WASHINGTON • Directly contradicting much of the Trump administration's position on climate change, 13 federal agencies have unveiled an exhaustive scientific report that says humans are the dominant cause of the global temperature rise that has created the warmest period in the history of civilisation.

Over the past 115 years, global average temperatures have increased by about 1 deg C, leading to record-breaking weather events and temperature extremes, the report says.

The long-term global warming trend is "unambiguous", it says, and there is "no convincing alternative explanation" that anything other than humans - the cars we drive, the power plants we operate, the forests we destroy - are to blame.

The report was approved for release by the White House, but the findings come as the Trump administration is defending its climate change policies.

The United Nations convenes its annual climate change conference this week in Bonn, Germany, and the United States delegation is expected to face harsh criticism over President Donald Trump's decision to walk away from the 195-nation Paris climate accord and top administration officials' stated doubts about the causes and impacts of a warming planet.

"This report has some very powerful, hard-hitting statements that are totally at odds with senior administration folks and at odds with their policies," said Dr Philip Duffy, president of Woods Hole Research Centre. "It begs the question, where are members of the administration getting their information from? They're obviously not getting it from their own scientists."

While there were pockets of resistance to the report in the Trump administration, according to climate scientists involved in drafting the report, there was little appetite for a knockdown fight over climate change among Mr Trump's top advisers, who are intensely focused on passing a tax reform Bill - an effort they think could determine the fate of his presidency.

The climate science report is part of a congressionally mandated review conducted every four years known as the National Climate Assessment.

The product of hundreds of experts within the government and academia, and peer-reviewed by the National Academy of Sciences, the report is considered the US' most definitive statement on climate change science.

The White House put out a statement last Friday that seemed to undercut the high level of confidence of the report's findings. "The climate has changed and is always changing," said spokesman Raj Shah.

"As the Climate Science Special Report states, the magnitude of future climate change depends significantly on 'remaining uncertainty in the sensitivity of Earth's climate'" to greenhouse gas emissions, he said.

NYTIMES