4 stars

It was a tip-off that got him started. When filmmaker Nathan Grossman heard that a schoolgirl was campaigning for climate action outside the Swedish Parliament, he went along to take a look.

Fifteen-year-old Greta Thunberg had set up camp on the footpath with her handmade sign "School Strike for Climate". In her regulation shirt and jeans and with her long plaits, she did not necessarily look the tough campaigner but she would eventually demonstrate an amazing ability to cut through.

She had an armoury of facts and figures to support her protest and was fully prepared to engage anyone who wanted to hear more. Or wanted to take her on. An elderly woman asked why she wasn't in school. Greta retorted that there was no point in going to school if there was no future for her generation. The world's leaders were ignoring the climate crisis.

Grossman could see how things might get interesting. Greta's father Svante, an actor, was nearby and he allowed the filmmaker to begin recording. The start was impromptu start, but the filmmaker eventually acquired funding so he could work on the project full-time.

In the year that followed, Greta became, of course, a celebrity who made speeches and met world leaders. A controversial figure who famously chose to sail the Atlantic rather than fly across it, so she could take up an invitation to speak at the UN Climate Action Summit in NYC in September, 2019.

Grossman's doco is by turns intimate and inspiring. While it documents the rise of teenage climate activism that Greta spearheaded across the globe, it is at the same time a character study. We observe Greta take the stage and speak unscripted in front of thousands of supporters, but not before she has quarrelled with her father. He simply wants her to eat something healthy, like a banana, or to forget about perfecting her speech.

I Am Greta is a blend of private and public moments, some of it guided by voiceover. Greta confides quite freely that she has little interest in socialising, has no time for small talk, and has been beset with anxiety and depression since she was very young. When she heard that the world was facing a new mass extinction, and that there was little or no time left to bend the emission curve.

She stopped flying, and stopped eating meat years ago. She was doing her bit, and had convinced her family, including her mother, an opera singer with an international career, to do theirs too. There are several scenes, briefly sketched, that hint at the impact of Greta's personality and activism on the Thunberg family.

Svante appears to have a significant presence in his elder daughter's life, as chaperone on her travels and a full-time companion-carer. As is now well known, Greta has Asperger's, but he confides that she also has OCD and selective mutism that has caused her in the past to stop communicating for many months on end. There is a hint of these traits, those aspects of her personality that some political opponents have cruelly and immorally used against her in ad hominem attacks.

The responses to Greta from world leaders compiled here in montage are either memorable, or they are shocking. Arnold Schwarzenegger declares himself early as a fan of someone who acts rather than complains. Pope Francis and the UN Secretary General are also encouraging. A meeting is arranged for her with a slightly bemused President Macron who can't disguise his surprise that he is the first leader she has met. No, she hadn't even met the Prime Minister of her own country.

This delightful episode is followed by clips of other responses. Vladimir Putin imparts his views, as does Donald Trump, and Brazil's Jair Bolsonaro. On this occasion, the Russian president's observations seem more temperate than some. Various media personalities, including Alan Jones and Andrew Bolt, also have their say on Greta Thunberg, and it reflects badly on them.

I Am Greta is a fascinating portrait of an angry young climate justice activist who calls out the global inertia in climate policy for what it is.