California and Washington state have joined five nations on the Pacific coast of the Americas to agree to step up the use of a price on carbon dioxide emissions as a central economic policy to slow climate change.
On Tuesday, the US states were acting in defiance of President Donald Trump who doubts 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 US 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".
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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 a step to implement the 2015 Paris agreement to limit rising temperatures.
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.
