A Fox News guest called 16-year-old climate activist Greta Thunberg "mentally ill" during a segment Monday night, prompting the network to issue an apology. Conservative commentator Michael J. Knowles brought up the Swedish activist on "The Story with Martha MacCallum" after the teen gave an impassioned speech about the dangers of climate change at the United Nations.

In the segment with Fox News host Harris Faulkner and progressive pundit Christopher Hahn, Knowles criticized climate activists before targeting Thunberg in particular.

"None of that matters because 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."

Get Breaking News Delivered to Your Inbox

Hahn interrupted Knowles and said, "Shame on you." Faulkner, who was filling in for MacCallum, let Hahn continue. "You're a grown man and you're attacking a child," Hahn said. "Shame on you."

Fox News later apologized in a statement released to multiple news outlets. "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," the statement said.

The Hollywood Reporter said the network has "no plans" to book Knowles for future shows. CBS News has reached out to Fox News for comment.

Knowles on the other hand, he continued to defend his comments and explain himself on Twitter. "Her mother wrote a book about her mental issues," he wrote. "There is nothing shameful about living with mental disorders."

Thunberg has spoken out about having Asperger's syndrome, a developmental disorder characterized by difficulties in social interaction and nonverbal communication. She, however, sees her condition as an advantage in many ways.

"Especially in a big crisis like this one, we need to think outside the box, we need to think outside our current system, we need people who think outside the box and who aren't like everyone else," Thunberg told "CBS This Morning" earlier this month.

On Monday, she scolded world leaders at the United Nations Climate Summit for not doing enough.

"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 as she fought back tears. "How dare you!"

Her powerful speech even caught the attention of President Trump, who appeared the mock the teen on Twitter, where he wrote: "She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!"

Thunberg trolled him back with subtle change in her Twitter bio on Tuesday. On the site, Thunberg now describes herself as: "A very happy young girl looking forward to a bright and wonderful future."