DAVOS (NYTIMES) - Neither uttered the other's name. But United States President Donald Trump and Swedish climate campaigner Greta Thunberg took unmistakable aim at each other on Tuesday (Jan 21) at the 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 Davos 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 criticised 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," he declared, adding, "fear and doubt is not a good thought process."

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

An hour later, 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," Thunberg said. "You say, 'Just leave this to us. We will fix this. We promise we won't let you down.'"

Then, a line that did not appear in her prepared remarks: "Don't be so pessimistic."

The last time the two encountered each other, at the United Nations in September, she glared as she watched him pass before her in the General Assembly building.

Photos and a video of her expression spread widely, only to be followed by what was widely seen as a sarcastic Twitter message from the president.

"She seems like a very happy young girl looking forward to a bright and wonderful future," he wrote. "So nice to see!"

Thunberg struck back immediately with her nearly two million Twitter followers, briefly changing her bio: "A very happy young girl looking forward to a bright and wonderful future," she wrote.

There was nothing so direct on Tuesday when Mr Trump and Thunberg spoke to separate audiences.

Mr Trump celebrated his deregulatory agenda, which he said had unshackled the American economy, allowing the US to build profitable new energy businesses and wean itself from energy dependence on what he labelled as hostile countries.

His impact on environmental protections has been wide-ranging. He has withdrawn from the Paris climate accord, rolled back a wide range of emissions regulations and empowered a bureaucracy that has sought to undermine the science of climate change.

The US is the largest emitter of greenhouse gases in history.

The closest Mr Trump's speech at Davos came to environmental issues was his claim that the US had the cleanest air and drinking water on Earth. In fact, the Trump administration has pushed through a plan to weaken clean-water regulations. And air pollution in the US has worsened since 2016, reversing decades of improvements, according to researchers at Carnegie Mellon University.

Mr Trump dismissed the activists as advocating a form of "radical socialism" that Americans would reject. While he got only perfunctory applause, his message resonated with some in the business-heavy audience.

Critics pointed to a contradiction that they said the corporate world had been unable to resolve: how to assuage the appetite for economic growth, based on gross domestic product, with the urgent need to check carbon emissions.

"It's truly a contradiction," said Mr Johan Rockstrom, director of the Potsdam Institute for Climate Impact Research. "It's difficult to see if the current GDP-based model of economic growth can go hand-in-hand with rapid cutting of emissions."

Thunberg, for her part, largely repeated the warning she issued at the United Nations last year and for which she drew widespread global attention.

Earlier in the day, Thunberg, speaking on another panel, brushed off criticism, saying it was more important to pay attention to the findings of the Intergovernmental Panel on Climate Change, which has warned how dangerously close the world is to overspending its carbon budget and triggering irreversible climate effects.

Though Mr Trump barely mentioned climate change, he did commit the US to a World Economic Forum initiative to plant a trillion new trees as a way to mitigate carbon dioxide emissions. The president pledged that the US would work to manage and preserve its forests.

On this, Thunberg withheld her praise, warning that such initiatives were often an excuse for inaction.

"We are not telling you to offset your emissions by just paying someone else to plant trees in places like Africa, while at the same time forests like the Amazon are being slaughtered at an infinitely higher rate," she said.

"Planting trees is good, of course," Thunberg said. "But it's nowhere near enough of what is needed and it cannot replace real mitigation or rewilding nature."