California plans to sue the EPA over its recent decision not to regulate greenhouse gas emissions from vehicles and other sources. Nine other states and environmental groups will join the suit. They argue that the Clean Air Act gives the EPA authority to regulate greenhouse gases. A victory in this lawsuit could set a precedent that the US government can regulate these emissions. The suit may also determine if greenhouse gases are considered air pollutants under the Clean Air Act. California has been more aggressive than the federal government on air pollution rules and wants to protect its authority to regulate vehicles in the state.