PHOENIX - Governors of five Western states signed a pact Monday to reduce greenhouse gases, an accord an aide to Gov. Janet Napolitano acknowledged is more aspirational than enforceable and her environmental director said she may not have the power to enforce.

The agreement requires the five states to set overall regional goals for emissions of carbon dioxide and other similar gases linked to global warming within six months, with specific reductions for each state. And with 18 months the states would set up a system of caps and trades designed to achieve thos goals.

Napolitano told Capitol Media Services the lack of action on the federal level requires governors to step in.

"At the regional level we're looking at fairly aggressive action,'' she said. "We need to do this, both to clean up the air and wean our way from foreign oil, so it's got dual advantages.''

But there is nothing in the measure that actually requires states to actually achieve those goals.

"You have to start with aspiration,'' said gubernatorial press aide Jeanine L'Ecuyer. "As the states work together, the goals - and how they'll be enforced - will be fleshed out.''

Potentially more significant, Steve Owens, director of the state Department of Environmental Quality, told Capitol Media Services he is unsure whether Napolitano actually has the authority t make companies monitor and report greenhouse gas emissions, much less actually force them to curtail what they emit.

Napolitano brushed aside questions of whether she need legislative approval to commit to reduce carbon dioxide emissions in Arizona. "Every governor in the Western states is doing it by executive order,'' she said.

In September, Napolitano ordered the state to purchase only vehicles for its fleet that produce low greenhouse gas emissions.

The governor said at that time she want to cut greenhouse gases to 2000 levels by 2020 - and half of that by 2040.

That followed the recommendations of the governor's Climate Change Advisory Group, which also suggested the state set up a reporting mechanism for private companies to detail greenhouse gas emissions, as well as something to regulate those emissions and trade pollution "credits'' with other companies.

But Owens said his staff is still researching whether state law allows his agency to require companies to do any of that.

"We have some fairly broad regulatory authority here in Arizona to cover a wide range pollutants,'' Owens told Capitol Media Services. Owens noted state law specifically mentions carbon as chemical that can be regulated.

Owens conceded, though, carbon is as different from carbon dioxide as sodium is from sodium chloride - more commonly known as salt. And he said there may be some "gaps that might requir legislation.''

He said New Mexico and Oregon, whose governors also signed th accords, are pursuing limits by regulation. But Owen acknowledged that took a change in state law in California approved by Gov. Arnold Schwarzenegger, another signer of th accord, to cap that state's greenhouse gas emissions at 1990 levels by 2010.

Napolitano's September order also included a provision which she said would eventually lead the state to follow California's lead to impose new emission standards on vehicles that can be sold in the state - emission standards based not on the current list of pollutants but on carbon dioxide.

Owens said that whole issue also is in legal limbo.

The first surrounds a lawsuit brought by California and 11 other states demanding that the U.S. Environmental Protection Agency allow regulation of carbon dioxide. That case is currently pending before the U.S. Supreme Court.

Owens said if California does get the go-ahead, that raises another question.

Existing state law says vehicles sold in Arizona must meet EPA standards. But he said it is unclear whether that allows the governor to conclude it could be the stricter standards EPA might allow California to impose.