Bloomberg said while Obama sees climate change as an urgent problem that threatens the planet, Romney does not.
The endorsement by Bloomberg provides a crucial boost to Obama just days ahead of the November 6 presidential election, which promises to be a tightly fought race.
The support by Bloomberg was unexpected, with both the Obama and Romney campaigns seeking the mayor's endorsement, given that he is in a position to influence independent voters around the country.
Bloomberg said if Obama listens to people on "both sides of the aisle, and builds the trust of moderates, he can fulfill the hope he inspired four years ago and lead our country toward a better future for my children and yours. And that's why I will be voting for him."
A lifelong democrat who had turned Republican before becoming an independent, Bloomberg said Romney is a "good and decent man" who as Massachusetts governor had taken important steps to tackle climate change, including signing on to a regional cap-and-trade plan.
The plan was designed to reduce carbon emissions 10 per cent below 1990 levels. However, Bloomberg expressed disappointment that Romney has since "reversed course, abandoning the very cap-and-trade program he once supported"
Among the other reasons cited by Bloomberg for his endorsement of Obama was the president's support for abortion rights and for same-sex couples.
Obama issued a statement welcoming the endorsement and pledged to continue to stand with New York in its time of need.
"While we may not agree on every issue, Mayor Bloomberg and I agree on the most important issues of our time -- that the key to a strong economy is investing in the skills and education of our people, that immigration reform is essential to an open and dynamic democracy, and that climate change is a threat to our children's future, and we owe it to them to do something about it," Obama said.
Bloomberg had not endorsed a presidential candidate in 2008, when Obama ran against Senator John McCain. He had endorsed Republican President George W Bush's re-election in 2004.
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