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The Trump administration is expected to deal another blow to President Barack Obama's signature climate rule this week. As early as Wednesday, the Environmental Protection Agency could release the final version of the Affordable Clean Energy rule—the administration's replacement for the Obama-era Clean Power Plan, which sought to limit emissions from coal-fired power plants and spur investment in renewable energy sources.

The United States' energy sector accounts for about a third of total U.S. emissions, and Obama's Clean Power Plan was hailed in 2015 as the strongest climate action of any sitting president. The plan was expected to cut greenhouse gas emissions from power plants by 32 percent by 2030, bolster renewable energy sources, and save tens of billions of dollars in health-care and climate-related costs—all while saving Americans an average of $85 per year on their electricity bills

President Donald Trump took aim at the Clean Power Plan early on the campaign trail, promising to repeal the rule that he called a "crushing attack on American industry." His administration's replacement, which is expected to look similar to the draft ACE rule released last summer, gives states more control over emissions reductions targets for coal plants.

Obama's Clean Power Plan set strict, state-wide emissions reduction targets and outlined three main pillars to achieve them: efficiency improvements at existing coal plants, and replacing coal with both natural gas and renewable resources. It sought to reduce energy-related emissions by encouraging a transition to low- and then zero-carbon energy sources. But Trump's EPA has argued that it doesn't have the authority to force states to alter their energy mixes

The EPA's authority to regulate power plant emissions comes from the Clean Air Act, which mandates that the EPA regulate emissions from coal plants with the "best system of emissions reduction." Obama's EPA decided the best system was the broad, three-pillared approach. Trump's EPA defined the best system as only on-site improvements to existing facilities, which means that, rather than pushing for states to add more low- and zero-carbon sources to their energy mixes, the ACE rule focuses only on what individual facilities can do to improve efficiency and bring down emissions.

While the ACE rule allows states to set their own emissions reduction targets, it also limits the technology options—called heat-rate efficiency improvements—that coal plants can choose from to increase efficiency.

One study published earlier this year in Environmental Research Letters, which evaluated the draft version of the rule released last summer, found that the ACE rule would lead to increased emissions in more than a dozen states and Washington, D.C., thanks to what the authors called "emissions rebound,"—a phenomenon in which power facilities that implement efficiency improvements operate more often or for longer periods of time, ultimately leading to more pollution. "The key takeaway is that ACE is a free pass for carbon emissions," study author Kathleen Lambert of the Center for Climate, Health and the Global Environment at the Harvard T.H. Chan School of Public Health said in January.

Lambert and her co-authors found that carbon dioxide, sulfur dioxide, and nitrogen oxide pollution would all be higher under ACE than the Clean Power Plan. The Trump administration's own analysis found that ACE would lead to an additional 1,400 early deaths per year by 2030, and some 15,000 extra cases of respiratory illnesses due to increases in air pollution. When the analysis was released, EPA officials said that the rule would achieve many of the goals of the Clean Power Plan, but in a more cost-effective and legally binding way. "However much people may want E.P.A. to demand that renewables be built instead of coal plants, we do not have that authority," William J. Wehrum, the EPA's air chief, told the New York Times.

The Trump administration is expected to nix one of the more controversial provisions of the draft rule in the final version released this week: a weakening of the air pollution permitting requirements for upgrades to power plants. The EPA's New Source Review program requires facilities to get specific permits for upgrades that could lead to more emissions. Under current law, companies have to obtain the new permits if the upgrades will cause a significant increase in a plant's annual emissions; under the draft ACE rule, the NSR program would have been triggered if an upgrade increased the plant's hourly emissions rate, which critics say is a more lenient measure. While the new ACE rule does not overhaul the permitting system, EPA officials always said that reforming the NSR program might be done separately.

Environmental groups have already signaled that they will challenge the administration's new rule in court.

Both the ACE rule and reforming the NSR program are part of the Trump administration's broader push to boost the nation's declining coal industry, but it may be too late for coal-fired plants on the verge of closing down.

More so than stringent environmental regulations, it is market forces—such as the glut of cheap natural gas and the declining cost of renewable technologies in the U.S.—that have led dozens of coal plants to close in recent years. A report out this week from the sustainability non-profit Ceres found that, although when combined, coal and natural gas still make up more than half of the nation's electricity, zero-carbon sources—including solar, wind, and nuclear plants—now generate a greater percentage of the nation's electricity than either fossil fuel individually.