President Trump called Greta Thunberg 'very angry' in a new interview, after saying climate activists were out-of-touch 'alarmists' and 'heirs of yesterday's fortune tellers' during his appearance Tuesday morning at the World Economic Forum.

Thunberg had been seated in the audience looking on, but Trump told the Wall Street Journal afterward that, 'I don't really know anything about her,' before calling her angry.

Portions of Trump's remarks seemed to be aimed at Thunberg and her supporters.

'This is not a time for pessimism. This is a time for optimism,' Trump said, which was in stark constrast to Thunberg and others, who used the conference to warn that the Australian wildfires and other disasters were only the beginning of the climate chaos that lie ahead.

To 'embrace the possibilities of tomorrow,' Trump said, 'we must reject the perennial prophets of doom and their predictions of the apocalypse.'

'They are the heirs of yesterday's fortune tellers,' Trump stated. 'They predicted an over-population crisis in the 1960s, mass starvation in the 70s, and an end of oil in the 1990s.'

'These alarmists always demand the same thing - absolute power to dominate, transform and control every aspect of our lives,' Trump continued. 'We will never let radical socialists destroy our economy, wreck our country or eradicate our liberty.'

The bulk of Trump's speech was touting the American economy under his leadership, a curtain-raiser as he works more seriously toward his re-election bid.

Tuesday's speech was also counter-programming with the Senate beginning its impeachment trial of Trump. House Democrats will start making their case as Trump dines with world leaders.

President Trump used his appearance at the World Economic Forum to tout the economic gains the U.S. has seen during his administration

President Trump sits onstage at the World Economic Forum in Davos, Switzerland Tuesday as WEF head Klaus Schwab introduces the president

President Trump gives a wave as he arrives on Air Force One in Zurich, Switzerland. The president will then head to Davos, Switzerland to give a speech and hold meetings at the World Economic Forum's 50th anniversary conference

President Trump is due to speak mid-morning Tuesday on the main stage at the World Economic Forum. He appeared at the conference in 2018, a year into his presidency, but skipped in last year due to the ongoing government shutdown

President Trump heads to Marine One, which will ferry him to the remote town of Davos, Switzerland, where the World Economic Forum takes place every year. His trip Tuesday and Wednesday will mark the second time as president he's attended

As President Trump was arriving, teen climate activist Greta Thunberg was making her first of two appearances Tuesday at the World Economic Forum

Greta Thunberg read climate statistics during her early morning appearance at the World Economic Forum

Greta Thunberg (right) has been mocked by President Trump on Twitter. She'll bookend his Davos appearance Tuesday, making two appearances

At a second appearance in Davos, Greta Thunberg said that while planting trees 'is good, of course' it's 'nowhere good enough to what is needed'

'He has a full day here in Davos, but will be briefed by staff periodically,' White House press secretary Stephanie Grisham said in a statement.

But thanks to the time difference, the bulk of the Senate proceedings will happen after Trump returns to his hotel, a ripe environment for live-tweeting.

Trump's day at Davos started with his speech.

'America's thriving, America's flourishing, and yes America is winning again like never before,' Trump told the crowd.

His appearance began and ended with polite applause.

He boasted of the 'blue collar boom' he had brought on, while touting the two trade deals he signed last week, comparing his record to the 'rather dismal state' of the economy under his predecessor, President Obama.

And while he snarked at climate activists, he said upon entering the forum that he was a 'very big believer in the environment,' and did announce that the U.S. would join the 'One Trillion Trees' initiative taking root at the conference.

The effort aims to do exactly what it says - plant a trillion trees by 2050. On Wednesday in Davos, environmentalists including the famed Jane Goodall will discuss the program at greater length.

Trump's arrival in Switzerland early Tuesday morning created a splitscreen moment as Thunberg was making her first of two appearances of the day.

President Trump (left) met with Swiss President Simonetta Sommaruga (right) Tuesday afternoon. Earlier in the day, Sommaruga had taken the stage to warn the world about climate change, while Trump, minutes later, criticized climate 'alarmists'

'We are all fighting for the environment and climate. If you see it from a bigger perspective, basically nothing has been done. It will require much more than this. This is just the very beginning,' Thunberg said during an early morning panel discussion. 'I'm not a person that can complain about not being heard,' Thunberg also said at the event, cueing laughs.

Later, Thunberg gave a more traditional address.

'One year ago, I said I wanted you to panic,' she said. 'I've been warned that telling people to panic about the climate crisis is a very dangerous thing to do.'

THE US DELEGATION AT DAVOS President Donald J. Trump Treasure Secretary Steven Mnuchin Commerce Secretary Wilbur Ross Labor Secretary Eugene Scalia Transportation Secretary Elaine Chao U.S. Trade Representative Robert Lighthizer Under Secretary for Growth, Energy and the Environment Keith Krach Assistant to the President Ivanka Trump Assistant to the President Jared Kushner Assistant to the President Christopher Liddell Advertisement

'Don't worry, it's fine, trust me, I've done this before and it doesn't lead to anything,' again blasting leaders for not enough action.

In her remarks, Thunberg also noted that while 'planting trees is good, of course' it's 'nowhere good enough to what is needed.'

Her remarks were in line with what World Economic Forum Founder and Executive Chairman Klaus Schwab said at the beginning of Tuesday's programming, as he and Swiss President Simonetta Sommaruga took the stage before Trump.

'The world is in a state of emergency and to not act is crazy,' Schwab said. 'The world is on fire,' is how Sommaruga greeted participants.

Trump has done the opposite of making climate change a priority during his three-year presidency.

Early on, he pulled the U.S. out of the Paris climate agreement, while he's deregulated domestic oil and gas. In the past, he's called climate change a Chinese 'hoax,' though he's also touted to supporters that he's an environmentalist because he cares about clean air and clean water.

Over the past few months, Trump has made Thunberg a target on Twitter.

Last month, Trump reacted to the teenager being named Time magazine's 2019 'Person of the Year' by mocking her serious demeanor.

Trump shared a congratulatory tweet about Thunberg's win from actress Roma Downey.

'So ridiculous,' the president responded. 'Greta must work on her Anger Management problem, then go to a good old fashioned movie with a friend! Chill Greta, Chill!'

Thunberg responded by briefly changing her Twitter bio to read: 'A teenager working on her anger management problem. Currently chilling and watching a good old fashioned movie with a friend.'

Trump had also weighed in September after Thunberg made a passionate plea to world leaders during the United Nations General Assembly to do something about the warming earth.

Greta Thunberg's (center) expression when she saw President Trump (left) enter the United Nations in September went viral and became a meme

President Trump took to Twitter last month to call it 'ridiculous' that the climate activist got the honor. Trump was Time's person of the year in 2016 after his shock win of the presidential election

Greta Thunberg replied by briefly changing her Twitter bio to mimic the words President Trump had used calling herself 'A teenager working on her anger management problem'

President Trump had also tweeted about Greta Thunberg in September when he mocked her for being hysterical during her address to the United Nations General Assembly

'I shouldn't be up here,' Thunberg said at the time. 'I should be back in school on the other side of the ocean. Yet you all come to us young people for hope? How dare you!' she exclaimed.

Trump then tweeted a video of Thunberg's speech. 'People are suffering, people are dying, entire ecosystems are collapsing. We are in the beginning of a mass extinction and all you can talk about is money and fairytales of eternal economic growth,' Thunberg had said.

'She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!' Trump said sarcastically.

Thunberg changed her Twitter bio then as well, calling herself 'a very happy young girl looking forward to a bright and wonderful future.'

However Thunberg's expression when she saw Trump entering the U.N. the day she gave her speech was not-so-happy.

The stink eye Thunberg gave the American president went viral online.

Trump has gotten a mixed reaction on the world stage, including at Davos.

His criticism of the media during his 2018 appearance elicited a mix of boos and groans from the audience.

Several months later, during an appearance at the United Nations General Assembly - the gathering where he ran into Thunberg a year later - members of the audience, which included heads of state, laughed out loud at Trump's bombast.

Preparations are being made in Davos, Switzerland for the World Economic Forum's 50th year. President Trump is due at the conference on Tuesday and Wednesday

Thousands of people - including heads of state, business leaders and celebrities - descend on Davos, Switzerland every year for the World Economic Forum. President Trump will make his second appearance as president on Tuesday

Climate activists march in Switzerland in advance of the World Economic Forum set to kick off Tuesday in Davos, Switzerland

'In less than two years, my administration has accomplished more than any administration in the history of out country,' Trump touted to laughter.

'So true,' he added. 'Didn't expect that reaction, but that's OK.'

And then there was the incident at the NATO summit where Canadian Prime Minister Justin Trudeau was caught on hot mic discussing Trump's behavior at the summit.

Trudeau was describing to U.K. Prime Minister Boris Johnson, French President Emmanual Macron and Dutch Prime Minister Mark Rutte how Trump essentially holds a press conference before his bilateral meetings with world leaders. The group was seen having a laugh.

Trump called the Canadian PM's behavior 'two-faced.' Trudeau fessed up to it.

'Last night I made reference to the fact that there was an unscheduled press conference before my meeting with President Trump. I was happy to be part of it but it was certainly notable,' Trudea said in a press conference the next day.

The moment inspired a 'Saturday Night Live' skit that featured actor Paul Rudd as Macron and Jimmy Fallon as Trudeau, who bully Trump - played by Alec Baldwin - in the U.N. cafeteria - and ask Kate McKinnon's Angela Merkel to sit with them instead.

The Trumps had some hiccups on the world stage in 2019, including when the French government showed this clip of Ivanka Trump being snubbed by world leaders including French President Emmanual Macron

Another incident, from the NATO summit in December, inspired this 'Saturday Night Live' skit, in which Jimmy Fallon as Justin Trudeau, Paul Rudd as Emmanual Macron and James Corbin as Boris Johnson pick on Alec Baldwin's Donald Trump in the U.N. cafeteria

The SNL skit concludes with the leaders taping an 'Impeach Me!!!' sign to Alec Baldwin's back. Trump was impeached later that month and his Senate trial begins Tuesday, the same day he'll give a main stage address at Davos

The incident also called back to the video shared by the Frech government that showed Ivanka Trump being snubbed by leaders including Macron, Christine Lagarde, who now serves as the president of the European Central Bank, and then-U.K. Prime Minister Theresa May.

WHO IS TRUMP MEETING WITH AT DAVOS Dr. Barham Salih, President of the Republic of Iraq Imran Khan of Pakistan, Prime Minister of the Islamic Republic of Pakistan Simonetta Sommaruga, President of the Swiss Confederation Ursula von der Leyen, President of the European Commission Nechirvan Barzani, President of the Kurdistan Regional Government World Economic Forum Founder and Executive Chairman Klaus Schwab Advertisement

As the first daughter tries to engage in the discussion, talking excitedly with her hands, she's largely ignored, the video clip from the July G-20 showed.

Both Trumps are heading to Davos.

Merkel and Lagarde will also be on hand.

Ivanka Trump is a member of the seven-person delegation announced to represent the U.S. at Davos before even the president committed to attend.

She tweeted about her participation Monday.

'Headed to Davos to call on the world’s largest employers to sign our #PledgetoAmericasWorkers and join us in unleashing the potential of our people and accelerating the historic wave of opportunity, wage growth and job creation in the United States,' Ivanka Trump wrote.

The president can use the opportunity to steer heads away from the Senate impeachment trial, which will kick off in the hours following his Davos speech.

Trump is also due to host a dinner and will participate in several bilateral meetings with leaders during his two-day stay in Switzerland.

He'll have the benefit of not having an awkward run-in with Iranian Foreign Minister Mohammad Javad Zarif who dropped out Monday.

The Iranians are blaming organizers who they claim 'abruptly changed its agenda' for the summit.

Trump has taken a victory lap for the American drone strike in Iraq that killed Iranian general Qassem Soleimani, a move that brought the countries to the brink of war.

And while Ukrainian President Volodymyr Zelensky is attending the conference, he wasn't on the list of leaders Trump planned to meet.

At the summit Trump will, however, be meeting with Iraqi President Barham Salih and Nechirvan Barzani, the president of the Kurdistan Regional Government - two important meetings that could impact America's future in the Middle East.

The White House also announced Monday that Trump would sit down with Pakistani Prime Minister Imran Khan, Sommaruga, European Commission President Ursula von der Leyen and Schwab.