The United Nations on Friday named former New York City mayor Michael Bloomberg as a special envoy for cities and climate change. The financial news tycoon will help UN leader Ban Ki-moon "raise political will and mobilize action among cities as part of his long-term strategy to advance efforts on climate change," said a UN statement. Ban is organizing a major climate summit in New York on September 23 and has stepped up warnings on the impact of rising temperatures. Bloomberg stood down as mayor of New York on January 1 after holding the post for 12 years during which he introduced a plan to cut the city's greenhouse emissions by 30 percent by 2030. Bloomberg brought in hybrid taxis and hundreds of miles of bicycle lanes. He is already head of the C40 Cities Climate Leadership Group, a network of major cities around the world taking action to cut harmful greenhouse gases. The group is to meet in Johannesburg next week. Bloomberg's appointment was immediately welcomed by World Bank president Jim Yong Kim as "a huge boost" to efforts to counter climate change. Kim said that building low-carbon cities will be critical for efforts to contain global warming. "The UN special envoy for cities and climate change needs to be a visionary who ruthlessly pushes to reach bold targets as quickly as possible," Kim said. Ban has two other climate envoys, former Ghana president John Kufuor and Norway's former prime minister Jens Stoltenberg.