At last December’s UN Climate Change Conference in Poland, Greta Thunberg declared to delegates: “You are not mature enough to tell it like it is – even that burden you leave to us children.” Dressed in a grey check shirt and lilac trousers, her signature plaits framing her face, the 16-year-old Swede faced down the room of suits – and the world responded.

Democratic congresswoman Alexandria Ocasio-Cortez and an array of climate scientists were among those who expressed their support. The icon the climate movement desperately needed had arrived.

Thunberg’s name is now synonymous with the mass school walkouts spreading across the world. Each Friday since August the teenager has missed class to sit on the steps outside the Swedish parliament in Stockholm in protest at her government’s inadequate action on climate change. Her parents thought she should be in lessons, as did the politicians, but Thunberg was undeterred and thousands of young people, from Australia to Uganda, are now following her example as part of the “Fridays for Future” movement.

On 15 February, the first nationwide action of this kind took place in Britain. Rather than attending school, pupils in as many as 40 UK towns and cities demanded that the government declare a climate emergency. “Students have drawn massive inspiration from Thunberg’s actions late last year,” Jake Woodier of Britain’s Youth Climate Coalition told me.

Thunberg’s ascent to the global stage has not been an uncomplicated one, however. At the age of 11, she fell into depression, stopped talking and eating, and lost ten kilos in weight. In a TEDx talk last year, Thunberg attributed her despair to the lack of attention the existential threat of climate change received from politicians and the media.

Yet rather than simply hoping for change, she resolved to take action. In the following years, she convinced her parents to stop flying, become vegan, install solar batteries, grow their own vegetables and only use their electric car when necessary – a series of steps that culminated, in the summer of 2018, with her decision to skip school.

Her motivations have been questioned; but on 2 February Thunberg addressed her detractors in a passionate and eloquent post on Facebook. She explained how she formed the idea of a strike after winning an environmental writing competition and connecting with the Fossil Free Dalsland movement in Sweden. Inspired by pupils in Parkland, Florida, who commemorated a mass shooting with a walkout, she maintained her plan even as other campaigners backed a protest march instead.

Thunberg denies being exploited by the wider climate movement, or being paid for her activism. And although she asks others, including scientists, for input into her speeches, she still claims them as her own: “Don’t you think that a 16 year old can speak for herself?”

Thunberg’s parents – Svante Thunberg, an actor, and Malena Ernman, a prominent opera singer – are familiar with performing to large audiences, and environmental consciousness runs deep in the family. Her father’s distant relative, Svanta Arrhenius, won the 1903 Nobel Prize for chemistry after calculating the effect of carbon dioxide emissions on the temperature of the earth.

Scenes from the Heart, a book by Thunberg’s mother on her daughters’ struggles with special needs, has also helped spread the family’s story. Thunberg has been diagnosed with Asperger syndrome, obsessive-compulsive disorder and selective mutism, meaning she only speaks when she feels compelled to do so. But Thunberg views the condition as a gift. “We aren’t very good at lying and we don’t normally enjoy participating in the social game that the rest of you seem so fond of,” she drily quipped in her TEDx talk.

The teenager credits the “black and white” viewpoint characteristic of autism with helping her confront the stark reality of climate change. “Either we limit the warming to 1.5°C over pre-industrial levels, or we don’t,” she wrote. “There are no grey areas when it comes to survival.”

This unapologetic realism extends to her critique of the global economy. “At places like Davos, people like to tell success stories, but their financial success has come with an unthinkable price tag,” she warned in a speech at the World Economic Forum.

But her message transcends political and national divides. By invoking the moral power of the intergenerational contract – the vow of present generations to conserve the Earth for future ones – the strikes have resonated across borders. Thunberg resembles a real-world version of Hunger Games protagonist Katniss Everdeen, speaking truth in the face of political failure. “I’ve learnt you are never too small to make a difference,” she told the UN conference. “And if a few children can get headlines all over the world just by not going to school, then imagine what we could all do together if we really wanted to.”