Davos, Switzerland CNN  — 

President Donald Trump attacked climate activists as “perennial prophets of doom” on Tuesday while addressing the World Economic Forum, in Davos, Switzerland, where the agenda is focused on tackling the climate crisis.

Trump’s remarks underscored the chasm between his denialist view of climate change and the overwhelming scientific consensus driving the rest of the developed world to action. Speaking shortly after the teenage climate activist Greta Thunberg accused world leaders of not taking action, Trump rejected calls for urgent action and encouraged the world to instead embrace “optimism.”

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

“They are the errors of yesterday’s fortune tellers and we have them and I have them and they want to see us do badly, but we don’t let that happen,” he added.

Trump later reiterated his previous criticism of Thunberg as “angry” in an interview with the Wall Street Journal.

“I don’t really know anything about her,” the President told the paper, later adding that teen climate activist is “very angry.”

The Trump administration has rolled back key environmental regulations and the President withdrew from the Paris climate accord, a landmark effort to reduce global greenhouse gas emissions.

Speaking on Tuesday as the US Senate is set to begin the impeachment trial back in Washington, Trump touted the US’s newly-minted position as the No. 1 producer of oil and natural gas – a boast that flew in the face of sustainability efforts underway at Davos, where the theme of the conference was focused on “a cohesive and sustainable world.”

While Trump rejected the science behind climate change and its irreversible effects, he insisted that he wants “clean” air and water, and he announced that the US would be joining the “One Trillion Trees” initiative, a program with the goal of restoring a trillion trees by 2050.

Trump said that in joining the initiative, America “will continue to show strong leadership and restoring, growing and better managing our trees and our forests.”

“This is not a time for pessimism. This is a time for optimism. Fear and doubt is not a good thought process because this is a time for tremendous hope and joy and optimism and action,” he said.

Trump, who missed the World Economic Forum’s annual meeting last year due to the government shutdown, is expected to sit down with business leaders and a slate of foreign counterparts, including the Iraqi and Swiss presidents later on Tuesday.

But impeachment remained on his mind as the President arrived at the economic summit.

When asked about the impeachment trial back home, Trump paused briefly to repeat his months-long defense that “it’s just a hoax.”

“It’s a witch hunt that’s been going on for years, and frankly, it’s disgraceful,” he added.

In his speech to the forum, the President focused on what he says his administration has done for the American middle class.

“America is winning again like never before,” the President told the forum. He said before he took office, the “shrinking middle class … felt neglected, betrayed, forgotten.” He touted his trade deals, tax cuts, and slashes to regulations.

He arrived at the gathering with a strong US economy at his back, one that has bolstered the global economy amid economic uncertainty in Europe and other parts of the world and helped enrich many of the billionaires and other businessmen attending the summit.

This story has been updated with additional details from the forum.