By Michelle Nichols

UNITED NATIONS (Reuters) - U.N. Secretary-General Ban Ki-moon on Friday appointed former New York City Mayor Michael Bloomberg as his special envoy for cities and climate change, in a bid to build momentum ahead of a planned U.N. summit meeting in September.

Ban said Bloomberg will assist him in "consultations with mayors and related key stakeholders, in order to raise political will and mobilize action among cities as part of his long-term strategy to advance efforts on climate change."

Ban is seeking to re-energize the global climate change debate and boost the United Nations' role. The U.N. role for Bloomberg - a billionaire philanthropist who left office last month - was reported by Reuters on Thursday.

In a statement, Bloomberg said cities had emerged as a leading force in the battle against climate change. His appointment as U.N. special envoy is for two years.

"Cities account for more than 70 percent of global greenhouse gas emissions and two-thirds of the world's energy use today, and their total population is projected to double by 2050," Bloomberg said.

"So the steps they take now to combat climate change will have a major impact on the future of our planet. Cities have shown they have the capacity and the will to meet this challenge," he said.

U.S. Secretary of State John Kerry congratulated Bloomberg on the appointment, saying: "I can't think of a person better suited for this important new role."

"Mayor Bloomberg has hands-on experience confronting climate change in one of the largest cities in the world, while, at the same time, seizing the many opportunities that sustainable city management creates," Kerry said in a statement.

Samantha Power, U.S. ambassador to the United Nations, was also quick to welcome Bloomberg's appointment, posting on Twitter: "Mayor @MikeBloomberg knows how to get things done. We need more leaders like him here @UN."

Bloomberg made combating climate change a key focus during his 12 years leading the United States' most populous city. He also advocated for national climate change legislation.

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DRUMMING UP SUPPORT

Bloomberg has played a leading role in the C40 Cities Climate Leadership Group, an international group of mayors created in 2005 and dedicated to reducing greenhouse gas emissions. The C40 group, of which Bloomberg is president of the board, is to meet in Johannesburg next week.

He announced last month that New York City's greenhouse gas emissions have dropped by 19 percent since 2005, putting the city nearly two-thirds of the way to meeting the goal he set five years ago.

In the blueprint he launched in 2007, called PlaNYC 2030, Bloomberg set a goal to slash citywide emissions 30 percent by 2030 through a number of initiatives, such as requiring hybrid taxi cabs, building bike lanes and retrofitting municipal buildings to make them more energy efficient.

The United Nations will host a one-day climate change summit in New York on September 23, 2014. Many developing nations want that to be a deadline for rich countries to outline planned cuts in greenhouse gases beyond 2020 as a key step towards a global climate deal in 2015.

Last month, Ban appointed former Ghana President John Kufuor and former Norwegian Prime Minister Jens Stoltenberg as special envoys on climate change to help drum up support for the September conference.

Ever since the 2009 U.N. climate summit in Copenhagen failed to secure a deal on a binding treaty on reducing carbon emissions, the United Nations has been sidelined, U.N. diplomats and officials say.

Climate discussions have shifted away from the world body to bilateral talks between key world powers and the Group of 20 club of major developed and developing nations.

Ban has long seen galvanizing support for global action on climate change as key to his legacy as secretary-general, the officials and diplomats say, and is eager to restore the United Nations' relevance to the climate negotiations.

(Reporting by Michelle Nichols; Additional reporting by Lesley Wroughton in Washington; Editing by Dan Grebler)