US President Donald Trump has reversed a key Obama-era scientific ruling that underpins all federal actions on curbing planet-warming gases. The so-called 2009 'endangerment finding' concluded that a range of greenhouse gases were a threat to public health. It's become the legal bedrock of federal efforts to rein in emissions, especially in vehicles.

The White House called the reversal the 'largest deregulation in American history', saying it would make cars cheaper, bringing down costs for automakers by $2,400 per vehicle.

Environmental groups say the move is by far the most significant rollback on climate change yet attempted and are set to challenge it in the courts.

Speaking in the Oval Office on Thursday, Trump said the 2009 ruling was 'a disastrous Obama era policy that severely damaged the American auto industry and massively drove up prices for American consumers'. He called the ruling a 'radical rule that became the legal foundation for the Green New Scam, one of the greatest scams in history'.

The Environmental Protection Agency (EPA) first took a stance on the impacts of greenhouse gases in 2009, finding that six key planet-warming greenhouse gases, including carbon dioxide and methane, were a danger to human health. This finding served as a lynchpin for US regulation of greenhouse gases and has been crucial in managing emissions across various sectors including motor vehicles, power plants, and more.

Administration officials stress that overturning the regulation will save more than $1 trillion and will help cut the price of energy and transportation. However, critics argue that the rollback will force Americans to spend an additional $1.4 trillion in fuel costs for less efficient vehicles and could lead to up to 58,000 premature deaths and 37 million more asthma attacks due to increased pollution.

This decision raises significant concerns about the direction of US climate policy, with environmentalists and opposition stakeholders preparing to challenge the repeal legally. Furthermore, uncertainties loom for the car industry, as reducing fuel efficiency might affect their sales internationally.

Legal experts suggest that the new proposal could be tested in the Supreme Court, setting a pivotal precedent for future environmental legislation in America.