Greta Thunberg has hit back at Donald Trump after her passionate speech at the United Nations summit on climate change.

The US president posted a seemingly sarcastic message about the 16-year-old activist alongside her UN address on Tuesday.

Tweeting to his 64.7 million followers, Mr Trump wrote: "She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!"

Image: Greta Thunberg changed her Twitter bio following the president's tweet

Not one to sit back and ignore the president's flippant comment, Thunberg changed her Twitter bio to: "A very happy young girl looking forward to a bright and wonderful future."

On Monday, a tearful Thunberg told world leaders gathered at a climate action summit in New York that "we are in the beginning of a mass extinction and all you can talk about is money and fairytales of eternal economic growth".


She said: "You have stolen my dreams and my childhood with your empty words. How dare you."

Mr Trump, who has previously described climate change as a "hoax", spent only a few minutes at the climate summit, a centrepiece of this year's UN General Assembly schedule.

Tearful Greta rips into world leaders at UN

Thunberg, who sailed across the Atlantic in a yacht to attend the event, was filmed glaring at the president as he walked past her on Monday.

In her emotional and charged speech, the teenager reprimanded those who she accused of ignoring science and being too slow in their approach to tackle the world's contributions to climate change.

Mr Trump is not the only one to take a dig at Thunberg. Fox News has apologised after one of its guests called her mentally ill - with the network saying he would never appear on the network again.

Image: Fox News has apologised to Greta Thunberg after Michael Knowles called her mentally ill

Conservative pundit Michael Knowles of The Daily Wire made his controversial comments during a programme on Monday evening.

He said Thunberg, who has Asperger's syndrome, was being exploited by her parents and the left wing.

"The climate hysteria movement is not about science," Mr 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."

Knowles continued: "She is mentally ill. She has autism. She has obsessive compulsive disorder. She has selective mutism. She had depression."

Thunberg glares at Trump

During her address, Thunberg told the audience: "You say you hear us and understand the urgency but no matter how sad and angry I am, I do not want to believe that.

"If you really understood the situation, and still kept on failing to act, then you would be evil and that I refuse to believe."

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She added: "You are failing us. But young people are starting to understand your betrayal."

Following her speech, Democratic presidential hopeful Julian Castro tweeted: "I think a lot of us can relate."