HAMPTON, N.H. (AP) — Since President Donald Trump vowed this summer to pull the United States out of an international climate accord, states looking to tackle carbon pollution have been forced to go it alone.

More than a dozen formed an alliance committed to reducing emissions in line with the Paris accord, an international agreement that aims to halt the rise in global temperatures. Several state leaders, including California Gov. Jerry Brown, were in Paris this week for a climate summit two years after the historic accord was signed. The summit offered updates on promised funding for poor countries to address climate change, the latest research on ways to cut emissions as well as the corporate world's response to the problem.

Others, wanting to be more ambitious, are taking a look at the country's only regional program that mandates emission reductions in the power sector. Known as the Regional Greenhouse Gas Initiative, it covers nine states in the Northeast and mid-Atlantic and is poised to expand.

"The clear signal from the Trump administration that they were going to pull back on environmental policy throws it back at the states and says, 'OK, it's your game,'" said William Shobe, a professor of public policy at the University of Virginia. "A number of states are responding, saying, 'OK, we're up to that and we're going to go ahead and implement policy.'"

The program sets limits on power plant emissions and requires them to purchase allowances equal to the amount of those emissions. Money raised through those allowances at quarterly auctions goes back to the states.

The only other program of its kind is in California , which covers the state's entire economy. It linked up with Quebec's cap-and-trade program in 2014 and will expand to include Ontario's next year.

Since it began in 2008, RGGI has led to a reduction in carbon dioxide emissions by 30 percent in the states stretching from Maine to Maryland — 16 percent more than other states without emissions programs. The program has also sent $2.8 billion back to the states, money that has funded a range of environmental programs, including energy efficiency upgrades at a middle school and library in Massachusetts, expansion of solar power in New York, and green measures at a brewery in New Hampshire.

"Young consumers don't have any patience for companies that are not consciously involved in reducing their footprint and their impact on the planet. I think that runs pretty deep," said Peter Egelston, as he led a tour of his Smuttynose Brewing Co. in Hampton, whose facility includes solar tubes that allow more natural light into the brewery and a high-tech milling system that gets more yield out of the malted barley.

Since the November elections, Virginia announced it is taking steps to become the first Southern state to join the regional cap-and-trade program, while New Jersey has promised to rejoin the pact. If they get on board, member states would represent a combined gross domestic product of $3.9 trillion, which if they were a country would displace Germany as the fourth-largest economy, according to the Acadia Center, a nonprofit that promotes clean energy.

Virginia's climate strategy, approved last month, would limit emissions from most power plants starting in 2020, followed by a 30 percent reduction over a decade. Eligible carbon emitters would have to participate in the regional greenhouse gas program.

"We do not have the luxury of waiting for Washington to wake up to this threat — we must act now," Democratic Gov. Terry McAuliffe said in a statement.

New Jersey is set to rejoin the regional program when Democratic Gov.-elect Phil Murphy takes office Jan. 16, succeeding GOP Gov. Chris Christie, who pulled the state out of the regional agreement, arguing it drove up electricity costs.

Still, RGGI has its doubters, many who contend its success has more to do with the changing energy landscape than government regulations.

They argue the nine states were already switching from coal-fired to natural gas power plants when they joined the program — which, according to a Duke University study , was responsible for a third of the reductions. Several states also instituted renewable energy goals that led to a shift toward wind, solar and hydro power that reduce emissions.

Americans for Prosperity, the conservative advocacy group founded by billionaire Koch brothers, said the program has offered very few benefits and that such models can't solve the climate problem. It has called for New Hampshire to pull out and opposes New Jersey rejoining.

"Our view is that RGGI and other ideologically driven energy policies cause energy prices to go up, which disproportionally impacts the poor," said Levi Russell, an Americans For Prosperity spokesman.

Still, supporters say the program has been a force for good — reducing emissions and improving air quality without hurting economic growth.

And with its emission cap set to become more stringent in 2020, said Dallas Burtraw, a senior fellow at the research institution Resources For The Future, RGGI could be even more ambitious than Obama's signature climate plan if ever went national. Obama's Clean Power Plan called for cutting emission in the power sector by 32 percent from 2005 levels by 2030. It was put on hold by the Supreme Court and is being scrapped by Trump.

"We have a great success story to tell," said Katie Scharf Dykes, who chairs the RGGI board and the Connecticut public utilities agency. "At this moment, this story is more relevant than ever before."