From Reuters:

Companies and governments around the world are turning to emissions trading as a weapon to fight climate change and join a global carbon market worth $144 billion last year.

Under cap-and-trade schemes, companies or countries face a carbon limit. If they exceed the limit they can buy allowances from others. They can also buy carbon offsets from outside projects which avoid greenhouse gas emissions, often from developing countries.

For a list of proposed and established schemes, go to: Carbon trading schemes abound around the globe