Monday's agreement is only the latest joint effort by the western states. Last year, Arizona and New Mexico formed the Southwest Climate Change Initiative, and the governors of California, Oregon and Washington issued a joint statement in 2003 calling for regional action to address global warming. Elsewhere in the country, several Northeastern states have created the Regional Greenhouse Gas Initiative, to reduce carbon dioxide emissions from power plants through a regional cap-and-trade program, and some Midwestern states signed on to the Lake Michigan Air Directors Consortium, to establish a voluntary registry for companies to report their emissions-reduction efforts.

Efforts to establish national emissions limits have gained traction in Congress since the Democrats became the majority party, with at least four major proposals emerging. President Bush opposes mandatory carbon dioxide (CO2) limits but has proposed reducing emissions through the use of alternative fuels. A coalition of businesses and environmental groups has called for federal legislation, including a cap-and-trade program, to limit emissions of CO2 and other greenhouse gases. In September, California became the first US state to restrict greenhouse gas emissions when Schwarzenegger signed a bill authorizing a state board to set emissions targets for various industries.