Ten states in the Northeast, led by New York, have agreed to begin talks about creating what would be the first market-based compact in the nation intended to reduce carbon dioxide emissions from power plants.

The idea, announced yesterday in Albany by Gov. George E. Pataki, is to curb emissions by creating a regional market in which power plants can buy and sell carbon dioxide credits among themselves as each state works toward reaching the target, or cap. Scientists consider carbon dioxide to be a major greenhouse gas contributing to global climate change.

Some environmentalists said the potential impact of the plan was huge but uncertain, because no details had been worked out, especially concerning how restrictive the caps would be and how many of the states expressing interest would ultimately participate.

Others expressed deep disappointment with the announcement because the timetable of the multistate talks does not call for an agreement until April 2005. It could be years after that before the rules and, in some states, laws take effect to actually begin reducing carbon dioxide in the atmosphere.

"I think it's a step forward," said Ashok Gupta, the director of the air and energy program at the Natural Resources Defense Council, a national conservation group based in New York. "We're hopeful that this will be the precursor to a national cap-and-trade program for carbon dioxide."

The process, begun by Mr. Pataki in April with a letter to 10 governors in New England and Middle Atlantic states, has some potential political wrinkles. Aides to Mr. Pataki, a Republican, said yesterday that the policies were farsighted and ambitious but not in conflict with the policies of President Bush. The Bush administration has been criticized by scientists who say it has failed to pay attention to global warming.

Within the initial group of states — Connecticut, New Jersey, Vermont, New Hampshire, Delaware, Maine, Pennsylvania, Massachusetts, Rhode Island and New York — there are also political divisions, with four Democratic governors and six Republicans. And several of the states responded with letters that expressed only lukewarm enthusiasm for Mr. Pataki's idea because they were moving ahead on ambitious reductions in power plant emissions on their own.

The response letter from Gov. James E. McGreevey of New Jersey, for example, called Mr. Pataki's plan modest in focusing only on carbon dioxide and not on other power plant pollutants, like mercury, sulfur dioxide and nitrogen oxides, which Mr. McGreevey, a Democrat, said the state was working to reduce through its own power plant rules.

"To the extent your proposal advances those initiatives, I am pleased to join and support," Mr. McGreevey wrote.

The central dynamic of the plan, and the great unanswered question of its environmental impact, revolve around the mixture of restriction, from the emissions caps, and voluntary incentives, in the trading system.

Power plants that keep their emissions under the cap would qualify for credits that they could then sell, and dirtier plants would have to buy credits to meet their cap obligations.

The idea is that developers and plant owners would have more financial incentive to build and operate clean, efficient plants, and modernize dirtier plants through cleaner fuels or new equipment.