President Trump said in an interview with the Wall Street Journal today that he doesn’t “really know anything” about Greta Thunberg, and added she’s “very angry.” Thunberg, in her speech at the World Economic Forum, warned that “our house is still on fire,” and told world leaders that their “inaction is fueling the flames by the hour”.

Neither uttered the other’s name. But President Trump and the Swedish climate campaigner Greta Thunberg took unmistakable aim at each other on Tuesday at this conference of business and government figures, reprising their roles as antagonists on the global stage.

The 73-year-old president and 17-year-old activist dominated the first full day of the gathering, painting starkly different visions of the future, and staking out opposite poles on the signature theme of this year’s forum: how best to manage a world of increasing temperatures, rising seas, and catastrophic wildfires.

Mr. Trump implicitly criticized Ms. Thunberg and other activists, saying they peddled warnings of doom at a time when his policies had ushered in a bright new era of economic prosperity for Americans.

“They are the heirs of yesterday’s foolish fortune tellers,” the president said. “They predicted an overpopulation crisis in the 1960s, a mass starvation in the 70s, and an end of oil in the 1990s.”

“This is not a time for pessimism,” Mr. Trump declared, adding, “Fear and doubt is not a good thought process.”

Ms. Thunberg listened, sitting with three other climate activists in the sixth row.

An hour later, Ms. Thunberg, addressing another Davos audience, rebuked leaders for failing to fix a problem of their own making. She said they had ignored pleas for the world to act on climate change. And she flatly rejected Mr. Trump’s assertion that there was much to be optimistic about.

“You say children shouldn’t worry,” Ms. Thunberg said. “You say, ‘Just leave this to us. We will fix this. We promise we won’t let you down.’”