US President Donald Trump has reversed a major 2009 EPA ruling that said greenhouse gases harm public health. This "endangerment finding" was the basis for federal climate actions, especially car emission rules. The White House calls the reversal the "largest deregulation in American history," promising cheaper cars and savings of $2,400 per vehicle. Trump said, "This radical rule became the legal foundation for the Green New Scam." Environmental groups warn this rollback is the biggest attack on climate change policies and plan to fight it in court. The EPA's 2009 decision covered six greenhouse gases, including carbon dioxide and methane. It enabled regulations on vehicles, power plants, and other sectors to limit emissions. Experts say undoing this could raise fuel costs by $1.4 trillion and cause thousands of premature deaths and millions of asthma attacks. Car makers face challenges too, as less efficient US vehicles may lose buyers abroad. The administration used a controversial 2023 report questioning climate science to justify the reversal, but courts found flaws in the report's formation. Legal experts expect fights in courts, possibly reaching the Supreme Court. Meghan Greenfield, a former EPA attorney, said if the reversal wins in court, future administrations may not easily restore the rule without new laws. This move shakes the foundation of US climate actions and sets the stage for heated legal battles ahead.