The Environmental Protection Agency on Monday released a draft rule to regulate carbon emissions from hundreds of fossil-fired power plants across the U.S., the cornerstone of President Barack Obama's climate-change agenda. The proposed rule mandates that power plants cut U.S. carbon-dioxide emissions 30% by 2030 from levels seen in 2005, an aggressive target that marks the first attempt at limiting such pollution.

The carbon framework seeks to strike a balance between what environmentalists want—an ambitious overall target—with what the utility industry wants—flexibility, a long compliance timeline and an earlier base-year calculation from which to meet the goal. Carbon emissions have dropped since 2005, making the overall reduction smaller than it would have been if the EPA had used a more-recent year for a baseline.