President Donald Trump is addressing the United Nations General Assembly on Tuesday, but overnight he took to Twitter to mock one of the unexpected figures to command attention during the gathering of world leaders: 16-year-old climate activist Greta Thunberg.

“She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!” Trump wrote on Twitter around midnight ET. He linked to a video in which an emotional Thunberg, speaking on Monday at the UN Climate Summit, sounded the alarm to international leaders about the climate crisis, saying, “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.”

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

“For 30 years the science has been crystal clear,” Thunberg said in her speech. “How dare you continue to look away then come here, saying you are doing enough.”

Thunberg, who has Asperger’s Syndrome, met last week with Trump’s predecessor, Barack Obama. The former president said on Twitter after the meeting that Thunberg is “already one of our planet’s greatest advocates. Recognizing that her generation will bear the brunt of climate change, she’s unafraid to push for real action. She embodies our vision at the @ObamaFoundation: A future shaped by young leaders like her.”

Thunberg drew attention throughout the day on Monday on cable news outlets, as protesters calling for urgent action on climate change shut down major intersections and snarled traffic.

She also has triggered some polarizing reactions. Political commentator Michael Knowles, appearing on Fox News’ The Story with Martha MacCallum, said that Thunberg was “a mentally ill Swedish child who is being exploited by her parents and by the international left.”

That clip went viral, and Fox News later issued an apology for the guest. “The comment made by Michael Knowles who was a guest on ‘The Story’ tonight was disgraceful,” a spokesperson said. “We apologize to Greta Thunberg and to our viewers.” The network confirmed that they have no plans to book him again.

Knowles defended his comment in a series of tweets.

“Her mother wrote a book about her mental issues,” he wrote. “There is nothing shameful about living with mental disorders. What is shameful is exploiting a child – particularly a child with mental disorders – to advance your political agenda.”