US Environmental Protection Agency (EPA) chief Scott Pruitt has said he “would not agree” carbon dioxide is a primary contributor to global warming.
He told CNBC that measuring human impact on the climate was “very challenging” and there was “tremendous disagreement” about the issue.
Mr Pruitt instead insisted that officials needed “to continue the debate” on the issue.
His remarks contradict his own agency’s findings on greenhouse gas emissions.
Data released in January by NASA and the US National Oceanic and Atmospheric Administration found that the planet’s rising temperature has been “driven largely by increased carbon dioxide and other human-made emissions in the atmosphere”.
The two US agencies added that the earth’s 2016 temperatures were the warmest ever.
The EPA’s website also notes that carbon dioxide is the “primary greenhouse gas that is contributing to recent climate change”.
In fact, there is consensus among the majority of scientists in the field that carbon emissions from human activities are a key driver of rising temperatures and that the impact of climate change will be severe.
Mr Pruitt, 48, sworn in last month, is considered one of President Donald Trump’s most controversial appointments due to his ties to the fossil fuel industry.
The former Oklahoma attorney general also spent years legally challenging the reach of the organisation he now heads.
During his confirmation hearing in January, Mr Pruitt did, however, say he believed humans had contributed to climate change, but he was not sure how much.
His comments broke with President Trump, who in 2012 asserted in a tweet that climate change was a “hoax”.
Environmental groups were swift to criticise Mr Pruitt’s latest comments.
“The arsonist is now in charge of the fire department, and he seems happy to let the climate crisis burn out of control,” said Sierra Club executive director Michael Brune.
Democratic Senator Brian Schatz said Mr Pruitt’s remarks suggested he is a “climate denier”.
“Anyone who denies over a century’s worth of established science and basic facts is unqualified to be the administrator of the EPA,” Mr Schatz said in a statement.