Guterres issued a robust defence of "clear-eyed" climate science and data, without which, he said, the world would never have understood the emergence of the "dangerous and existential threat of climate change".

"We must fight mis- and disinformation, online harassment, and greenwashing," Guterres said at the UN's World Meteorological Organization (WMO) weather and climate agency in Geneva.

"Scientists and researchers should never fear telling the truth."

Guterres's remarks will be seen in some quarters as a riposte to Trump's speech at the United Nations in New York last month, in which the Republican president championed fossil fuels and derided green technologies.

"Climate change -- it's the greatest con job ever perpetrated on the world, in my opinion," said Trump.

The "carbon footprint is a hoax made up by people with evil intentions", he said.

"We're getting rid of the falsely named renewables, by the way: they're a joke, they don't work, they're too expensive," he added, about his administration's war on solar and wind power, bolstered by a new law that ends clean energy tax credits.

- Planet on the 'brink' -

But Guterres insisted that in 2024, "almost all new power capacity came from renewables", and investment was surging.

"Renewables are the cheapest, fastest and smartest source of new power. They represent the only credible path to end the relentless destruction of our climate," he insisted.

The WMO is marking its 75 anniversary this year, and is leading the charge for all countries to be covered by extreme weather early warning systems by 2027.

"Global warming is pushing our planet to the brink," said Guterres.

"Every one of the last 10 years has been the hottest in history. Ocean heat is breaking records while decimating ecosystems. And no country is safe from fires, floods, storms and heatwaves.

"As always, the poorest and most vulnerable countries pay the highest price -- particularly small island developing states and the least developed countries."

The COP30 summit will be held in the Brazilian Amazon city of Belem from November 10-21.

The UN secretary-general said countries needed strong domestic climate action plans in time for the summit, urging nations to address the problem of climate disasters at source.

"By November's UN Climate Conference in Brazil, countries must deliver bold new national climate action plans that align with limiting global temperature rise to 1.5 degrees Celsius," Guterres said.

He said these had to include commitments to reduce greenhouse gas emissions over the next decade.

"Much greater ambition is required."