Donald Trump has mocked Greta Thunberg on Twitter, after the 16-year-old condemned world leaders for failing to combat climate change.

“You have stolen my dreams and my childhood,” the Swedish schoolgirl said during a speech at the UN climate summit on Monday.

“We are in the beginning of a mass extinction and all you can talk about is money and fairytales of economic growth. How dare you!”

“She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!” Mr Trump said while retweeting a clip of the teenager’s speech.

The president was not expected to attend Monday’s summit but dropped by briefly to listen to Angela Merkel’s remarks, before leaving without speaking.

Greta Thunberg inspires climate activists everywhere: In pictures Show all 12 1 /12 Greta Thunberg inspires climate activists everywhere: In pictures Greta Thunberg inspires climate activists everywhere: In pictures In the protest that started a movement, Greta skips school to sit outside of the Swedish parliament in Stockholm in order to raise awareness of climate change on 28 August 2018 Getty Greta Thunberg inspires climate activists everywhere: In pictures Greta speaks at the World Economic Forum in Davos on 25 January AFP/Getty Greta Thunberg inspires climate activists everywhere: In pictures Greta stages a protest at the World Economic Forum in Davos on 25 January Reuters Greta Thunberg inspires climate activists everywhere: In pictures Greta speaks at the House of Commons in London on 23 April PA Greta Thunberg inspires climate activists everywhere: In pictures Greta addresses to the occupation at Marble Arch in London on 21 April AFP/Getty Greta Thunberg inspires climate activists everywhere: In pictures Greta meets the pope on a visit to Rome Reuters Greta Thunberg inspires climate activists everywhere: In pictures Greta speaks at the senate in Rome on 18 April Reuters Greta Thunberg inspires climate activists everywhere: In pictures Greta addresses a debate of the EU Environment, Public Health and Food Safety committee at the European Parliament in Strasbourg on 16 April AFP/Getty Greta Thunberg inspires climate activists everywhere: In pictures Greta receives the Special Climate Protection Award at the German Film and Television awards in Berlin on 30 March AFP/Getty Greta Thunberg inspires climate activists everywhere: In pictures Greta attends a children's climate protest in Berlin on 29 March AFP/Getty Greta Thunberg inspires climate activists everywhere: In pictures Greta addresses a children's climate protest on 1 March in Hamburg Getty Greta Thunberg inspires climate activists everywhere: In pictures Greta attends a meeting for the Civil Society For rEUnaissance at the EU Charlemagne Building in Brussels on 21 February AFP/Getty

The 73-year-old is a climate sceptic, who has previously denied the existence of climate change and called it a Chinese hoax.

He has also repealed carbon-reduction policies in the US and withdrew the country from the landmark Paris Agreement.

Michael Bloomberg, the UN’s special climate envoy, thanked the president for stepping by.

He said the summit may prove useful when the president formulated climate policy, drawing laughter and applause from the audience.

Greta Thunberg watches Donald Trump speak to reporters (REUTERS)

The US did not ask to speak at the conference but the White House drew attention on the assembly floor.

Wang Yi, China’s foreign minister, said countries ”must honour our commitments and follow through on the Paris Agreement,” in a clear jibe at the Trump administration.

“The withdrawal of certain parties will not shake the collective goal of the world community,” Mr Yi said, to applause.

Ms Thunberg has been nominated for the Nobel Peace Prize by legislators in the Swedish government, in recognition for her efforts to combat climate change.

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