Greta Thunberg watches as U.S. President Donald Trump enters the United Nations to speak with reporters on Monday. (Reuters)

Greta Thunberg hit back at Donald Trump after he mocked her on social media by making his scathing tweet her Twitter bio.

The 16-year-old Swedish climate activist was ridiculed by the President for urging world leaders to combat climate change at a UN summit in New York.

Ms Thunberg scolded the audience at the UN Climate Action Summit on Monday, saying: “We are in the beginning of a mass extinction and yet all you can talk about is money. You are failing us.”

Tweeting a clip from her speech on Wednesday, Mr Trump said: “She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!”

She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see! https://t.co/1tQG6QcVKO — Donald J. Trump (@realDonaldTrump) September 24, 2019

But the activist hit back soon after by changing her Twitter bio to read: “A very happy young girl looking forward to a bright and wonderful future.”

Mr Trump had spent only a few minutes at the climate summit, a centrepiece of this year’s UN general assembly schedule.

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Ms Thunberg, who sailed across the Atlantic in a yacht to attend the event, used her speech to warn world leaders that the eyes of future generations are on them.

In an impassioned speech at the UN climate action summit, the 16-year-old, who has inspired a global climate strike movement, told delegates they would never be forgiven if they failed to tackle rising temperatures.

View photos The activist changed her Twitter bio after Mr Trump's comments. (Twitter/@GretaThunberg) More

The teenager, who travelled to the US by yacht to avoid flying, said she should not be up on stage, but should be in school on the other side of the ocean.

She said: “You come to us young people for hope – how dare you? You have stolen my dreams, my childhood with your empty words.

“People are suffering, people are dying, entire ecosystems are collapsing, we are in the beginning of a mass extinction and all you can talk about is money and the fairy tale of endless economic growth. How dare you?

“How dare you continue to look away, and come here saying that you are doing enough, when the politics and solutions needed are still nowhere in sight.”