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Secretary-General Ban Ki-moon says he is counting on former mayor Michael Bloomberg "to work for humanity" in his new job as United Nations Special Envoy for Cities and Climate Change.

Ban made the comments Friday as he welcomed Bloomberg to U.N. headquarters for the first time since his appointment on Jan. 31.

The U.N. chief said he was counting on Bloomberg's "dedicated and visionary leadership" to help countries around the world address the climate change phenomenon.

As a first priority, Ban said he wants the former mayor to help make the U.N. climate change summit he has called in September "a great success."

"Most importantly," Ban said, "we need to have a global, legal climate change agreement by 2015."