WASHINGTON (AP) — The Biden administration proposed new limits Thursday on greenhouse gas emissions from coal- and gas-fired power plants, its most ambitious effort yet to roll back planet-warming pollution from the nation’s second-largest contributor to climate change.

A rule unveiled by the Environmental Protection Agency could force power plants to capture smokestack emissions using a technology that has long been promised but is not in widespread use in the U.S.

"This administration is committed to meeting the urgency of the climate crisis and taking the necessary actions required,'' EPA Administrator Michael Regan said during Thursday's announcement.

The new rule will “significantly reduce greenhouse gas emissions from fossil fuel-fired power plants, protecting health and protecting our planet,'' Regan said. The plan would not only “improve air quality nationwide, but it will bring substantial health benefits to communities all across the country, especially our frontline communities ... that have unjustly borne the burden of pollution for decades,'' Regan said in a speech at the University of Maryland.

If finalized, the proposed regulation would mark the first time the federal government has restricted carbon dioxide emissions from existing power plants, which generate about 25% of U.S. greenhouse gas pollution, second only to the transportation sector. The rule also would apply to future electric plants and would avoid up to 617 million metric tons of carbon dioxide through 2042, equivalent to annual emissions of 137 million passenger vehicles, the EPA said.

Almost all the coal plants — along with large, frequently used gas-fired power plants — would have to cut or capture nearly all their carbon dioxide emissions by 2038, the EPA said. Plants that cannot meet the new standards would be forced to retire.

The plan is likely to be challenged by industry groups and Republican-leaning states, which have accused the Democratic administration of overreach on environmental regulations and warn of a pending reliability crisis for the electric grid. The power plant rule is one of at least a half-dozen EPA rules limiting power plant emissions and wastewater treatment.

“It's truly an onslaught” of government regulation “designed to shut down the coal fleet prematurely,'' Rich Nolan, president and CEO of the National Mining Association, said in an interview before the rule was announced.

In a call with reporters on Wednesday, Regan denied that the power plant rule — or any other regulation — was aimed at shutting down the coal fleet even though he acknowledged, “We will see some coal retirements.”

The proposal “relies on proven, readily available technologies to limit carbon pollution” and builds on industry practices already underway to move toward clean energy, he said.

Coal provides about 20% of U.S. electricity, down from about 45% in 2010. Natural gas provides about 40% of U.S. electricity. The remainder comes from nuclear energy and renewables such as wind, solar and hydropower.

Tom Kuhn, president of the Edison Electric Institute, which represents U.S. investor-owned electric companies, said the group will assess whether the EPA's proposal aligns with its commitment to provide reliable, clean energy.

Carbon emissions from the U.S. power sector are at the same level as in 1984, while electricity use has climbed 73% since then, Kuhn said.

The EPA rule would not mandate use of equipment to capture and store carbon emissions — a technology that is expensive and still being developed — but instead would set caps on carbon dioxide pollution that plant operators would have to meet. Some natural gas plants could start blending gas with another fuel source such as hydrogen, which does not emit carbon, although specific actions would be left to the industry.

Still, the regulation is expected to lead to greater use of carbon capture equipment, a technology that the EPA said has been "adequately demonstrated" to control pollution.

Jay Duffy, a lawyer for the Boston-based Clean Air Task Force, said the EPA rule is likely to “propel deployment of carbon capture” technology far above current usage. “It’s a way for (fossil fuel) plants to operate in a decarbonized world," he said before the rule was announced.

"Industry innovates and over-complies,'' Duffy said, citing a 1970s EPA rule that required power plants to use sulfur dioxide scrubbers. At the time, there were only three commercial scrubber units operating at U.S. power plants and just one vendor. Within a few years, there were 119 sulfur scrubbers installed and 13 vendors, Duffy said in an essay posted on the group's website.

More recently, the U.S. power industry exceeded emissions goals set by the Obama administration in its Clean Power Plan, even though the plan was blocked by the courts and never implemented.

Still, the scope of the power plant rule is immense. About 60% of the electricity generated in the U.S. last year came from burning fossil fuels at the nation’s 3,400 coal and gas-fired plants, according to the U.S. Energy Information Administration.

“These rules are a big deal,'' said David Doniger, senior strategic director for climate and clean energy at the Natural Resources Defense Council. The power plant rules are crucial to meeting President Joe Biden's goals to cut greenhouse gas emissions in half by 2030 and eliminate carbon emissions from the power grid by 2035, he and other advocates said.

“We need to do this to meet the climate crisis,'' Doniger said.

The proposal comes weeks after the Biden administration announced strict new tailpipe pollution limits that would require up to two-thirds of new vehicles sold in the U.S. to be electric by 2032 and months after Biden announced rules to curb methane leaks from oil and gas wells.

The rules follow climate action by the 2021 infrastructure law and billions of dollars in tax credits and other incentives from the Inflation Reduction Act, approved last year.

While Biden has made fighting global warming a top priority, he has faced sharp criticism from environmentalists — particularly young climate activists — for a recent decision to approve the contentious Willow oil project in Alaska. The massive drilling plan by oil giant ConocoPhillips could produce up to 180,000 barrels of oil a day on Alaska’s petroleum-rich North Slope. Environmental groups call Willow a “carbon bomb” and have mounted a social media #StopWillow campaign.

The new plan comes 14 years after the EPA declared that carbon dioxide and other greenhouse gases endanger public health. President Barack Obama tried to set limits on carbon pollution from U.S. power plants, but his 2015 Clean Power Plan was blocked by the Supreme Court and later was rolled back by President Donald Trump.

Last year, the Supreme Court limited how the Clean Air Act can be used to reduce climate-altering emissions from power plants. The 6-3 ruling confirmed the EPA's authority to regulate carbon emissions from power plants but said it could not force a nationwide transition away from the use of coal to generate electricity.

The EPA said its new rule will give plant operators flexibility to meet the new standards in a method of their choosing. And instead of creating one limit that all power plants must meet, the agency said it will set a range of targets based on the size of the plant, how often it is used and whether it is already scheduled for retirement.

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