The big idea: Climb into a car left in the Sun and it's like an oven inside — the windows let sunlight in but trap the heat. Some gases in the air do the same to the whole planet.
How the greenhouse effect works
- 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, CO2 | Burning fossil fuels (coal, oil, gas), deforestation | Its bonds vibrate at infrared frequencies, so it absorbs Earth's infrared |
| Methane, CH4 | Farming (cattle), rice fields, landfill, gas leaks | Strong infrared absorber per molecule — even small amounts matter |
| Water vapour, H2O | Evaporation from oceans, lakes and plants | The biggest natural contributor; absorbs over a wide infrared range |
| Nitrous oxide, N2O | Fertilisers, some industry | Absorbs infrared the other gases miss; long-lived in the air |
Every one of these gases has natural sources as well as human ones. The human part is what strengthens the effect:
| Gas | Natural sources | Human sources |
|---|---|---|
| CO₂ | Respiration, decay, volcanoes | Burning fossil fuels, deforestation, cement |
| CH₄ | Wetlands, termites | Cattle, rice fields, landfill, gas leaks |
| H₂O | Evaporation from oceans and lakes | Little directly; a warmer climate holds more water vapour |
| N₂O | Bacteria in soils and oceans | Fertilisers, some industry |
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.
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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).
Absorb infrared
- CO2, CH4, H2O and N2O have bonds that resonate at infrared frequencies.
- So they absorb Earth's infrared.
Let infrared through
- The main air gases N2 and O2 do not.
- Their simple two-atom bonds don't vibrate that way, so they let infrared straight through.
Two ways to explain the absorption: Resonance model: the bonds have natural frequencies in the infrared; radiation at those frequencies drives them and is absorbed.
Energy-level model: a molecule has fixed vibrational energy levels; it absorbs a photon only if hf equals the gap between two levels, then re-emits a photon of the same energy in a random direction.
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.
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How this is tested — this micro is mostly words, tested two ways:
Paper 1A
- A quick 'identify' MCQ — the main human cause of the enhanced effect (burning fossil fuels → more CO2).
Paper 2
- A short 'outline' of the mechanism: gases absorb the surface's infrared and re-emit some back down.
The 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 CO2 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 |
The words to know
| Word | What it means |
|---|---|
| Greenhouse gas | A gas that absorbs and re-emits infrared: CO₂, CH₄, H₂O, N₂O |
| Greenhouse effect | Warming of the surface by infrared re-emitted downwards by greenhouse gases |
| Enhanced greenhouse effect | The extra warming caused by greenhouse gases added by human activity |
| Resonance | Strong absorption when the radiation frequency equals a natural frequency of the molecule |
IB-style questionOutline[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.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
What if there were no atmosphere?: The solar intensity arriving would be the same (it depends only on the Sun and the distance). But with no back-radiation, the surface would be cooler and so would emit less: about 255 K instead of about 288 K.