2.3 What is the greenhouse gas effect?

Understanding the greenhouse gas effect is central to the human causes of global warming. The Earth receives energy from two main sources: the Sun and the atmosphere.

Energy from the sun in the form of radiation passes through the Earth's atmosphere and heats the surface of the Earth. Some of this radiation is absorbed and re-emitted back through the atmosphere. This radiation has a longer wavelength than incoming radiation. At this longer wavelength it is absorbed by certain gases in the atmosphere and re-emitted in all directions, some of it back to Earth. This warms the atmosphere. This is the greenhouse effect.

The gases involved are called greenhouse gases. Together they form a tiny fraction of total atmospheric gases – so small that we talk about their concentration as ‘parts per million’ (ppm – 10 000 ppm is 10 000/1 000 000 or 1%). The current concentration of all greenhouse gases in the atmosphere is about 470 ppm. That's much less than 1%, but they play an important role in the Earth's climate.

If there were no greenhouse gases in the atmosphere, then global average temperature would be about −18°C, but with them the average global temperature is about +15°C.

So, greenhouse gases may only comprise a small part of the atmosphere, but they have a very potent effect. Small changes have a big effect on global temperature and climate.

• A short animation from the BBC's Weather Centre explains the greenhouse effect in a few minutes.

• The answer to the question ‘What is the greenhouse effect?’ can be found at the IPCC's website.