By the end of this lesson
- Name the five greenhouse gases in the specification.
- Describe how greenhouse gases trap heat.
- Distinguish the natural greenhouse effect from the enhanced greenhouse effect.
BIG QUESTION
If greenhouse gases keep Earth warm, why are they a problem?
The natural greenhouse effect is essential for life; the concern is the increase in concentration caused by humans.
The five greenhouse gases
The specification names water vapourWater in the form of a gas; in the atmosphere it acts as a greenhouse gas., carbon dioxide, nitrous oxideA greenhouse gas released when soil bacteria act on nitrogen fertilisers, especially in waterlogged soils, and from some industrial processes., methaneA greenhouse gas released by cattle, rice fields and decomposing waste in landfill. and CFCsChlorofluorocarbons: man-made greenhouse gases, once used in fridges and aerosols, that also damage the ozone layer. (chlorofluorocarbons) as greenhouse gases.
Water vapour is the most abundant, but its concentration depends mainly on temperature. Some gases, such as methane and CFCs, trap far more heat per molecule than carbon dioxide.
How the greenhouse effect works
Short-wavelength radiation from the Sun passes through the atmosphere and warms the Earth’s surface. The warm surface emits longer-wavelength infrared radiationRadiation emitted by the warm surface of the Earth, which greenhouse gases absorb..
Greenhouse gases absorb some of this infrared radiation and re-emit it in all directions, including back towards the surface. This keeps the lower atmosphere warmer than it would otherwise be.
Natural and enhanced effects
Without the natural greenhouse effectGreenhouse gases in the atmosphere absorbing infrared radiation emitted by the Earth and re-radiating some back, keeping the Earth warm., the Earth’s average surface temperature would be well below freezing, and liquid water and most life would not be possible.
The enhanced greenhouse effectExtra warming of the Earth caused by human activities increasing the concentration of greenhouse gases, so more infrared radiation is trapped. refers to the additional warming caused by increased concentrations of greenhouse gases from human activities. This is considered in 5.14 and 5.15.
Greenhouse gases in the specification
Know a source for each gas.
| Gas | Example natural source | Example human source |
|---|---|---|
| Water vapour | Evaporation and transpiration. | Indirectly increased as warming raises evaporation. |
| Carbon dioxide | Respiration, volcanoes. | Burning fossil fuels, deforestation. |
| Methane | Wetlands, decomposition without oxygen. | Cattle, rice paddies, landfill. |
| Nitrous oxide | Soil bacteria. | Nitrogen fertilisers. |
| CFCs | None (synthetic). | Old refrigerants and aerosols. |
Language that earns precision
- greenhouse gas
- A gas that absorbs and re-emits infrared radiation in the atmosphere.
- greenhouse effect
- Warming of the Earth’s surface and lower atmosphere caused by greenhouse gases.
- infrared radiation
- Long-wavelength electromagnetic radiation emitted by warm objects.
Common misconceptions
Claim: The greenhouse effect is caused by the hole in the ozone layer.
Correction: These are different issues. The greenhouse effect involves infrared absorption by gases; ozone depletion affects ultraviolet.
Claim: The greenhouse effect is entirely harmful.
Correction: The natural effect keeps Earth warm enough for life; the problem is the enhanced effect.
Claim: Greenhouse gases block sunlight from reaching Earth.
Correction: Most sunlight passes through; the gases absorb infrared emitted by the warmed surface.
Match the answer to the command word
Learn all five gases exactly as listed; “nitrous oxide” is not the same as nitrogen.
Sequence: Sun’s radiation passes through → surface warms → infrared emitted → gases absorb and re-emit.
Recall, apply, then justify
Name three greenhouse gases.
Hint
Five are in the specification.
Suggested marking points
- First correct gas, for example carbon dioxide.
- Second correct gas, for example methane.
- Third correct gas from water vapour, nitrous oxide or CFCs.
Describe how greenhouse gases keep the Earth’s surface warm.
Hint
Follow the radiation.
Suggested marking points
- Radiation from the Sun warms the Earth’s surface.
- The surface emits infrared radiation.
- Greenhouse gases absorb the infrared and re-emit some back towards the surface.
Explain why water vapour is rarely targeted in climate policy even though it is the most abundant greenhouse gas.
Planning hint
What controls water vapour concentration? Consider feedback.
What a strong design includes
- Water vapour concentration depends mainly on air temperature.
- Extra water vapour added by humans rains out within days.
- So humans cannot directly control its long-term concentration.
- Warming caused by other gases increases evaporation.
- This adds more water vapour, amplifying warming (a positive feedback).
- So reducing CO₂ and methane indirectly limits water vapour increase.