Teenage climate change activist Greta Thunberg responded to US President Donald Trump's mocking of her online by embracing his words on her Twitter page Tuesday.

Trump stirred up fresh outrage on social media late Monday with a tweet that sneered at an impassioned speech made by the 16-year-old at the UN climate summit in New York.

Her voice shaking with emotion in an address that was the defining moment of the summit, Thunberg accused world leaders of betraying her generation by failing to act on rising emissions, repeating the words "how dare you" four times.

"You have stolen my dreams and my childhood with your empty words, and yet I'm one of the lucky ones," she said. "People are suffering. People are dying." "She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!" climate skeptic Trump tweeted a few hours later, alongside a clip of the speech.

The trolling of the activist -- who has been diagnosed with Asperger syndrome, a mild form of autism -- racked up more than 16,000 responses in three hours, many of them attacking the president.

"DonaldTrump picking on an innocent young girl is absolutely disgusting!!" wrote one user.

Thunberg later changed her Twitter biography to read: "A very happy young girl looking forward to a bright and wonderful future." "When your troll president gets out-trolled," tweeted American businessman and columnist Jeff Yang, alongside an image of Thunberg's new bio.

Thunberg and Trump were briefly in the same small room as they arrived for the summit, with video showing the teen glowering at the president as he passed by with his entourage.

Thunberg has become the global face of a growing youth movement against climate inaction that mobilized millions in a worldwide strike on Friday.

Her struggle began quietly in August 2018 when she skipped school for the first three weeks, and then on Fridays to spend the day outside Sweden's parliament with a sign labeled "School strike for climate."

In August, she crossed the Atlantic in a two-week journey on a sailboat to attend the climate summit in New York. She refuses to fly because of the carbon emissions caused by planes.