 2 — re-emit some back down. The molecules re-emit infrared in all directions, so some is sent back down to the surface, keeping it warmer than with no atmosphere.
Final answer
Greenhouse gases absorb the infrared the surface emits, then re-emit it in all directions — some returns to the surface, so the surface stays warmer. (Note: sunlight still passes straight in — only the outgoing infrared is trapped.)