Climate activist Greta Thunberg spoke at the United Nations Climate Action Summit on Monday, where she scolded world leaders for failing to address climate change. The 16-year-old has become one of the leading voices for a generation confronting the consequences of a warmer planet.

"People are suffering. People are dying and dying ecosystems are collapsing. We are in the beginning of a mass extinction, and all you can talk about is the money and fairy tales of eternal economic growth," she said Monday, as she fought back tears. "How dare you! For more than 30 years the science has been crystal clear. How dare you continue to look away and come here saying that you're doing enough when the politics and solutions needed are still nowhere in sight."

Late Monday night, President Trump took to Twitter, citing the first half of that statement then saying Thunberg "seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!"

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Thunberg recently celebrated the one-year anniversary of the start of her climate change movement. Last August, she began striking by herself outside the Swedish parliament, and soon, students around the world began walking out of school, demanding action from their governments. She's been called "the voice of the planet," and has even been nominated for the Nobel Peace Prize.

Greta Thunberg at U.N.: "You are still not mature enough to tell it like it is. You are failing us. Young people are starting to understand your betrayal. The eyes of all future generations are upon you." https://t.co/5ZPcsQ1Bbr pic.twitter.com/58gwYMqBQV — CBS News (@CBSNews) September 23, 2019

"This is all wrong. I shouldn't be up here," said Thunberg, whose lone protest culminated in Friday's global climate strikes. "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."

She told the U.N. that even the strictest emission cuts being talked about only gives the world a 50% chance of limiting future warming to another 0.4 degrees Celsius (0.72 degrees Fahrenheit) from now, which is a global goal. Those odds are not good enough, she said.

"We will not let you get away with this," Thunberg said. "Right now is where we draw the line."

Environmentalists have grown increasingly alarmed over warming trends that have exceeded scientists' models. For instance, a climate study in January showed the world's oceans are warming significantly faster than previously thought. Since 1970, the ocean has warmed 40% more than previous estimates.

The U.N. noted on its website that global emissions "are reaching record levels and show no sign of peaking," and that the last four years were the the hottest recorded." And that change, the U.N. said, is beginning to have a "life-threatening impact" as it brings more air pollution, heatwaves and greater risk to food security.

Environmental activist Greta Thunberg, of Sweden, addresses the Climate Action Summit in the United Nations General Assembly, at U.N. headquarters, Monday, Sept. 23, 2019. AP

An emotional Thunberg sat in front of the General Assembly and plainly told those in attendance, "You have stolen my dreams."

"How dare you. You have stolen my dreams and my childhood with your empty words," she said. "You say you hear us and that you understand the urgency. But no matter how sad and angry I am, I do not want to believe that, because if you really understood the situation and still kept on failing to act, then you would be evil and that I refuse to believe."