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The Natural Resources Defense Council, meanwhile, said the proposal was a step toward ending an era when power plants were allowed to dump limitless amounts of carbon pollution into the air. The group had threatened to sue the EPA when it failed to finalize the March 2012 proposal within a year but decided against pursuing litigation when Obama vowed to take action on climate.

Obama has given the EPA until next summer to propose a rule controlling heat-trapping pollution at existing power plants.