After Thunberg, a 16-year-old from Sweden, excoriated global leaders for their inaction and a tweet showing her glaring at Trump as he arrived at the UN headquarters went viral, Trump tweeted his own take on the speech.
On Tuesday, Thunberg changed the banner on her Twitter page to showcase Trump's description of her.
Climate change activists from around the world have praised Thunberg's remarks, which did not name Trump or any world leader by name. The U.S. president has faced criticism from activists and scientists for calling climate change a hoax.
”People are suffering, people are dying, entire ecosystems are collapsing," Thunberg said in her speech, a video of which has also gone viral. "We are in the beginning of a mass extinction and all you can talk about is money and fairytales of eternal economic growth.”
The president briefly attended the UN Climate Summit, although he arrived after Thunberg's remarks.
Later on Monday, conservative activists tried to dismiss Thunberg as naive and invoked her Asperger's syndrome, which Thunberg has described as a "superpower."
Michael Knowles of the conservative news site the Daily Wire, attacked Thunberg on Fox News, labeling her has a “mentally ill Swedish child.”
"The climate hysteria movement is not about science," Knowles said. "If it were about science, it would be led by scientists, rather than by politicians and a mentally ill Swedish child who is being exploited by her parents and by the international left."
Critics have denounced Knowles' comment and Fox News has apologized for the “disgraceful” comment, according to the Washington Post.
“The comment made by Michael Knowles who was a guest on The Story tonight was disgraceful — we apologize to Greta Thunberg and to our viewers,” a spokesperson for the network told The Washington Post in a statement.