Teenage climate activist Greta Thunberg has changed her Twitter bio to a quote about herself from US President Donald Trump.

‘A very happy young girl looking forward to a bright and wonderful future,’ the new bio reads.

The move follows a tweet from Trump on Monday responding to a video of Thunberg giving an emotional speech at a UN summit on climate change.

‘She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!’ Trump tweeted. The tweet has been retweeted more than 39,000 times.

Trump and Thunberg have never met in person, even as they are both in New York for the UN General Assembly. Trump walked past the 16-year-old Swedish activist on Monday, appearing not to notice her behind him as he addressed reporters.

Thunberg's speech sounded the alarm about climate change and was sharply critical of world leaders.

‘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 fairy tales of eternal economic growth,’ Thunberg said in the clip.

The US president is a known climate sceptic, but he did briefly make a surprise visit to the climate summit and clapped as speakers addressed carbon emissions and rising sea levels.

Trump prioritizes economic growth over climate measures and has cast scorn on some proposed reforms to reduce emissions, saying they could hurt workers and leave people worse off financially.

Some US media outlets took Trump's tweet about Thunberg as a sign he was mocking her.

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