China has become the latest country to sign an international treaty to reduce global warming.

The treaty, adopted in the Japanese city of Kyoto in December, requires industrialised nations to reduce carbon dioxide emissions by the year two-thousand-and-twelve.

It comes into effect once fifty-five countries accounting for more than half of the world's emissions have ratified it.

Nearly forty countries have signed the agreement, but none has yet ratified it.

Because it's classed as a developing country, China is requested only to make voluntary reductions -- although it's one of the world's main polluters.