What's being done about it?

Every five years, the United Nations has a meeting called the Earth Summit, where world leaders agree on what to do about global warming.

In 1997, an important agreement was made at the UN Conference on Climate Change in Kyoto, Japan, to cut the amount of gases that industries make.

That agreement came into force in February 2005.

Some people say it won't have any real impact because America - one of the world's biggest polluters - has pulled out.

Leaders agree the world can cut the amount of carbon dioxide that's released into the atmosphere by changing the way power is produced too.