With federal action against climate change stagnating, some state leaders are taking matters into their own hands. Ten Northeastern states agreed yesterday to begin discussing the creation of the nation’s first market-based plan to reduce carbon dioxide emissions from power plants. Under the plan, plants would be able to buy or sell CO2 credits in order to reach emissions targets set by the participating states, which tentatively include Connecticut, Delaware, Maine, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, and Vermont. Environmentalists said the plan showed great promise but lacked details; some expressed disappointment in the slow timeline, which does not call for a final agreement until 2005.