Greenhouse gases — AQA GCSE Combined Science
Test yourself on Greenhouse gases with AQA GCSE practice questions.
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Greenhouse gases explained
Short-wavelength radiation from the Sun passes through the atmosphere and is absorbed by the Earth's surface, warming it.
Read the full explanation
The warmed surface emits longer-wavelength infrared radiation. Greenhouse gases in the atmosphere absorb some of this outgoing infrared radiation and re-emit it in all directions, including back towards the Earth. This trapping of energy keeps the average surface temperature higher than it would otherwise be, maintaining conditions suitable for life. Water vapour, carbon dioxide and methane are the greenhouse gases named in this specification. Without this natural greenhouse effect, the Earth would be far too cold for liquid water and for most life. Human activities that add carbon dioxide or methane to the atmosphere strengthen the effect and contribute to global warming.
Students should be able to describe the greenhouse effect in terms of the interaction of short and long wavelength radiation with matter.
The Sun emits mainly short wavelength radiation, including visible light and ultraviolet, which passes through the atmosphere and is absorbed by the Earth's surface. The warmed surface re-emits energy as longer wavelength infrared radiation. Greenhouse gases such as carbon dioxide, methane and water vapour absorb some of this outgoing infrared, so the energy is re-radiated in all directions, including back towards the surface. This trapped energy keeps the mean surface temperature higher than it would otherwise be. A useful model is a car parked in sunshine: short wavelength light enters through the glass, the interior warms and emits infrared, and the glass reduces the escape of that infrared. The effect is natural and essential; the enhanced greenhouse effect occurs when extra gases strengthen it.
Your focus
- State that water vapour, carbon dioxide and methane are greenhouse gases.
- Explain how greenhouse gases absorb and re-emit infrared radiation to maintain Earth's temperature.
- Describe why the natural greenhouse effect is essential for life on Earth.
Show all 6 objectives
- Describe the pathway of short wavelength solar radiation to the Earth's surface and its absorption.
- Describe how the warmed surface emits long wavelength infrared radiation.
- Explain how greenhouse gases absorb and re-radiate infrared radiation, warming the surface.
Greenhouse gases exam tips
Marking Points
- The Sun emits radiation that passes through the atmosphere and warms the Earth's surface.
- The warmed Earth's surface emits infrared radiation.
- Greenhouse gases absorb outgoing infrared radiation and re-emit it in all directions, including back towards the Earth.
- This trapped energy keeps the average global temperature higher than it would be without the atmosphere.
- Water vapour, carbon dioxide and methane are greenhouse gases named in the specification.
- The natural greenhouse effect maintains temperatures high enough for liquid water and life on Earth.
- Short wavelength radiation from the Sun (visible and ultraviolet) passes through the atmosphere and is absorbed by the Earth's surface, warming it.
- The warmed Earth's surface emits longer wavelength infrared radiation.
- Greenhouse gases in the atmosphere absorb some of this outgoing long wavelength infrared radiation.
- Absorbed energy is re-radiated in all directions, including back towards the Earth's surface, so the surface and lower atmosphere stay warmer than they would without those gases.
- The natural greenhouse effect is essential for life; adding more greenhouse gases enhances the effect and tends to raise the mean global temperature.
Examiner Tips
- 💡Use the sequence 'Sun warms surface → surface emits infrared → greenhouse gases absorb and re-emit → energy trapped' to structure an explanation.
- 💡Name water vapour, carbon dioxide and methane exactly, and avoid adding gases not required by the specification.
- 💡When explaining why life is supported, refer to temperatures high enough for liquid water rather than simply saying 'it keeps Earth warm'.
- 💡Use the phrases 'short wavelength radiation' and 'long wavelength infrared radiation' explicitly, because the command word is 'describe' and the specification names both.
- 💡Sequence the account: incoming short wavelength, surface warming, outgoing long wavelength, absorption by greenhouse gases, re-radiation in all directions.
- 💡Link the final point to the consequence for surface temperature rather than stopping at absorption.
Common Mistakes
- Saying greenhouse gases trap heat from the Sun: they mainly absorb infrared radiation emitted by the warmed Earth, not incoming solar radiation.
- Confusing the natural greenhouse effect with global warming: the natural effect keeps Earth warm enough for life, while global warming is the additional warming caused by extra greenhouse gases.
- Listing oxygen or nitrogen as greenhouse gases: the specification names water vapour, carbon dioxide and methane.
- Saying the greenhouse effect is caused only by human activity: correct this by stating that it is a natural process and that human activity enhances it.
- Claiming that greenhouse gases form a solid barrier or 'hole' that heat cannot cross: correct this by describing absorption and re-radiation of infrared in all directions.
- Stating that the atmosphere absorbs incoming short wavelength radiation as the main mechanism: correct this by explaining that short wavelength radiation mainly reaches the surface, while outgoing long wavelength infrared is what greenhouse gases absorb.