Seven north-eastern US states have signed the country's first plan setting Kyoto-style legal limits on greenhouse gases from power stations.
They agreed to take steps to curb CO2 emissions starting in 2009.
The plan was endorsed by the governors of New York, New Jersey, Delaware, Connecticut, Maine, New Hampshire and Vermont. It is open to other states.
Known as the Regional Greenhouse Gas Initiative (RGGI), it is seen as a break with the Bush administration.
The US is the world's largest producer of greenhouse gases.
It pulled out of the Kyoto Protocol on global warming in 2001, saying it would hurt the US economy.
President George Bush has said he backs voluntary, not mandatory, cuts in the human production of the greenhouse gases that most scientists believe are driving the global climate outside its natural cycle.
On the market
The RGGI - initiated in 2003 by New York Governor George Pataki, a Republican - gives power stations binding targets for cutting their CO2 emissions.
They can either do that by installing new, cleaner technology, or by buying CO2 allowances from other companies who have cut their CO2 beyond the target.
Carbon allowances thus become a tradable commodity, and there is money to be made for firms that clean up their act.
Under the RGGI, each of the seven signatories must proceed with required legislative or regulatory approvals to adopt the programme.