Greta Thunberg says she wasn't surprised by Donald Trump's sarcastic response to her impassioned speech on climate change this week.

"Of course he was going to write that," she told a Scandinavian talk show on Wednesday.

"You can interpret it (the tweet) in many different ways, but I knew that at some time he was probably going to say something about me."

The US President tweeted that Ms Thunberg seemed like a "very happy young girl looking forward to a bright and wonderful future", following her speech at the United Nations summit on climate change on Tuesday.

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During the address, Ms Thunberg became visibly emotional as she urged world leaders to do more to combat global warming.

The 16-year-old climate activist has given an interview declaring what she really thought about the US President's tweet after her impassioned speech. Photo / AP
The 16-year-old climate activist has given an interview declaring what she really thought about the US President's tweet after her impassioned speech. Photo / AP

"You have stolen my dreams and my childhood with your empty words," she said. "How dare you. People are suffering. People are dying. Entire ecosystems are collapsing."

Mr Trump's response was the first time the president had tweeted about the 16-year-old Swedish activist, who led worldwide climate protests last Friday, news.com.au reports.

"It doesn't make any difference in a way," she told the Norwegian-Swedish television production Skavlan, which will air the full interview on Friday.

In an earlier, cheekier response to the tweet, Ms Thunberg changed her bio on Twitter to read "a very happy young girl looking forward to a bright and wonderful future".

It had previously read "16-year-old climate activist with Asperger's".

Ms Thunberg's campaign began on August 20 in 2018, when she held solitary demonstrations outside Sweden's parliament, skipping classes once a week to protest climate change.

Since then, her solo protest has inspired millions across the world to stage protests urging leaders to tackle global warming. But she told the UN summit she should never have had to skip school.

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"This is all wrong. I shouldn't be up here. I should be back in school on the other side of the ocean. Yet you all come to us for hope. How dare you!" she said.

Mr Trump and Ms Thunberg did not meet in person while in New York for the UN General Assembly. The US President also appeared not to notice as she stared daggers at him while he addressed reporters.