Mexico’s under-secretary for the environment added his voice to former French president Nicolas Sarkozy’s in musing that a border carbon tariff on United States imports might be a response to the Trump administration’s expected back-pedaling in climate. American voices aligned with the president-elect promptly threatened a trade war against any country that tried.

“A carbon tariff against the United States is an option for us,” Mexico’s Rodolfo Lacy Tamayo, told the New York Times at the tail end of the United Nations climate conference in Marrakech. “We will apply any kind of policy necessary to defend the quality of life for our people, to protect our environment and to protect our industries.”

“It’s an empty threat,” scoffed Thomas J. Pyle, president of the climate-denialist Institute for Energy Research, an entity, the paper says, “which Mr. Trump has cited as influential in shaping his energy and climate proposals.”

“The idea that other nations might punish the United States with a pollution import tax is a switch,” the Times observes. Under President Barack Obama, the United States at one stage “hoped to use trade sanctions to punish other countries, particularly China, for polluting, and push them toward global climate talks. Now the policy could be reversed and used against the United States, as much of the rest of the world economy moves ahead with pricing carbon, while Washington prepares to roll back its climate change plan.”

Indeed, Lacy Tamayo was echoing Sarkozy’s earlier suggestion to the European Union.

Other US voices cited by the Times advised other nations not to take threats of a retaliatory trade war lightly. “Is [Trump] the sort of person who would back down, or would he retaliate?” asked Robert N. Stavins, who leads Harvard’s environmental economics program. “He seems like the kind of person who would retaliate. And then you’d have a trade war.”

“And a trade war may be a price too high for countries whose economies depend on American consumers and suppliers,” concludes the Times.