Democratic presidential hopeful Mike Bloomberg has unveiled a piece of his climate plan that would boost the energy efficiency of buildings, which are responsible for 12 percent of U.S. greenhouse gas emissions.

Bloomberg, a former mayor of New York City, has vowed to cut the country’s emissions by 50 percent by 2030. He said he would slash the use of fossil fuels, including natural gas, for cooking, heat and hot water by promoting pollution-freeappliances and zero-carbon furnaces through a combination of federal incentives and standards, including a “Bucks for Boilers” program.

By 2025, all new buildings would be required to meet zero-carbon emissions on site, so that they would be powered by electric utilities using increasing amounts of renewable sources.

“Gas isn’t as clean as we thought. It’s a much bigger problem than we anticipated,” Bloomberg said in an email. “Gas is now a bigger source of climate pollution than coal, and emissions from gas are growing. For those reasons, my clean energy plan calls for replacing all U.S. coal plants, replacing existing gas plants, and stopping the construction of new gas plants as well.”

“We don’t exactly ban the use of gas, but we are looking to set building codes, help them get adopted locally, and offer incentives for pollution-free appliances to get gas out of as many homes and buildings as we can,” he said.

Buildings are a major source of pollution, said Antha N. Williams, the Bloomberg campaign’s senior adviser on climate, energy and environment. She said that if all the buildings in the United States were a country, it would be the third largest emitter of greenhouse gases after China and the United States.

The Energy Information Administration says that 8.5 trillion cubic feet, or 29 percent, of all natural gas used in the United States is burned at residential and commercial buildings.

The latest piece of the Bloomberg climate policy comes as climate change has become a controversial portion of the campaigns of other candidates.

Sen. Elizabeth Warren (D-Mass.) — who favors zero-carbon buildings by 2028, zero-carbon vehicles by 2030, and an increase in the existing loan guarantee program — has pledged to ban hydraulic fracturing, or fracking, as part of a $3 trillion 10-year plan. Sen. Bernie Sanders (I-Vt.) is offering a $16.3 trillion version of the “Green New Deal,” including a section that would nationalize the utility industry as well as ban fracking. Sanders also wants to end fossil fuel use in buildings by 2030.

Sen. Amy Klobuchar (D-Minn.) promises to achieve 100 percent clean energy by 2050.

A fracking ban would particularly affect parts of Ohio and Pennsylvania, which are rich in shale gas and are key states for President Trump or his eventual election opponent.

“We’re not flipping a switch and shutting down production overnight,” Bloomberg said. “We’re going to help people get cleaner, pollution-free homes and buildings, and we’ll put thousands of people to work installing that infrastructure. Already, there are more renewable energy jobs in western Pennsylvania than jobs in fossil fuels. That gives people working in the shale gas industry some options for how to use their skills.”

Bloomberg said he could not estimate the cost of his proposal until the rest of his plan was complete.

He faces competition from another billionaire, Tom Steyer, who has said he would declare a national climate emergency upon taking office, allowing the use of military funds for climate action and pressing Congress to adopt far-reaching legislation. Steyer has advocated “strong standards” for new buildings and retrofits for old ones as part of a $2 trillion infrastructure program.

Williams pointed to initiatives Bloomberg undertook as mayor, including an effort to train building superintendents so they could install and repair “green” equipment. Bloomberg has also given generously to the Sierra Club’s Beyond Coal campaign, which is devoted to shutting down U.S. coal-fired power plants. Last week, Beyond Coal announced imminent closure of the 300th and 301st coal-fired power plants since the campaign began in 2010.

She said Bloomberg did not favor a fracking ban or the nationalization of utilities, though the insistence on zero-carbon new buildings would apply pressure to natural gas drillers. Nor does he favor any “whiz-bang new technology,” Williams added.

Instead, Bloomberg’s plan provides block grants to states to support clean energy, tax credits for new appliances, low-interest loans, and federal programs to help people with their home mortgages. It would also set efficiency standards for utilities and provide weatherization assistance for lower-income earners.

The plan would also make use of federal buildings. “The federal government is the landlord of nearly 377 million square feet of commercial real estate,” the plan says. Bloomberg would use federal buildings as models.

As president, Bloomberg, like others seeking the Democratic Party nomination, would also be a sharp change from President Trump, who rejects the notion that climate change is linked to human activity and has declared that the United States is withdrawing from the Paris climate accord forged in 2015. Trump has also rolled back a wide variety of regulations that would have raised energy-efficiency standards for many items, includingmotor vehicles and appliances.

The former mayor’s proposals, released Wednesday, face potential conflicts with local and state governments, which generally set building codes.

But Bruce Nilles, a former Sierra Club lawyer now at the Rocky Mountain Institute working on energy use in buildings, said that gas-fired furnaces, boilers and appliances generate traditional pollutants as well as greenhouse gases, both of which can be regulated by the Energy Department’s appliance standards and the Environmental Protection Agency under the Clean Air Act.

Nilles, who also works part-time for the Bloomberg campaign, said that as mayor of New York, Bloomberg cut local emissions by 5 percent. Recently, about two dozen cities, including San Jose, Calif., have begun to regulate natural gas emissions at the local level.

He said that Bloomberg became involved in the Sierra Club’s Beyond Coal campaign in 2011 when it was still a nascent campaign. “Bloomberg changed the entire conversation,” Nilles said.