Mayor Bloomberg has reversed course and is now pushing through a mandate to turn New York’s iconic yellow taxicabs green.

Mr. Bloomberg announced yesterday that the city would, over the next five year, require owners of the city’s 13,000 medallion (yellow) taxicabs to replace their vehicles with environmentally friendly hybrids. The new standards are expected to reduce carbon emissions by 50% over the next 10 years and create the cleanest fleet of taxis in the nation.

“The idea is to make our cabs more efficient. If we can do that we’ll save the equivalent of 32,000 cars on the street,” Mr. Bloomberg said on NBC’s “The Today Show.”

The idea of mandating that all yellow taxis go hybrid — rather than offering incentives for individual owners to volunteer — was first proposed about five years ago by City Council Member David Yassky, a Democrat of Brooklyn. It was not adopted as a priority by the Bloomberg administration until now.

Mr. Bloomberg has expanded the number of taxi medallions for alternative fuel taxicabs and replaced thousands of city-owned vehicles with hybrids. But he did not push his Taxi and Limousine Commission to adopt the mandate.

Council Member John Liu, a Democrat of Queens, called Mr. Bloomberg ‘s hybrid announcement a “welcomed change of heart.”

“It’s pretty much an about-face, but it’s a welcomed about-face,” Mr. Liu said.

When asked why he had not been behind a mandate until now, Mr. Bloomberg said: “Maybe David’s been persuasive and he convinced me that it’s a great idea. Nothing wrong with that.”

Mr. Yassky praised Mr. Bloomberg for making the environment a hallmark issue during his second term.

Yesterday’s plan, which still needs to be officially approved by the TLC, would be a sweeping change for a fleet that has only 375 hybrids now.

It establishes a timeline that would phase in the hybrids between 2008 and 2012. Medallion owners currently have to replace their taxicabs every three years. Starting in October 2008, all new vehicles entering the fleet would have to get at least 25 miles to the gallon. The number of hybrids would then go up each year.

While drivers would have to pay slightly more to buy their hybrid vehicles, Mr. Bloomberg said, they would make up it up in savings in lower gas bills. Currently, livery cabs would not be affected by the plan.

The hybrid plan comes on the heels of Mr. Bloomberg ‘s 127-point sustainability plan to reduce carbon emissions by 30% and just a week after the mayor hosted an international climate summit. Environmental groups have been lining up behind the mayor in recent weeks and he has become something of an overnight environmental trailblazer.

Mr. Bloomberg has prided himself on being a pragmatic leader who is open to changing his mind when he feels the evidence warrants it.

Yesterday, his hybrid plan thrust him to the national stage again, this time with appearances on “Today” and on CNN — where Matt Lauer and Wolfe Blitzer each brought up his presidential ambitions.

Mr. Bloomberg, as usual, denied that he has plans to run.