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Barring any last minute changes, U.N. Secretary-General Ban Ki-moon - who is seeking to re-energize the global climate change debate and boost the United Nations' role - could make the announcement as early as Friday, the sources said on the condition of anonymity.
Bloomberg, a billionaire philanthropist who left office last month, 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.
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 due 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 that he set five years ago.
In the climate change 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.
Bloomberg pledged to continue focusing on promoting his key causes - combating climate change, gun control and immigration - after leaving office through his philanthropic work.
The United Nations will host a one-day climate change summit in New York on September 23, 2014. Many developing nations want it 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 drum up support for the planned 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.
But 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.
(Additional reporting by Louis Charbonneau; Editing by Bernard Orr)