UNITED NATIONS — Former New York mayor Michael Bloomberg has been chosen by Secretary-General Ban Ki-moon to be the U.N. special envoy for cities and climate change.

U.N. deputy spokesman Farhan Haq announced the appointment of the billionaire businessman and philanthropist turned politician to the U.N. post on Friday.

He said Bloomberg will assist the secretary-general in his consultations with mayors and other key parties "to raise political will and mobilize action among cities as part of his longer-term strategy to advance efforts on climate change."

Bloomberg served three terms as New York's mayor before handing the reins of the America's largest city to Bill de Blasio on Jan. 1.

The billionaire businessman and philanthropist turned politician made combating climate change a major focus of his 12 years as mayor. He was also very outspoken on how cities should be run to cope with ever increasing populations.

Bloomberg is scheduled to co-host the Feb. 4-6 mayors summit of the C40 Cities Climate Leadership Group in Johannesburg.

The group is a network of large cities from around the world committed to taking measures that reduce climate-warming greenhouse gases and climate risks.

At the summit, he will hand over the chair and presidency of the group to Eduardo Paes, the mayor of Rio de Janeiro.

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Last year, Bloomberg boasted that New York city's air quality hit its highest levels in 50 years and now has the cleanest air of any major American city.

He said the level of sulfur dioxide in the air has gone down by 69 percent since 2008. The level of soot pollution has gone down by 23 percent since 2007 — achievements officials attributed to a combination of factors, including buildings burning lower-pollution heating oils or switching over to cleaner burning natural gas.

Associated Press Writer Alexandra Olson contributed to this report from the United Nations