Massachusetts v. EPA (2007)
Twelve states and environmental groups sued the EPA to force it to regulate greenhouse gases. The Supreme Court ruled 5-4 that CO2 qualifies as an air pollutant under the Clean Air Act and that EPA had to decide whether it endangers public health.
EPA issued its endangerment finding in 2009, creating the legal basis for all federal climate regulations.
Established that greenhouse gases fall under the Clean Air Act, making regulation EPA's statutory duty.
The current lawsuit rests on the same legal foundation: the Clean Air Act's mandate to regulate dangerous air pollutants.
