The big idea: Certain gases in the atmosphere act like a blanket — they let the Sun's energy in, but trap some of the heat trying to escape back into space.
This is the greenhouse effect, and the gases that do it are greenhouse gases.
A natural greenhouse effect keeps Earth warm enough for life. The problem is that human activity has added extra greenhouse gases, trapping more heat and raising the average global temperature — this is the enhanced greenhouse effect, the cause of global warming.
- Greenhouse gas
- An atmospheric gas that absorbs longwave (infrared) radiation and re-radiates heat, warming the atmosphere — e.g. carbon dioxide, methane and water vapour.
- Greenhouse effect
- The warming of the atmosphere caused by greenhouse gases absorbing longwave infrared radiation that would otherwise escape into space.
- Enhanced greenhouse effect
- The extra warming caused when human activity adds more greenhouse gases (mainly CO₂) to the atmosphere.
- Global warming
- The rise in average global temperature that results from the enhanced greenhouse effect.
- Longwave (infrared) radiation
- The heat energy re-emitted by the warm Earth's surface — this is the radiation that greenhouse gases absorb.
Natural vs enhanced — keep them apart: The greenhouse effect is not a bad thing in itself — without it Earth would be far too cold for life.
What causes global warming is the enhanced greenhouse effect: the extra gases humans add on top of the natural amount.
Two facts unlock almost every exam question here: which gases trap heat, and which radiation they trap.
The greenhouse gases you must know are carbon dioxide (CO₂), methane (CH₄) and water vapour.
| Greenhouse gas | Main sources | Why it matters |
|---|---|---|
| Carbon dioxide (CO₂) | Burning fossil fuels (coal, oil, gas), deforestation | The main contributor to the ENHANCED (human-caused) greenhouse effect — released in huge amounts and stays in the air a long time |
| Methane (CH₄) | Cattle, rice paddies, landfill, leaking gas, melting permafrost | A far stronger absorber of longwave infrared per molecule than CO₂, although it is present in much smaller amounts |
| Water vapour (H₂O) | Evaporation from oceans, lakes and plants | The most ABUNDANT natural greenhouse gas; a warmer atmosphere holds more, which amplifies the warming |
The key detail: which radiation is absorbed: Greenhouse gases do not stop sunlight coming in.
Incoming sunlight is mostly shortwave radiation (visible light), and it passes straight through the greenhouse gases to warm the surface.
The warm Earth then re-emits energy as longwave infrared radiation (heat). It is this longwave infrared that CO₂, methane and water vapour absorb — so the heat is trapped instead of escaping to space.
If a 1-mark question asks which radiation CO₂ and methane absorb, the answer is longwave infrared, not visible light.
| Incoming from the Sun | Outgoing from the Earth | |
|---|---|---|
| Type of radiation | Mostly shortwave (visible light + UV) | Longwave INFRARED (heat) |
| Passes through greenhouse gases? | Yes — passes straight through to warm the surface | No — ABSORBED by CO₂, methane and water vapour |
| Net effect | Surface absorbs the energy and warms up | Heat is trapped and re-radiated back down, warming the atmosphere |
The mechanism, step by step
- Shortwave sunlight passes through the atmosphere and warms the Earth's surface.
- The warm surface re-radiates the energy as longwave infrared (heat).
- Greenhouse gases absorb this longwave infrared instead of letting it escape into space.
- The gases re-radiate the heat in all directions, including back down — so the atmosphere warms.
- Add more greenhouse gases → more infrared absorbed → less heat escapes → the average temperature rises (global warming).
Why methane is singled out: Methane absorbs longwave infrared just like CO₂, but per molecule it is a much stronger absorber — so even the smaller amounts in the air matter.
That is why a question can ask for two reasons methane contributes to the greenhouse effect: (1) it absorbs/traps longwave infrared radiation, and (2) per molecule it is a more powerful greenhouse gas than CO₂.
Reading a CO₂ vs temperature graph: A favourite 1-mark task gives a graph where atmospheric CO₂ and average global temperature both rise over the same years.
To explain the correlation: as CO₂ rises, more longwave infrared is absorbed, so less heat escapes and the temperature rises — the two lines climb together (a positive correlation).
If the graph instead shows fossil-fuel use rising, the link is: burning fossil fuels releases CO₂ → atmospheric CO₂ rises → stronger greenhouse effect → warming.
| Natural greenhouse effect | Enhanced greenhouse effect | |
|---|---|---|
| Cause | Naturally present CO₂, water vapour and methane | EXTRA CO₂/methane added by human activity (mainly burning fossil fuels) |
| Result | Keeps Earth warm enough for life (~+33 °C warmer than with no atmosphere) | Even more longwave infrared trapped → average global temperature RISES (global warming) |
| Is it a problem? | No — life depends on it | Yes — the rapid extra warming disrupts climates and ecosystems |
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How this is tested: On Paper 1 (1-mark items) you are asked to identify the main contributor to the enhanced greenhouse effect (CO₂), the radiation absorbed by CO₂ and methane (longwave infrared), or why methane contributes.
Graph-stimulus 1-markers ask you to explain a correlation between rising CO₂ (or fossil-fuel use) and rising temperature.
On Paper 2 a 3-mark Explain can ask you to link burning fossil fuels to a real consequence such as Arctic sea-ice loss — that is a cause-and-effect chain.
IB-style question — fossil fuels and shrinking sea ice
Satellite data show that the area of Arctic summer sea ice fell sharply over recent decades, while global fossil-fuel use rose over the same period. Explain how burning fossil fuels could cause this loss of sea ice. [3]
How to score all three marks
- Start with the gas released. Burning fossil fuels releases carbon dioxide (CO₂), so the concentration of CO₂ in the atmosphere rises.
- Link CO₂ to warming. The extra CO₂ absorbs more longwave infrared radiation (the enhanced greenhouse effect), so less heat escapes and the average temperature rises.
- Reach the sea ice. The warmer temperatures melt the Arctic sea ice, so its area falls. (A strong answer adds the ice-albedo positive feedback: bare ocean is darker, absorbs more heat and melts even more ice.)
Final answer
Burning fossil fuels releases CO₂ → atmospheric CO₂ rises → more longwave infrared is absorbed (enhanced greenhouse effect) → average temperature rises → the warmer Arctic melts, so sea-ice area falls (amplified by the ice-albedo feedback).
✓ Why this scores full marks: It is a chain of causes: fuel → CO₂ → infrared trapped → warming → ice melts. Each link is a separate scoring point.
A weak answer just says 'fossil fuels are bad for the climate' — it never names CO₂, the longwave infrared it traps, or the warming that melts the ice.