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, 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'. Former President Barack Obama said that repealing the finding would make Americans more vulnerable. The Environmental Protection Agency (EPA) first took a stance on the impacts of greenhouse gases in 2009. The agency decided that six key planet-warming greenhouse gases, including carbon dioxide and methane, were a danger to human health. Trump administration officials are stressing that overturning the regulation will save more than $1tn and will help cut the price of energy and transport. Many environmentalists are sceptical of the potential cost savings being touted by the Trump team. For some in the US car industry there will be uncertainty about the rollback as manufacturing less fuel-efficient vehicles might limit their sales overseas.