the greenhouse effect

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The Earth emits infrared radiation. This is absorbed by gases in the atmosphere called greenhouse gases: methane, water vapour and carbon dioxide. These absorb the radiation emitted by the Earth and prevent it from escaping into space. This greenhouse effect keeps the Earth warmer than it would otherwise be, and the temperature gradually increases over the decades.

Remember the Moon and Earth are the same distance from the Sun. The Moon has no atmosphere and an average surface temperature of  18°C, while the Earth has an average surface temperature of 14°C. So you can see that greenhouse gases are not a bad thing in themselves - they keep us from freezing! The concentration of greenhouse gases is increasing, leading to global warming.

Diagram of sun's action on a greenhouse   Diagram of greenhouse effect with Earth's atmosphere
The sun's rays pass through the greenhouse glass and heat the floor. The floor re-radiates this heat: some passes straight out through the glass, whereas the rest is trapped inside the greenhouse making it get hotter.
 
Visible sunlight passes straight through the clear atmosphere and heats up the ground. The ground absorbs this energy, then radiates this heat energy as infrared rays. Some of this passes back out into space,but the rest stays trapped in the atmosphere, making it stay warmer.

 

[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science ]