From the NYT:

President Obama could leave office with the most
aggressive, far­-reaching environmental legacy of any occupant of the White House. Yet it is very possible that not a single major environmental law will have passed during his two terms in Washington.

Instead, Mr. Obama has turned to the vast reach of the Clean Air Act of 1970, which some legal experts call the most powerful environmental law in the world. Faced with a Congress that has shut down his attempts to push through an environmental agenda, Mr. Obama is using the authority of the act passed at the birth of the environmental movement to issue a series of landmark regulations on air pollution, from soot to smog, to mercury and planet-­warming carbon dioxide.