PARIS (Reuters) - California and Washington state joined five nations on the Pacific coast of the Americas on Tuesday to agree to step up the use of a price on carbon dioxide emissions as a central economic policy to slow climate change.

People cool off at the beach during a Southern California heat wave in Oceanside, California, U.S., October 24, 2017. REUTERS/Mike Blake

The U.S. states were acting in defiance of President Donald Trump who says he doubts that man-made greenhouse gas emissions from burning fossil fuels are the prime cause of global warming and plans to quit the 2015 Paris climate accord.

Canada, Mexico, Costa Rica, Colombia and Chile agreed the measure at a climate summit in Paris along with the two U.S. states and provinces across Canada of British Columbia, Alberta, Ontario, Quebec and Nova Scotia.

Nations in the Americas, mostly the United States, account for about a quarter of all man-made greenhouse gas emissions.

During the One Planet summit hosted by French President Emmanuel Macron they declared “a commitment to implement carbon pricing as a central economic and environmental policy instrument for ambitious climate change action.”

They said they would also work to strengthen systems for “measurement, reporting and verification of greenhouse gas emissions, as a necessary foundation for the coordination of carbon markets within the Americas and globally.”

Signatories said the plan was an step to implement the 2015 Paris agreement to limit rising temperatures.

“Our economies must stop denying climate change,” Chilean President Michelle Bachelet said in a statement of the carbon pricing plan. “When we incorporate climate change through carbon pricing we allow market forces to push climate action.”

A 2016 World Bank report said greater cooperation through carbon trading could reduce the cost of cutting greenhouse gas emissions by almost a third by 2030.

World Bank President Jim Yong Kim welcomed the “Carbon Pricing in the Americas” plan, saying: “Carbon pricing provides the most stable, cost-efficient and predictable path for transitioning countries toward low-carbon economies.”

Environmental economist Nathaniel Keohane, of the Environmental Defense Fund, also predicted the plan could expand beyond carbon taxes and national markets.

“I see this as a first step towards, and a vision of, a carbon market of the Americas,” he told Reuters.