Hours after young environmental activist Greta Thunberg gave an emotional speech at the United Nations, President Donald Trump appeared to mock her on Twitter. Not to be outdone, Thunberg later quietly took a swipe of her own at him.

“She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!” Trump wrote late Monday, retweeting a video of the teen giving an impassioned plea at the United Nations Climate Action Summit.

Thunberg did not respond directly to Trump. But several hours later, she updated her Twitter bio to mimic his tweet.

"A very happy young girl looking forward to a bright and wonderful future," her bio read, an update from her previous description, which said "16 year old climate activist with Asperger's. Join the global climate strikes on Sept 20-27th! #FridaysForFuture."

In her speech, a tearful and sometimes trembling Thunberg accused world leaders of stealing her dreams and her childhood with their inaction on climate change — a cause she has championed and turned into a global movement through the Fridays for Future school strikes she launched.

"I shouldn't be up here. I should be back at school on the other side of the ocean," said the 16-year-old from Sweden. "Yet you all come to us young people for hope. How dare you! You have stolen my dreams and my childhood with your empty words."

"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 added. "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?"

After the speech, Thunberg’s name was trending worldwide on social media. The White House did not immediately respond to a request for comment from NBC News regarding Trump's comments.

Thunberg's appeal to the U.N. was widely praised. Among those who voiced support was 2020 Democratic presidential contender Sen. Kamala Harris, D-Calif., who wrote on Twitter that Thunberg “was right: we are currently failing our nation's youth by not taking swift action to combat the climate crisis.”

But it also became a flashpoint for some.

On Fox News on Monday night, The Daily Wire’s Michael Knowles called Thunberg — who has Asperger's syndrome, a diagnosis she has embraced as her "superpower" — mentally ill.

"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," he said. (The network later issued an apology to Thunberg, calling Knowles' comment "disgraceful.")

And Sebastian Gorka, former deputy assistant to Trump, compared Thunberg to a "victim of a Maoist 're-education' camp," tweeting that "the adults who brainwashed" her should be charged with child abuse.

Trump's stance on climate change, which he has referred to as a hoax, has drawn ire from many in the international community. In 2017, he announced the U.S. would pull out of the Paris agreement, a landmark global accord to curb emissions. Throughout his administration, he has rolled back and reversed a series of environmental protections.

Thunberg has not held back in the past about her contempt for Trump. Last month, she told The New York Times, "My message to him is just to listen to the science, and he obviously doesn’t do that."

A GIF of Thunberg glaring at Trump as he passed by her at the summit on Monday in a brief surprise appearance got almost as much attention online as her speech did.

Greta Thunberg’s glare at Donald Trump is giving me the energy to get through this Monday pic.twitter.com/xdM0rdI2ln — Sam Stryker (@sbstryker) September 23, 2019

Thunberg initially began her climate protest on her own, sitting outside Swedish Parliament in August 2018 following Sweden's hottest summer on record. She is a nominee for the Nobel peace prize, which is awarded in October.

Thunberg arrived in the United States late last month. Due to high levels of emissions from air travel, she abstains from flying, and instead traveled across the Atlantic Ocean by sailboat to attend the climate summit.