GENEVA -- A top UN scientist on Thursday shrugged off an online quip from U.S. President Donald Trump that questioned global warming, saying a U.S. government report will show the "fundamental impacts of climate change on the U.S. continent."

Officials at the World Meteorological Organization also said environmentally minded efforts by the state of California, in parts of the financial sector, among grassroots activists and others will have more of an impact to help the fight against climate change than "political disturbance" and "discourse" will impede it.

The science, they said, will have the last word.

Some of that science comes Friday in a new U.S. federal climate change assessment report, from agencies including the National Oceanic and Atmospheric Administration and the Department of Energy.

"It's very interesting what you will see tomorrow," said Pavel Kabat, WMO's chief scientist. "That is a major report coming supported by all agencies ... under the administration of President Trump ranging from NOAA to NASA, from NASA to DOE, showing the fundamental impacts of climate change on the U.S. continent."

Kabat was speaking at a news conference to present WMO's latest greenhouse gas bulletin, which said levels in the atmosphere climbed to a record last year. In a statement, WMO secretary-general Petteri Taalas warned "the window of opportunity" to act against climate change has nearly closed.

WMO says globally averaged concentrations of carbon dioxide reached 405.5 parts per million in 2017, up slightly from the previous year and from 400.1 in 2015. Concentrations of other heat-trapping gases, like methane and nitrous oxide, also rose.

The findings come ahead of next month's meeting of world leaders in Katowice, Poland, for the annual global climate summit where they hope to find ways to implement the 2015 Paris climate accord that aims to keep global warming at 1.5 degrees Celsius.

The Pacific island nation of Vanuatu, which lies about 2,000 kilometres northeast of Australia, is one of those countries already suffering severe effects of climate change.

Its foreign minister, Ralph Regenvanu, said his government is exploring the possibility of suing fossil fuel companies responsible for most man-made greenhouse gas emissions over the damage they have caused.

"By hiding what they knew and misrepresenting the dangers of unabated fossil fuel use, the fossil fuel industry and supportive governments protected their high-emissions profits and obstructed the changes needed to prevent or at least minimize the impacts of climate change," he told a 'virtual' summit of nations that are vulnerable to climate change.

WMO's Taalas said the last time Earth had such high levels of carbon dioxide concentration was 3-5 million years ago.

Without cuts in greenhouse gases, Taalas said, "climate change will have increasingly destructive and irreversible impacts on life on Earth."

That came just hours after Trump, who once called global warming a "hoax" and has pulled the United States out of the Paris climate agreement, floated a new question about climate change on Twitter: "Brutal and Extended Cold Blast could shatter ALL RECORDS - Whatever happened to Global Warming?"

Despite that, Kabat cited a "global transformation" underway in thinking about global warming, saying "the point of no return has been passed." He said efforts by some industries, in civil society and by local officials could "overtake" governmental agreements or statements on the issue.

"I think on the longer timescale we are not that much worried about the current political disturbance," he said.

"Despite the statements of President Trump, there is still a very concrete and specific movement by the agencies under the administration to actually make the case of action against climate change," Kabat said.

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Frank Jordans contributed from Berlin.