The U.S. Environmental Protection Agency is releasing the Clean Power Plan proposal today. This is the first attempt to cut carbon pollution from existing power plants, the single largest source of carbon pollution in the United States. According to the EPA, power plants account for roughly one-third of all domestic greenhouse gas emissions in the United States. While there are already standards for the level of arsenic, mercury, sulfur dioxide, nitrogen oxides, and particle pollution that power plants can emit, there are currently no national limits on carbon pollution levels.