Michael Knowles had just finished discussing why he believes meatless diets may be worse for the environment during a Monday Fox News segment about climate change when he suddenly pivoted to one of the day's biggest stories.

"None of that matters because the climate hysteria movement is not about science," the conservative pundit and Daily Wire podcast host 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."

Knowles, much to the dismay of many viewers and his fellow Fox News guest, was referring to teenage climate activist Greta Thunberg, who just hours earlier made headlines for delivering an impassioned message to global leaders at the United Nations. The 16-year-old has been open about being diagnosed with Asperger's syndrome, embracing it as her "superpower."

Amid the intense criticism over Knowles' remarks Monday night after a clip of the show went viral, Fox News apologized to Thunberg.

"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.

Fox News also said it has "no plans" to book Knowles.

Knowles did not respond to a request for comment late Monday but defended himself on Twitter, writing: "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."

Thunberg has called the condition a "gift" and credited it with sparking her activism.

"Some people mock me for my diagnosis. But Asperger is not a disease, it's a gift. People also say that since I have Asperger I couldn't possibly have put myself in this position. But that's exactly why I did this," she wrote on Facebook in February. "Because if I would have been 'normal' and social I would have organized myself in an organisation, or started an organisation by myself. But since I am not that good at socializing I did this instead. I was so frustrated that nothing was being done about the climate crisis and I felt like I had to do something, anything."

The network's apology comes as conservatives have ramped up their broadsides against Thunberg, who traveled to the United States earlier this month on a solar-powered sailboat and has since been chastising powerful politicians about their inaction on climate change. In a widely reviled tweet over the weekend, conservative commentator Dinesh D'Souza compared Thunberg to young people featured in Nazi propaganda, sharing a photo of the Swedish teen alongside an illustration of a Caucasian girl sporting braided hair and holding a Nazi flag. On Monday, Sebastian Gorka, the former Breitbart editor and White House aide, labeled Thunberg as an "autist child."

Even President Donald Trump weighed in late Monday with a tweet that many took as sarcastic.

"She seems like a very happy young girl looking forward to a bright and wonderful future," Trump tweeted shortly after 11:30 p.m., sharing a video of Thunberg delivering her fiery address at the U.N. climate summit. "So nice to see!"

Knowles made his statements about Thunberg while appearing on the "The Story with Martha MacCallum" and his description of the teenager instantly sparked a rebuke from liberal radio host Christopher Hahn, who had also been invited on as a guest.

"How dare you?" Hahn could be heard interjecting.

When Fox News anchor Harris Faulkner, who was filling in for MacCallum, gave Hahn the floor, he tore into Knowles.

"You're a grown man and you're attacking a child," Hahn said. "Shame on you."

The pair then began talking over each other as Knowles insisted he wasn't going after Thunberg but rather "attacking the left for exploiting a mentally ill child."

"Now, relax, skinny boy," Hahn barked, eliciting a noise of surprise from Faulkner and a chuckle from Knowles. "Have some couth when you're on television. Maybe on your podcast you get away and say whatever you want because nobody's listening. You're on national television. Be a grownup when you're talking about children."

Hahn later added: "You are despicable for talking about her like that. You should apologize on national television right now."

Meanwhile, Knowles stuck to his talking point that Thunberg is "mentally ill," pointing to a family memoir that recounted details from the teenager's childhood.

"She has autism," he said. "She has obsessive compulsive disorder. She has selective mutism. She had depression. Her mother wrote about this in a book."

Ignoring Hahn's barbs, Knowles told the radio host, "You can inveigh all you want, but unfortunately, you don't have many facts on your side."

The heated exchange, which lasted more than two minutes, quickly spread on social media Monday night. By early Tuesday, one video of the short segment had amassed more than 1.6 million views.

Critics denounced Knowles' comments, which also drew swift backlash from autism advocacy organizations.

"To intentionally demean any person, regardless of neuro-diversity, is both cruel and wrong," the Autism Society said in a statement to the Hollywood Reporter. "Certainly some might disagree with Ms. Thunberg on policy issues, but it is shameful to issue a derogatory statement to a youth advocate."

Julia Bascom, executive director of the Autistic Self Advocacy Network, told THR that it is "absolutely unconscionable to attack someone for their disability, especially when that person is a child."

"History is full of autistic people and people with other cognitive disabilities who were and are compelling, credible activists and leaders," Bascom said. "Greta is a part of that tradition, and our community is lucky to have her. Period."

Others supported Knowles, including fellow conservative pundit Ann Coulter.

"SPECTACULAR!!!!" tweeted Coulter about Monday's segment.

Despite Fox News's apology, the segment featuring Knowles and Hahn wasn't the last time Monday night that Thunberg was mentioned on the network.

Hours later on her show, Fox News host Laura Ingraham played a clip of Thunberg speaking at the climate summit and compared the teen to characters in the 1984 horror film "Children of the Corn," based on a novel by Stephen King.

"I can't wait for Stephen King's sequel, 'Children of the Climate,' " Ingraham said.