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The greenhouse effect is the name given to the natural process that causes the Earth to be warmer than it would be in the absence of an atmosphere.

The surface of the Earth is heated by the Sun. Greenhouse gases in the atmosphere trap this heat, keeping the planet warm.

1. Solar radiation at most wavelengths passes through the Earth's atmosphere.
2. The Earth absorbs most of the radiation and warms up.
3. The Earth radiates heat energy.
4. Some of the heat escapes into space.
5. Some of the heat is absorbed by greenhouse gases in the atmosphere.
6. Greenhouse gases radiate heat in all directions
7. The lower atmosphere warms up.

Greenhouse gases can occur naturally. They trap heat in the Earth’s atmosphere. Greenhouse gases present in the atmosphere include:

• water vapour
• carbon dioxide
• methane
• nitrous oxide

Water vapour is the largest contributor, responsible for 98 per cent of the natural greenhouse effect.

Without the greenhouse effect, the surface of the Earth would have an average temperature of -18 °C (Source: World Meteorological Organization) and be covered in ice. Life as we know it would not be able to survive.

Although these gases have always been present in the world's atmosphere, their concentration is increasing because of human activity.

Global warming (or global heating) is attributed to the enhanced greenhouse effect. This is caused by the increased concentration of greenhouse gases in the atmosphere.

Greenhouse gases e.g. carbon dioxide (CO2), methane and nitrous oxides, act like a blanket and trap heat inside the Earth's atmosphere.

As concentrations of these gases build up, they are more effective at preventing heat being lost into space. The amount of heat being lost from the atmosphere is less than the energy entering the atmosphere. As a result the temperature of the atmosphere increases.

1. Greenhouse gases allow more solar radiation to pass through Earth's atmosphere
2. The Earth absorbs most of the radiation and warms up.
3. The Earth radiates heat energy.
4. More heat is absorbed by greenhouse gases in the atmosphere.
5. Less heat escapes into space.
6. Greenhouse gases radiate heat in all directions
7. The lower atmosphere heats up.

Human activity is responsible for increasing amounts of greenhouse gases in the atmosphere. The main causes of this are:

• burning fossil fuels for heat and electricity
• fossil fuel use in transport
• agriculture
• deforestation
• cement manufacture