Greta Thunberg has told the World Economic Forum in Davos that planting trees is not enough to address climate change, an apparent rebuke of a pledge made moments earlier by US President Donald Trump.

Key points: Donald Trump called climate activists "heirs of yesterday's foolish fortune tellers" but said he was a "very big believer in the environment"

Donald Trump called climate activists "heirs of yesterday's foolish fortune tellers" but said he was a "very big believer in the environment" Greta Thunberg said the inaction of world leaders was hastening climate change

Greta Thunberg said the inaction of world leaders was hastening climate change Nobel-winning economist Joseph Stiglitz also criticised Mr Trump's speech.

"Our house is still on fire," Ms Thunberg said, echoing remarks she made at the annual meeting a year ago.

"Your inaction is fuelling the flames."

The ongoing row between the teenage activist and 73-year-old US President around climate change appeared an attempt by both to frame the argument at Davos.

Ms Thunberg called for an immediate end to fossil fuel investments in front of a packed audience less than a hour after watching Mr Trump make his keynote address in the Swiss ski resort.

Mr Trump announced the US would join an existing initiative to plant 1 trillion trees, but also spoke at length about the economic importance of oil and gas and called climate change activists "the perennial prophets of doom" who were predicting an "apocalypse".

Ms Thunberg responded by referring to "empty words and promises" by world leaders.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 1 minute 12 seconds 1 m 12 s Ms Thunberg says simply planting trees is not enough to address climate change

"You say children shouldn't worry … don't be so pessimistic and then, nothing, silence," Ms Thunberg said.

Earlier, she called on world leaders to listen to young activists, who have followed her to Davos this year.

She and Mr Trump have been sparring for months.

Last month, the President told Ms Thunberg in a tweet to "chill" and to "work on her anger management problem".

It prompted a dry response from Ms Thunberg, who then changed her Twitter caption to read: "A teenager working on her anger management problem. Currently chilling and watching a good old-fashioned movie with a friend."

"I'm not a person that can complain about not being heard," she said, prompting laughter from the audience on the first day of the annual WEF meeting.

"The science and voice of young people is not the centre of the conversation, but it needs to be."

Among the young "climate heroes" being celebrated by the WEF is Irish teenage scientist Fionn Ferreira, who created a solution for preventing microplastics from reaching oceans.

They also include South African climate activist Ayakha Melithafa, 17, and Canadian Autum Peltier, who has been advocating for water conservation since she was eight.

Activists take part in a march to highlight issues surrounding climate change at the World Economic Forum in Davos. ( REUTERS: Arnd Wiegmann )

Trump focuses on economy

In his address, Mr Trump mostly avoided environmental issues, instead talking up the US economy.

But he later told reporters: "I'm a very big believer in the environment. I want the cleanest water and the cleanest air".

He attended the event despite his impeachment trial in the US getting underway today.

Nobel Prize-winning economist Joseph Stiglitz criticised the President's swipe at climate "pessimists".

US President Donald Trump has taken a swipe at "climate pessimists" at the World Economic Forum at Davos, Switzerland. ( APL: Markus Schreib )

"As if what we are seeing with our eyes are not there," Mr Stiglitz said. "It's astounding."

This was the second time Mr Trump has taken the stage at the WEF meeting. Two years ago, he urged companies to invest in America after passing the first tax cuts to encourage business spending.

This year he thanked overseas companies for investing in the US, which he said was now on a better economic standing than he could have imagined when he took office three years ago.

"The time for scepticism is over," he said.

"To every business looking for a place to succeed — there is no better place than the US."

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He also told a packed auditorium that trade deals struck this month with China and Mexico represented a model for the 21st Century.

Mr Trump also took his biggest swipe yet at the Federal Reserve, whose policies he believes are holding back the US economy.

"The Fed raised rates too quickly and has lowered them too slowly," he said.

ABC/AP