In 2007, the United States Supreme Court ruled in what would become a landmark case (Massachusetts v. EPA, 549 U.S. 497) that greenhouse gas emissions do in fact pose a danger to human health and therefore can be regulated under the Clean Air Act. At the time of the ruling, there was instead strong bipartisan support for Congressional action in a market-based approach setting rather than this more traditional command and control type of regulation to address carbon emissions. However, after the ACES failed to make it through Congress and no new legislative attempts gained any traction, this was still in the lawmaking back pocket.
This ruling paved the way for the Obama Administration's Clean Power Plan proposal.
You can read more about the Endangerment Finding at these sources: