The Greenhouse Effect (Cambridge (CIE) IGCSE Physics): Revision Note

The greenhouse effect

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Temperature of Earth

• Without greenhouse gases in its atmosphere, the average surface temperature of the Earth would be about −18 °C

  • This is because the infrared radiation emitted by the Earth's surface would escape directly into space, instead of being absorbed by gases in the atmosphere

• The temperature of the Earth is affected by factors controlling the balance between incoming radiation and radiation emitted

• The Earth receives the majority of its thermal energy in the form of thermal radiation from the Sun (opens in a new tab)

  • At the same time, the Earth emits its own thermal radiation at a much longer wavelength than the radiation it receives — the Sun emits mainly short-wavelength radiation, while the Earth emits long-wavelength infrared

  • This is caused by the difference in surface temperature of the Earth and the Sun

How the greenhouse effect works

• Some gases in the atmosphere, such as water vapour, methane and carbon dioxide (greenhouse gases), absorb longer-wavelength infrared radiation emitted by the Earth and prevent it from escaping directly into space

  • These gases absorb the radiation and then emit it back in all directions, some of it back towards the surface

  • These gases keep the Earth's temperature at a reasonable level, and the process is called the greenhouse effect

• This process makes the Earth warmer than it would be if these gases were not in its atmosphere

Radiation and the greenhouse effect

The Earth receives thermal radiation from the Sun but emits its own thermal radiation at the same time. The greenhouse effect is where this radiation is trapped by gases

• The temperature of the Earth, therefore, depends on several factors, such as the rate that infrared radiation is:

  • Reflected back into space

  • Absorbed by the Earth’s atmosphere or by the Earth’s surface

  • Emitted from the Earth’s surface and from the Earth’s atmosphere into space