Sometimes, as a journalist or documentary maker, a big story will, if you’re lucky, unexpectedly land in your lap. This is what happened to a young filmmaker from Sweden called Nathan Grossman, 29, when he followed a friend’s tip to go down to the parliament building in Stockholm and film a 15-year-old girl’s strike to demand that the government live up to the commitment it made to lower carbon emissions under the Paris Agreement. Although he didn’t know it then, Grossman was witnessing the seed of a new global movement.

The petite teenager with lank Pippi Longstocking-like pigtails, a reserved demeanour, and a laser-like focus on the way that the world seemed to be going to hell in a handbasket was, of course, Greta Thunberg. At the time, she was unknown. Within a matter of months, however, thanks to her school strikes — initially aimed at putting the climate on the agenda at the 2018 Swedish general election — she would be a household name around the world.

In I Am Greta, filmed between August 2018 and September 2019, Grossman has intimately, compellingly and upliftingly captured Thunberg’s whirlwind journey from the impeccably clean streets of the Swedish capital to the UN Climate Action Summit in New York City, and almost everything in between.

To prove, in her own words in the film, the impossibility of living sustainably today, she travelled to the United States by sailing the Atlantic in a 60ft racing yacht, along with Grossman, her father Svante, and a two-man crew. At the UN, perhaps fired-up by the arduousness of the 15-day crossing (“That was a bit more rocky than most of her other activism,” says Grossman), she blisteringly denounced the assembled representatives of governments, businesses and civil society for their inadequate response to a crisis that she warned, for the umpteenth time, was threatening to take away her generation’s future.

“This is all wrong,” she rails in a clip from the event, with white-hot indignation. “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 young people for hope. How dare you!”

A screening of the film at Limmud Stockholm last year was followed by a discussion between Grossman and Yonatan Neril, an interfaith enviromental activist and rabbi, about the connection between Judaism and the climate crisis. One of the concepts they discussed was tikkun olam, “healing the world” which can mean different things to different people. Grossman, who describes himself as coming from a “very, very super-secular family”, when we talk via Zoom, recalls that Neril said it’s the idea that it’s “a mitzvah to do good”.

“In a sense, what we’re looking at right now,” says the filmmaker, “is a world that needs more of that mitzvah. I think we probably need to do more and be more kind of active for the sake of the world than we are at the moment.”

For me, Thunberg, through her tireless and dogged efforts to wake people up to the consequences if nations do not treat the climate crisis seriously, effectively emerges from the documentary as the very embodiment of tikkun olam. Not that that was necessarily ever on Grossman’s mind. In fact, when he met Thunberg on his initial “recce”, he wasn’t even sure whether there was a story he could tell.

“I felt she was fairly shy,” he says, “and didn’t really know what to do with this kind of shooting.”

Was he impressed by her? “No,” replies Grossman, laughing. “Not immediately. But during that first day, passers-by started to talk to her, and other kinds of journalists came and asked her questions. It was then I started getting more and more impressed by her, because she had this way of compressing this complicated issue of climate change into something understandable. I stood with my headphones on and I listened to conversations, and I didn’t turn the camera off.”

Grossman’s own interest in climate change was sparked by watching Al Gore’s Oscar-winning 2006 documentary, An Inconvenient Truth, in which the former US vice-president graphically illustrated the destructive effects of global warming. “So it comes more from there than Judaism,” he says. Similarly, Thunberg, who has Asperger’s, watched a documentary at school featuring malnourished polar bears, floods and droughts, and scientists saying time was running out. In I Am Greta, she says the bleak prognosis for the environment if we don’t act made her depressed and anxious. She stopped eating and speaking, and almost starved to death. It took several years for her to start feeling better again, but she eventually came out the other side, filled with a new determination to fight for the health of the planet.

After three months of filming Thunberg, Grossman still had very little. He was thinking he could do a short film limited to public broadcast in Sweden and Norway, but then something incredible happened: the school strikes spread to other countries, and Thunberg’s lonely crusade exploded into a global phenomenon.

“I remember one day I saw in the news that there were tens of thousands of children striking in Australia,” says Grossman, “and then I, of course, felt, ‘Oh, this is moving seriously outside of Scandinavia.’ It was at that point I got permission from my boss to work full-time on this.”

As early as the first week of her strike in Sweden, he noticed that Thunberg was polarising people. “I was a bit surprised by that, because I couldn’t understand why people were so provocated (sic) by her, in the sense that she wasn’t saying anything new. She was just referencing reports and figures from the UN that were publicly available.”

As her fame grew and her message spread wider, the attacks on Thunberg became more vitriolic and personal. Examples in Grossman’s film include an unimpressed Putin; an Australian commentator who calls her “a depressed and extremely anxious girl”; Piers Morgan saying there are people who find her “annoying”, “very hyperbolic”, “over emotional”; Trump sarcastically name-checking her at a rally and everyone booing; and Brazil’s president, Jair Bolsonaro, insisting that it’s “a catastrophe that this brat is given time in the media” after she called out the killing of indigenous people for defending the Amazon rain forest. They’re all men. Is there misogyny at play here? “Maybe,” says Grossman. “There’s probably a cocktail of elements.”

As ever, it is probably just easier for people to attack the messenger than to address the message. “We get provocated, I personally get provocated, seeing these figures of how we live outside the planetary boundaries... And, of course, being young and female, as she is, probably, for some people, becomes even more provocating.”

Thunberg’s youth, and that of the activists she inspires, underscores how climate change has, to an extent, become a “generational conflict”. This wasn’t something Grossman had really thought about before, but was crystallised during filming.

“The older you are, the less affected you will be. And that, at least in studies in Sweden, also relates to how worried people are about the issue,” he says.

“And then you have young people who don’t have the political and economical power, but they will be super-affected by this issue and they tend to worry about it very much.”

Thunberg is greeted like a rock star at protests not because what she is saying is new, but, Grossman suggests, because she is “the first very public Generation Z person who went out and said, ‘This is my future at stake,’ and by doing that she raised awareness for that divide.”

She is, though, a reluctant activist. She would prefer to stay home with her family, dogs and routines, but feels a responsibility to keep speaking out and repeating her warnings. Even so, her frustration is clear. When she gets invited to events like the UN conference, she believes it is just to give the impression that people are listening and intending to act, when in fact very little is being done.

“One of the hard things about being a whistleblower, in a sense, in this issue,” says Grossman, “is that you need to go to the arenas where the political discussions are taking place, but sometimes in these political arenas they say, ‘Now we have done our job addressing the issue just by inviting you.’ But the invitation is not the thing here; the thing is the action.”

The way that countries have responded to coronavirus has given Grossman hope by showing that governments will throw money at an issue and change behaviours when something is treated as a crisis. This, he realised, is why Thunberg talks about the “climate crisis”. Until recently he didn’t understand what she meant, and even considered cutting references to it from the documentary. Now he has a context, he gets it. But when will our leaders?

If we don’t find effective ways to tackle climate change soon, society’s problems, including antisemitism, will be exacerbated, he says.

“I think every person in the world who knows anything about climate change will understand that it puts lots of stress on any community, and I think the Jewish community will be affected by climate change very much in the future, as well as all people in the world,” says Grossman.

“Sadly, we know that if there’s a lack of resources, if there’s changing demographics, that will result in more conflicts. And anyone who’s been affected by antisemitism can definitely understand how important it is to focus on not getting more conflict in between people and in between land areas . . . The climate science shows us that we’re looking towards a future where we can actually go the opposite way, and that’s something which is making me very worried.”

I Am Greta is in UK and Irish cinemas from today