Greenhouse gases: Greenhouse gases in the atmosphere maintain temperatures on Earth high enough to support life. Water vapour, carbon dioxide and methane are greenhouse gases.

The idea

Short-wavelength radiation from the Sun passes through the atmosphere and is absorbed by the Earth's surface, warming it. 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.

What earns the marks

• 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.
• 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.

Common mistakes and how to put them right

• Saying greenhouse gases trap heat from the Sun.

  Put it right: They mainly absorb infrared radiation emitted by the warmed Earth, not incoming solar radiation.

• Confusing the natural greenhouse effect with global warming.

  Put it right: 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.

  Put it right: The specification names water vapour, carbon dioxide and methane.

Exam 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'.