On October 29, the U.S. Supreme Court agreed to hear an appeal of the D.C. Circuit’s decision to vacate the Affordable Clean Energy (ACE) rule, which was the Trump administration’s response to reduce greenhouse gas emissions and replace the Obama administration’s Clean Power Plan (CPP). The lower court decision struck down the ACE rule. It said the Trump administration had erred in concluding that only certain on-site emission-reduction measures can be the “best system of emission reduction” for existing power plants under the Clean Air Act.

This is the latest development regarding the Environmental Protection Agency’s (EPA) authority to regulate greenhouse gas (GHG) emissions, which has now spanned more than six years and three presidential administrations.

The Supreme Court said it expects “Congress to speak clearly if it wishes to assign to an agency, decisions of vast economic and political significance.”

The Supreme Court previously blocked the CPP rule in 2016, preventing it from taking effect. The rule would have given expansive powers to the EPA to regulate the carbon dioxide emissions of coal-fired power plants.

The D.C. Circuit Court vacated the ACE rule in January and required the EPA to start over on a new rule.

A coalition led by West Virginia Attorney General Patrick Morrisey and 19 other states field suit in April to get on the Supreme Court’s docket.

The National Mining Association (NMA applauded the Supreme Court’s decision.

“We are encouraged that the Supreme Court recognized the importance of the question at issue in deciding to consider whether Congress empowered the EPA the authority to remake the energy economy,” the NMA said. “It has been and remains our position that the Clean Air Act does not provide the EPA with near unfettered authority to issue rules that fundamentally remake the nation’s electricity grids, causing massive electricity reliability and affordability repercussions across our economy — yet the decision by the D.C. Circuit to vacate the ACE rule mandates the EPA to consider generation shifting as a legal alternative.