Former New York mayor Michael Bloomberg has been appointed to be the UN special envoy for cities and climate change.
It's a position that will give the billionaire businessman and philanthropist an international stage to press for action to combat global warming.
Secretary-General Ban Ki-moon chose Bloomberg, who made combating climate change a major focus of his 12 years as mayor and was outspoken on how cities should be run to cope with ever increasing populations without harming the environment.
UN deputy spokesman Farhan Haq said on Friday that Bloomberg will assist the UN chief in his consultations with mayors and other key parties "to raise political will and mobilise action among cities as part of his longer-term strategy to advance efforts on climate change."
The secretary-general also wants Bloomberg to bring solutions to the climate summit he is hosting in New York on September 23 to try to galvanise action to combat climate change, Haq said.
Bloomberg, 71, tweeted that he was honoured by the appointment.
"Cities are taking measurable action to reduce emissions, emerging as leaders in the battle against climate change," he tweeted.
"I look forward to working with cities around the world and the UN to accelerate progress" to combat global warming.
Bloomberg served three terms as New York's mayor before handing the reins of America's largest city to Bill de Blasio on January 1.
He is scheduled to co-host the February 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 locally that reduce climate-warming greenhouse gases and global climate risks.
At the summit, Bloomberg will hand over the chair and presidency of the group to Eduardo Paes, the mayor of Rio de Janeiro.
His appointment as special climate envoy drew widespread praise.
US Secretary of State John Kerry tweeted: "We need your energy and hard work to help address climate change."
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 per cent since 2008. The level of soot pollution has gone down by 23 per cent 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.