A Canadian scientist who helped author an exhaustive U.S. draft report on climate change says the study makes it clear dramatic action is needed to stop global temperatures from rising, but that her team has no idea how the Trump administration will react to it.

The report, written by scientists from 13 federal agencies, concludes that the United States is already feeling the effects of climate change, with a stark increase in the frequency of heat waves, heavy rains and other extreme weather over the last four decades.

The report is now awaiting approval from U.S. President Donald Trump's administration.

"The reality is we need that policy mechanism to point us in the right direction as well, because otherwise we, as humans, are just resistant to change," said Katharine Hayhoe, director of the Texas Tech University Climate Science Center, in an interview with CBC Radio's The House.

"We're like, 'Why can't we do it the way we've always done it?" Well, we can't because we've found out there are side-effects," she said.

The report, part of the National Climate Assessment, concludes that the long-term evidence that global warming is being driven by human activities is "unambiguous."

That runs counter to Trump's views.

The president has called climate change a "total con job" and "hoax" perpetrated to harm U.S. economic competitiveness, while his Environmental Protection Agency Administrator Scott Pruitt has cast doubt on whether human behaviour has contributed to the planet's warming.

"Because this document is based on the solid, peer-reviewed science of climate change, it does flatly contradict many of the statements people in the U.S. administration have made over the past year," Hayhoe said.

The U.S. administration has until Aug. 18 to review the final stage of the report.

EPA Administrator Scott Pruitt, who has said he doesn't believe carbon dioxide is the primary contributor to climate change, is one of the Trump officials who will decide the report's fate. (Pablo Martinez Monsivais/Associated Press)

The EPA is one of 13 agencies that now have three options: approve the legally mandated report, reject it or conditionally approve it with revisions.

"We don't have any indication of which one of those three options is going to happen," she said.

Hayhoe said the language used in the almost 700-page report — for example, "extremely likely" — is as close as scientists will get to saying human-driven climate change is a sure thing.

"'Extremely likely' to a scientist is standing out there, looking up at the sky, seeing the asteroid approaching you and saying it's extremely likely it's going to hit the Earth," she said.

Report available online

Although a version of the report was available online for months, it made headlines on Monday when the New York Times published a story about it.

Hayhoe said the report wasn't leaked.

"The article was actually written about the third-order draft that has been available publicly [online] since December," Hayhoe said.

The Times has since obtained a fifth draft of the report, which is now online.

The climate assessment has generally been released every four years under a federal initiative mandated by Congress in 1990. The current draft, targeted for release later this year, largely builds on the conclusions of the 2014 assessment released under the Obama administration.

Hayhoe, who was raised in Toronto, said Canada is on the right path with a national carbon tax.

"Which of these policies is best is not really a scientific question. As a human being, though, I do support a price on carbon, because it actually allows the free market to then kick in and say, 'OK, you have a choice. You pay for your choices, but if you really want to drive a gas guzzler you still can. Nobody is trying to tell you you can't.'"

Canada's Liberal government has set a starting price of $10 a tonne on carbon dioxide emissions in 2018, increasing to $50 a tonne by 2022. All of the money raised will be returned to the respective provinces, which can decide what to do with those revenues.