US President Donald Trump has overturned a 2009 scientific ruling that formed the legal basis of federal efforts to regulate greenhouse gas emissions, marking one of the biggest climate policy rollbacks in US history.

The decision reverses the “endangerment finding” issued by the Environmental Protection Agency under former President Barack Obama, which concluded that greenhouse gases threatened public health.

The White House said the move would cut vehicle costs by about $2,400 and reduce regulatory burdens on automakers. Trump called the ruling “a disastrous Obama era policy” and described it as the foundation of the “Green New Scam.”

Obama warned that repealing the finding would weaken public safety, saying, “Without it, we’ll be less safe, less healthy and less able to fight climate change.”

Press secretary Karoline Leavitt said the rollback could save more than $1 trillion and lower energy and transport costs.

Environmental groups have condemned the move and plan legal action.

Experts say the ruling had long been central to regulating emissions from vehicles, power plants, and industry.

The reversal is partly based on a report commissioned by the United States Department of Energy, which critics say is flawed. Legal analysts believe the case could reach the Supreme Court of the United States, potentially determining the long-term future of US climate regulation.