Britain on Tuesday announced the swiftest, most ambitious climate target of any industrial economy. In so doing, it not only laid out the economic case for clean energy, but also sought to chart a very different course from its longtime ally across the pond, the United States. Prime Minister Keir Starmer of Britain told delegates at the international climate talks in Baku, Azerbaijan, that his country would aim to reduce its emissions of planet-heating greenhouse gases by 81 percent by 2035, compared with levels in 1990. “The race is on for the clean energy jobs of the future, the economy of the future,” he said. “The economic prize on offer to the U.K. is enormous,” Ms. Clarkson said.