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Former New York Mayor Michael Bloomberg, in his new U.N. job, says cities hold the key to confronting climate change because they account for 75 percent of the heat-trapping gases and mayors have executive powers to reduce emissions.

The three-term mayor and billionaire businessman was a keynote speaker at Tuesday's opening of a three-day U.N. meeting on making urban areas — where about 70 percent of the world's population is expected to reside by 2050 — more livable, sustainable, economically successful and environmentally friendly.

Bloomberg, now the U.N. special envoy for cities and climate change, urged cities to take action to reduce greenhouse gas emissions that are driving climate change and "climate risks" such as flooding — and he encouraged all governments to empower their cities to take climate actions.