The earth is naturally warmed by rays from the sun which pass through the atmosphere and are reflected back out to space again.

The atmosphere's made up of layers of gases - some of which are called greenhouse gases.

They're mostly natural and make up a kind of thermal blanket over the Earth.

This lets some of the rays back out of the atmosphere, keeping the earth at the right temperature for animals, plants and humans to survive (60°F/16°C). So some think global warming is good.

But most scientists think if extra greenhouse gases are made, the thermal blanket gets thicker and too much heat is kept in the earth's atmosphere - that's when global warming's bad.

Greenhouse gases are made out of:

water vapourcarbon dioxidemethanenitrous oxideozonechlorofluorocarbons (CFCs)

They are all natural gases, but extra greenhouses gases can be made by humans. Environment experts are especially concerned about carbon dioxide.

How are extra greenhouse gases made?

Although plants absorb carbon as they grow, extra carbon is released into the atmosphere when humans cut down trees.

Extra greenhouse gases are produced through activities which release carbon dioxide, methane, nitrous oxide and ozone CFCs (chlorofluorocarbons).

So there you have the definition for global warming.

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