The U.S. Supreme Court on Oct. 29 agreed to hear legal challenges to a federal appeals court ruling that vacated the Trump administration's signature carbon rule for coal-fired power plants.

In a ruling issued on former U.S. President Donald Trump's last full day in office in January, the U.S. Court of Appeals for the D.C. Circuit determined that the Affordable Clean Energy, or ACE, rule rested on an impermissibly narrow reading of the Clean Air Act.

The ACE rule was meant to replace the Obama-era Clean Power Plan, a far more sweeping U.S. Environmental Protection Agency climate rule that centered on the use of "outside the fence line" emissions reduction measures, such as power plant coal-to-gas switching, emissions trading and renewable energy development.

Unlike the Clean Power Plan, the ACE rule would have required coal plants to curb emissions by making efficiency upgrades to their facilities based on a menu of different technologies.

Critics of the ACE rule pointed to modeling that projected it would increase carbon emissions and air pollution in 18 states and Washington, D.C., by enabling retrofitted coal plants to run more often. But the Trump EPA maintained that the ACE rule represented the only permissible reading of Section 111(d) of the Clean Air Act, which covers existing fossil fuel-fired generators.

The D.C. Circuit disagreed, stating in its Jan. 19 ruling that the Trump EPA "reads the statute to require the agency to turn its back on major elements of the systems that the power sector is actually and successfully using to efficiently and cost-effectively achieve the greatest emission reductions."

The coal-producing states of West Virginia and North Dakota, as well as The North American Coal Corp. and Westmoreland Mining Holdings LLC, filed four separate petitions asking the Supreme Court to review the D.C. Circuit's decision.

West Virginia asked the high court to consider whether Congress intended to authorize the EPA to issue Clean Air Act rules "without any limits on what the agency can require so long as it considers cost, non-air impacts, and energy requirements."

North Dakota's petition focused on the question of whether the EPA can issue Clean Air Act rules that effectively deprive states of their ability to create their own Section 111(d) implementation plans.

North American Coal asked the Supreme Court to consider whether the Clean Air Act authorizes the agency to develop industrywide emission reduction programs such as cap-and-trade regimes.

Westmoreland's petition raised two questions. The first centered on the so-called 111(d)/112 exclusion theory, a line of legal reasoning that argues the EPA cannot regulate sources of hazardous air pollution already covered by Section 112 of the Clean Air Act. The coal producer also asked the high court to consider whether the Clean Air Act gives the EPA authority to "decide matters of vast economic and political significance as to whether and how to restructure the nation's energy system."

The Supreme Court accepted and consolidated all four petitions but said it will only consider Westmoreland's second question dealing with the EPA's authority as it relates to the U.S. power grid. All of the questions in the other three petitions will be considered as well.

The Supreme Court's decision to address the EPA's authority to combat climate pollution came on the same day U.S. President Joe Biden heads to Glasgow, Scotland, for a crucial United Nations climate conference.

The court has a 6-3 conservative tilt, and several judges appointed by Republican presidents have been reluctant to defer to agencies' expertise on matters dealing with ambiguous statutes.

In its Oct. 29 order, the Supreme Court allotted one hour for oral arguments. It has yet to set a date.

EPA Administrator Michael Regan, known as a consensus-builder, has meanwhile promised that the agency will issue a strong replacement for the ACE rule.

The four consolidated cases are West Virginia v. EPA (No. 20-1530), North American Coal Corp. v. EPA (No. 20-1531), Westmoreland Mining Holdings v. EPA (No. 20-1778), and North Dakota v. EPA (No. 20-1780).