On June 19, 2019, the Environmental Protection Agency issued the Affordable Clean Energy Rule (ACE Rule), replacing the Obama administration’s Clean Power Plan (CPP) for limiting greenhouse gas emissions from the power sector. The ACE Rule takes a fundamentally different approach. While the CPP sought to shift the nation’s electricity generation away from fossil-fuel sources toward natural gas and renewables, the ACE Rule seeks to limit emissions by improving the efficiency of (and thereby maintaining) existing coal-fired power plants.