US President Donald has taken a subtle jab at a young activist, just hours after she ripped into him and other world leaders at the United Nations Climate Action Summit in New York.

But climate activist, Greta Thunberg, has made sure she had the final word.

Ms Thunberg gave a powerful speech on Monday at the summit in which she accused world leaders of ignoring climate change.

Tears streamed down the 16-year-old's face as she chided them with the repeated phrase, "How dare you", in an emotional appeal for action.

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Mr Trump later shared a video of her speech on Twitter and appeared to take a swipe at the teen.

"She seems like a very happy young girl looking forward to a bright and wonderful future. So Nice to see!" he wrote above a video of a visibly distressed Ms Thunberg.

But the Swedish schoolgirl took it in her stride, reports news.com.au, changing her Twitter bio just hours later to describe herself as: "A very happy young girl looking forward to a bright and wonderful future."

The president's jab came after Ms Thunberg stared him down as the pair crossed paths in the United Nations building before the Summit began.

Mr Trump β€” who has denied climate change, called it a Chinese hoax and repealed US carbon-reduction policies β€” was not scheduled to attend but made the surprise visit before leaving to attend a religious freedoms meeting on site.

Video footage of the frosty exchange shows Mr Trump appearing to ignore Ms Thunberg as he walks straight past her with his entourage. She can be seen with her eyes fixed on him, holding her steely gaze as he moves through the corridor.

Greta Thunberg stares down Donald Trump. Photo / Supplied
Greta Thunberg stares down Donald Trump. Photo / Supplied

Heads of state from around the world, including French President Emmanuel Macron and German Chancellor Angela Merkel, descended on the Big Apple this week to make new pledges to curb global-warming emissions.

At the summit, Ms Thunberg accused them of ignoring 30 years of "crystal clear" science behind the climate crisis, saying: "We are in the beginning of a mass extinction and all you can talk about is money and fairy tales of eternal economic growth β€” how dare you."

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Ms Thunberg, who travelled from Europe to New York on a zero-emissions sailboat to speak at the event, first came to worldwide attention when she started a lone protest outside her country's parliament more than a year ago. It was that very decision which culminated in Friday's global climate strikes.

"This is all wrong. I shouldn't be up here," she told the international heads of state.

"I should be back in school on the other side of the ocean.

"Yet you have come to us young people for hope. How dare you. You have stolen my dreams and my childhood with your empty words, and yet I'm one of the lucky ones. People are suffering. People are dying. Entire ecosystems are collapsing.

"You say you hear us, and that you understand the urgency … I do not want to believe that. "Because if you really understood the situation, and still kept on failing to act, then you would be evil. And that I refuse to believe."

Greta Thunberg made the impassioned plea at the UN. Photo / AP
Greta Thunberg made the impassioned plea at the UN. Photo / AP

She told the UN that even the strictest emission cuts being talked about only gives the world a 50 per cent chance of limiting future warming to another 0.4C from now, which is a global goal. Those odds are not good enough, she said.

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"We will not let you get away with this," Ms Thunberg continued. "Right now is where we draw the line."

Her speech was met with a stunned silence β€” then a rapturous applause.

Following Ms Thunberg's speech, she and 15 other children filed a complaint with the UN alleging that five of the world's major economies have violated their human rights by not taking adequate action to stop the unfolding climate crisis.

The 2019 Climate Action Summit kicked off at the UN on Monday, where world leaders gathered to discuss serious strategies to mitigate climate change. Representatives of participating nations were told by UN Secretary General Antonio Guterres to come up with "concrete, realistic plans" to further their commitments to reducing greenhouse gas emissions and get to net zero emissions by 2050.

Leader after leader told the UN that they will do more to prevent a warming world from reaching even more dangerous levels. But as they made their pledges, they conceded it was not enough.