Greta Thunberg said the weight of having to sound the alarm on climate change was "too much" for her or any child after a hugely surprising fly-on-the-wall film about her rise was premiered at the Venice film festival.

The Swedish teenager allowed film-maker Nathan Grossman to follow her for a year after he met her in 2018 on the very first day of her schools' strike, sitting alone outside parliament in Stockholm with her homemade placard.

In that time she went from being a self-confessed ''shy nerdy person" to a global icon. 

The resulting film, "Greta", reveals not only the inside story of the pain and risk Thunberg has put herself through for the climate cause -- braving death threats and a hair-raising North Atlantic crossing in a racing yacht -- but her love of breaking into dance and her gift for comedy.

"In the movie, you can see that is not actually true, that I do decide for myself," said the activist, who has been dismissed as a "brat" by Brazil's far-right President Jair Bolsonaro, who claimed she was being manipulated.

The film shows how she dances in her pyjamas to relieve stress as she crisscrosses Europe on trains and in her father's electric car, living off baked beans and pasta as she urges leaders to act to save the planet.

"It is such a responsibility. I don't want to have to do all this," she said. 

Yet when far-right critics vilified her as "mentally ill" in the film, Thunberg, who has Asperger syndrome, laughed it off saying, "Sometimes I think it might be good if everyone had a bit of Asberger's.
"I don't see the world in black and white, just the climate crisis."

Despite the adoration, Thunberg now receives at demonstrations and on social media, in the film, she admits that "kids were mean to me" when I was younger. "I was never invited to parties and was left out."

The activist, now 17, said she was relieved the documentary does away with the idea that she is an "angry naive child who sits in the United Nations General Assembly screaming at world leaders because that is not the person I am."

Indeed, she drew a laugh from reporters and Grossman by admitting that at one stage she "doubted his seriousness" as a film-maker because he worked on his own.

Source: AFP