Despite being just 16 years old, climate activist Greta Thunberg was not afraid to address a group of the world's most powerful people, angrily warning them that they're not doing their jobs when it comes to the climate crisis. Her speech during the United Nations Climate Action Summit gained widespread attention — even from President Trump, who skipped the speech but later appeared to mock the teen on Twitter.

At the U.N. on Monday, Thunberg scolded world leaders for caring more about money and economic growth than about the world's collapsing ecosystems. "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."

Thunberg told the leaders, "You have stolen my dreams," and she fought back tears as she said, "People are suffering! People are dying!"

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President Trump retweeted a video clip of her remarks with what appears to be a sarcastic comment: "She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!"

She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see! https://t.co/1tQG6QcVKO — Donald J. Trump (@realDonaldTrump) September 24, 2019

Thunberg certainly did not appear happy during her speech. In fact, her serious tone verged on fury at times, while at other moments tears welled up in her eyes. She also did not seem to be looking toward a bright future. Instead, she explicitly warned the leaders that the odds of success were not in the world's favor.

But if President Trump's "happy young girl" comment seemed dismissive to some, Thunberg stuck to her guns. She even appeared to troll him right back by adopting the president's comment into her own Twitter bio, which she updated to read: "@GretaThunberg A very happy young girl looking forward to a bright and wonderful future."

Greta Thunberg changed her Twitter bio to read: "A very happy young girl looking forward to a bright and wonderful future." Twitter/Greta Thunberg

Mr. Trump himself only attended the climate summit for about 10 minutes, dropping in unexpectedly after the White House press secretary said he probably wouldn't have time. He and Thunberg did cross paths at the U.N., and in a brief moment caught on camera, Thunberg is seen staring intently at Mr. Trump has he talks to reporters. They did not appear to interact.

Swedish environmental activist Greta Thunberg watches as President Trump enters the United Nations to speak with reporters in a still image from video taken in New York City, on Monday, September 23, 2019. Reuters

This is not the first time Mr. Trump has skipped international climate discussions. He declined to appear at annual G-7 climate sessions in France this year and in Quebec in 2018. He has also undone many of President Obama's environmental initiatives, withdrawing the U.S. from the Paris climate agreement and rolling back emissions regulations like the Clean Power Plan.