Moving forward with a controversial cornerstone of President Barack Obama's recently announced Climate Action Plan, the Environmental Protection Agency has proposed updated Clean Air Act standards that limit carbon emissions for new power plants

The centerpiece of the President's Climate Action Plan is the establishment of carbon pollution standards for new and existing power plants. The EPA proposed source performance standards for new power plants in the spring of 2012, with carbon dioxide emission standards that were initially very stringent, treating coal and natural gas plants the same. After receiving more than 2.5 million public comments,the EPA decided to address a common concern and re-propose the rule with separate standards for coal and natural gas.

Under the proposal, newly constructed  large natural gas-fired turbines would need to meet a limit of 1,000 pounds of CO2 per megawatt-hour, while new small natural gas-fired turbines would need to meet a limit of 1,100 pounds of CO2 per megawatt-hour. New coal-fired units would need to meet a limit of 1,100 pounds of CO2 per megawatt-hour, with an option to meet a somewhat tighter limit if they choose to average emissions over multiple years.

“These standards… ensure that the power plants of the future use cleaner energy technologies, such as efficient natural gas, advanced coal technology, nuclear power, and renewable energy like wind and solar,” a statement by the EPA says.

Power plants are the largest concentrated source of emissions in the United States, together accounting for roughly one-third of all domestic greenhouse gas emissions, according to the EPA. While the U.S. has limits in place for arsenic, mercury and lead pollution that power plants can emit, there are currently no national limits on the amount of carbon pollution new power plants can emit.

The EPA is seeking comment and information on the proposal. A comment period will be open for 60 days following publication in the Federal Register.

Separately, the EPA is reaching out to states, industry, and the public as it develops emission guidelines for existing power plants. In accordance with a June 25 Presidential Memorandum , EPA will issue proposed standards for existing power plants by June 1, 2014.