The president shared a video of Thunberg delivering a blunt and emotional speech alongside other youth environmental activists who castigated officials for not doing enough to prevent a catastrophic heating of the planet.
"People are suffering. People are dying. Entire ecosystems are collapsing. We are in the beginning of a mass extinction and all you can talk about is money and fairy tales of eternal economic growth,"
Thunberg said as tears filled her eyes and her voice shook with frustration. "How dare you."
"She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!" Trump tweeted sarcastically in response.
Thunberg, whose appearance at the summit came after a worldwide strike largely inspired by her activism, has become a high-profile figure in the youth-led fight against climate change since she protested Swedish parliament over a lack of climate action and crossed the Atlantic to the U.S. on a zero-emissions sailboat to attend the U.N. meeting.
The tweet followed Trump's brief and unexpected 15-minute appearance Monday at the daylong climate summit, where leaders from more than 60 countries presented plans to step up emissions reductions. He did not speak but listened to remarks from German Chancellor Angela Merkel and Indian Prime Minister Narendra Modi. Summit organizers had hoped the summit would help build momentum to pressure national leaders indifferent to climate change, including those from the United States, Brazil and Australia — world powers who were noticeably absent from the summit stage.
Thunberg's speech comes amid a weeklong youth-led campaign surrounding the U.N. summit in New York that will be capped by another global strike on Friday.
"You all come to us young people for hope. How dare you," Thunberg said in her speech Monday, "You have stolen my dreams and my childhood with your empty words."
She added: "If you choose to fail us, I say we will never forgive you. We will not let you get away with this."