President Donald Trump retweeted a video of a teenage activist who gave an impassioned speech at the United Nations urging world leaders to do more to combat climate change, saying she seemed to be a "happy young girl" with a "bright and wonderful future."

Swedish 16-year-old Greta Thunberg, fought back tears as she scolded the audience at the U.N. Climate Action Summit on Monday, repeatedly asking, "How dare you?"

"This is all wrong. I shouldn't be up here," said Thunberg, whose mission to fight climate change started when she sat alone with a homemade sign and some leaflets outside of Swedish parliament more than a year ago. That lone protest culminated in Friday's global climate strikes in which an estimated 4 million people worldwide took to the streets.

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"I should be back in school on the other side of the ocean. Yet you have come to us young people for hope. How dare you. You have stolen my dreams and my childhood with your empty words," Thunberg told the panel.

"We are in the beginning of a mass extinction and yet all you can talk about is money," Thunberg said. "You are failing us."

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Later, she and 15 other youth activists filed a formal complaint with an arm of the U.N. that protects children, saying that governments' lack of action on warming is violating their basic rights.

In a tweet late Monday, Trump says: "She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!"

Trump had spent only a few minutes at the climate summit, a centerpiece of this year's U.N. General Assembly schedule.Trump has previously called climate change a hoax. Speaking to reporters Monday, he said, "I'm a big believer in clean air and water."

Thunberg and Trump briefly crossed paths at the U.N. Monday. Cameras caught Thunberg looking at Trump from several feet away before Trump spoke to reporters about non-climate topics. It's not clear if Trump saw her.

World leaders agreed in 2009 to try to keep warming to just 2 degrees Celsius (3.6 degrees Fahrenheit) since pre-industrial times. Then in 2015 they added a secondary, tougher goal, at the urging of small islands, to keep warming to just 1.5 degrees Celsius (2.7 degrees Fahrenheit).

The U.N.'s World Meteorological Organization released a science report Sunday showing that in the last several years, warming, sea level rise and carbon pollution have all accelerated.

The new weather agency report showed that the world has warmed already by 1.1 degrees Celsius (2 degrees Fahrenheit). So that means the goals are to limit further warming to 0.9 degrees Celsius (1.6 degrees Fahrenheit) from now or even 0.4 degrees Celsius (0.72 degrees Fahrenheit) from now.

Efforts to reduce carbon pollution need to be tripled to keep from hitting the 2-degree Celsius mark and must increase fivefold to limit warming to 1.5 degrees Celsius since pre-industrial times, the World Meteorological Organization report said.