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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.

Peter Lehner, executive director of the Natural Resources Defense Council, called Bloomberg “a perfect choice for the U.N. special envoy.”

Bloomberg was “a leader in tackling climate change pollution” during his tenure as mayor,” Lehner said, and he “understands first-hand that our cities must act now to safeguard residents from the impacts of climate change, and that it can be done in a way that will deliver economic benefits at the same time.”