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Despite a growing mountain of evidence that shows the world is careening towards a climate catastrophe, symbolized this month by the devastating bushfires in Australia, President Donald Trump thinks that “this is not a time of pessimism, this is a time for optimism.”

In his remarks at the World Economic Forum in Davos, Switzerland on Tuesday morning, Trump spent the vast majority of his speech reeling off a laundry list of economic indicators that showed just how strong the U.S. economy is. But he finished his address by attacking climate change activists and scientists who have been raising the alarm about the plight of the planet.

“Fear and doubt is not a good thought process,” Trump said. “Because this is a time for tremendous hope and joy and optimism in action. But to embrace the possibilities of tomorrow, we must reject the perennial prophets of doom, and their predictions of the apocalypse.”

Trump once again repeated the lie that the U.S. has “the cleanest air in the world” when, in fact, his administration has made the air quality in the U.S. worse. According to the Environmental Performance Index, a metric from environmental scientists at Yale and Columbia, the U.S. ranks 10th when it comes to clean air quality.

Trump also said that today’s climate scientists are simply repeating errors of the past when “foolish fortune tellers...predicted an overpopulation crisis in the 1960s, mass starvation in the 70s, and an end of oil in the 1990s.”

Climate change is a hot topic at Davos this year, and an hour before Trump took the stage, Greta Thunberg, the 17-year-old Swedish activist who the U.S. president has criticized for her outspoken opinions and “anger management issues,” told the world’s elite that they have done “basically nothing” to avert a climate catastrophe.

“Pretty much nothing has been done since the global emissions of CO2 has not reduced,” Thunberg said. “If you see it from that aspect, what has concretely been done, if you see it from a bigger perspective, basically nothing, it will require much more than this, this is just the very beginning.”

Trump labeled activists like Thunberg “alarmists” who are simply seeking “absolute power to dominate, transform and control every aspect of our lives.”

“We will never let radical socialists destroy our economy, wreck our country, or eradicate our liberty,” Trump added.

Thunberg said the world needed to pay attention to the findings of the 2018 Intergovernmental Panel on Climate Change report, which shows that in order to have the best chance at keeping the rise in global temperatures to under 1.5 degrees, countries must limit carbon emissions to a collective 420 gigatons.

She added that people often assume that “future generations will somehow suck hundreds of billions of tonnes of CO2 out of the atmosphere, even though such technology doesn’t exist yet.”

One of those people appears to be Trump, who made a cryptic reference to some unknown scientific breakthrough that would solve the current crisis.

“We're continuing to work on things that you'll be hearing about in the near future,” Trump said. “You'll be hearing about it but we have found the answers to things people said, would not be possible certainly not in a very short period of time.”