Michael R. Bloomberg, the billionaire former mayor of New York City, has a new job: United Nations special envoy for cities and climate change. The United Nations secretary general’s office said Friday that Mr. Bloomberg would be responsible for nudging cities around the world to take steps to combat climate change, helping to “raise political will and mobilize action among cities” in the months before a climate summit meeting that is scheduled to take place in New York in September. The United Nations climate agenda has been entangled in arguments between rich and poor nations over who should more urgently take steps to reduce carbon emissions. As mayor, Mr. Bloomberg set a citywide goal to slash emissions by 30 percent by 2030. He tried to penalize taxi owners who did not use fuel-efficient hybrid cars, but the courts blocked that effort in 2011, calling it a de facto regulation of emissions standards — a power that, under existing laws, belongs to the federal government. Reuters first reported the news of Mr. Bloomberg’s appointment early on Friday. “I can’t think of a person better suited for this important new role,” Secretary of State John Kerry said in a statement.