In 2007, the Supreme Court had ruled that greenhouse gases were pollutants under the Clean Air Act. According to that act, if a pollutant was found to endanger public health and welfare, the EPA was obligated to regulate it. Under President Obama, the EPA concluded that greenhouse gases were such a danger, prompting the EPA to promulgate greenhouse gas regulations in the following years.
LINK TO THIS ARTICLE
https://www.apeoplesepa.org/modules/greenhouse-gas-endangerment-finding-december-15-2009SOURCE
CITATION
EPA, "Endangerment and Cause or Contribute Findings for Greenhouse Gases Under Section 202(a) of the Clean Air Act," Federal Register, December 15, 2009.