Lord Stern, the world’s leading climate change economist, said nations that were taking strong action on emissions could start imposing restrictions on “dirty” US exports by 2020.

Lord Stern, who advises several G20 leaders and is one of the key players in the international negotiations seeking a deal on emissions, made his comments 10 days before the annual United Nations climate change conference opens in Cancun, Mexico. They reflect the feeling in many countries that a lack of action on emissions in the US is delaying progress in the talks.

“The US will increasingly see the risks of being left behind, and 10 years from now they would have to start worrying about being shut out of markets because their production is dirty,” Lord Stern said. “If they persist in being slow about reducing emissions, US exports will start to look more carbon intensive. [countries] will start measuring the carbon content of exports.”