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The US Environmental Protection Agency (EPA) is moving to repeal former president Barack Obama's landmark regulations to reduce greenhouse-gas emissions from power plants.

The plan, introduced on 10 October, is a step towards fulfilling President Donald Trump's promises to reverse Obama-era climate regulations and end the “war on coal”. But any attempt to repeal the power-plant rule is certain to face lawsuits from environmental groups and many states that support Obama's climate policies.

“The Trump Administration’s persistent and indefensible denial of climate change — and their continued assault on actions essential to stemming its increasing devastation — is reprehensible,” said Eric Schneiderman, attorney general for the state of New York, in a prepared statement. “I will use every available legal tool to fight their dangerous agenda.”

US emissions from electricity generation have been falling in recent years as energy utilities have shifted away from coal, and towards cheap natural gas and renewables. The Obama administration established the power-plant regulations to hasten that progress, and to help the United States to meet its commitments under the 2015 Paris climate accord.

The power-plant rule would reduce greenhouse-gas emissions to 32% below 2005 levels by 2030 — but it is mired in legal challenges. In 2016, the US Supreme Court blocked the regulations from taking effect. Legal challenges from 27 state governments are still pending, although a federal appeals court has put the case on hold while the Trump administration reviews the rule.

Trump has shown no fear of challenging environmentalists on climate issues: he has already announced plans to pull the United States out of 2015 Paris climate pact. But his administration's attempts to roll back various environmental regulations have faced legal setbacks. One of the latest rebukes came on 4 October, when a federal court rejected an effort by the Department of the Interior to delay implementing curbs on methane emissions from oil and gas operations on public lands.

A long fight

The power-plant rule that Trump's administration plans to challenge was made possible by the Supreme Court's decision in 2007 that carbon dioxide and other greenhouse gases are pollutants under the terms of the Clean Air Act. Two years later, the EPA ruled that these gases are a threat to human health and the environment — a decision known as an 'endangerment finding'. That allowed the agency to draft regulations to limit greenhouse-gas output from various sources.

EPA administrator Scott Pruitt sued to overturn the endangerment finding in his former role as Oklahoma's attorney general, before Trump took office. More recently, as EPA's chief, he has questioned his own agency's authority to regulate CO 2 . Environmentalists fear that he will attempt to repeal the endangerment finding, which would inevitably prompt a flurry of lawsuits.

The legal fight over the EPA's new plan to repeal the Obama power-plant regulations will almost certainly focus on whether the Clean Air Act allows the agency to require that utilities alter their energy portfolios to reduce emissions. The Obama administration set limits on emissions and then allowed states and utilities to decide how to meet those limits, with options that included expanding efforts to reduce energy consumption and developing new sources of renewable energy.

The Trump administration's proposal says that the EPA overstepped its legal authority when it finalized the Obama-era rules. The administration argues that the Clean Air Act limits the EPA to crafting regulations that can be implemented at power plants themselves. The proposal also says that the EPA is still considering whether and how to craft alternative regulations for power-plant emissions.

Jonathan Adler, who heads the Center for Business Law and Regulation at Case Western Reserve University School of Law in Cleveland, Ohio, says the Trump administration can reasonably argue — as many states have — that the Clean Air Act was not designed to regulate greenhouse gases. Courts often give a certain amount of deference to federal agencies on regulatory matters, he says, but only if the agencies show that they have followed all legal and procedural requirements for finalizing new rules.

“Some of the same legal doctrines that helped the Obama administration defend its regulatory decisions will now help the Trump administration defend its decisions going in the opposite direction,” Adler says. “This will certainly be a test for whether this administration is capable of engaging in this sort of heavy lift.”