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US President Donald Trump has decried climate "prophets of doom" in a speech at the World Economic Forum in Davos, where sustainability is the main theme.
He called for a rejection of "predictions of the apocalypse" and said America would defend its economy.
Mr Trump did not directly name the climate activist Greta Thunberg, who was in the audience.
Later, she excoriated political leaders, saying the world "in case you hadn't noticed, is currently on fire".
Environmental destruction is at the top of the agenda at the annual summit of the world's decision-makers, which takes place at a Swiss ski resort.
In his keynote speech, Mr Trump said that it was a time for optimism, not pessimism in a speech that touted his administration's economic achievements and America's energy boom.
Speaking of climate activists, he said: "These alarmists always demand the same thing – absolute power to dominate, transform and control every aspect of our lives."
They were, he said, "the heirs of yesterday's foolish fortune tellers".
He was speaking hours before his impeachment trial gets under way in the US Senate.
Soon after he spoke, Ms Thunberg, the 17-year-old Swedish climate activist who has led a global movement of school strikes calling for urgent environmental action, opened a session on "Averting a Climate Apocalypse".
"I've been warned that telling people to panic about the climate crisis is a very dangerous thing to do," she said, without naming Mr Trump or any other leader. "But trust me it's fine."
She excoriated politicians and business leaders for what she said were continuous "empty words and promises".
"You say, 'We won't let you down. Don't be so pessimistic.' And then, silence."
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Although the world's nations, through the Paris climate deal, have committed to keeping temperatures well below 2.0C (3.6F) above pre-industrial times and endeavoured to keep the rise to 1.5C, scientists say we are well off that target.
Last year, the Intergovernmental Panel on Climate Change said the Earth was heading instead for 3C and that "rapid, far-reaching and unprecedented changes in all aspects of society" would be needed to keep to the preferred target of 1.5C.
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