Greta Thunberg cut a frail and lonely figure when she started a school strike for the climate outside the Swedish parliament building last August. Her parents tried to dissuade her. Classmates declined to join. Passersby expressed pity and bemusement at the sight of the then unknown 15-year-old sitting on the cobblestones with a hand-painted banner.

Eight months on, the picture could not be more different. The pigtailed teenager is feted across the world as a model of determination, inspiration and positive action. National presidents and corporate executives line up to be criticised by her, face to face. Her skolstrejk för klimatet (school strike for climate) banner has been translated into dozens of languages. And, most striking of all, the loner is now anything but alone.

On 15 March, when she returns to the cobblestones (as she has done almost every Friday in rain, sun, ice and snow), it will be as a figurehead for a vast and growing movement. The global climate strike this Friday is gearing up to be one of the biggest environmental protests the world has ever seen. As it approaches, Thunberg is clearly excited.

“It’s amazing,” she says. “It’s more than 71 countries and more than 700 places, and counting. It’s increasing very much now, and that’s very, very fun.”

A year ago, this was unimaginable. Back then, Thunberg was a painfully introverted, slightly built nobody, waking at 6am to prepare for school and heading back home at 3pm. “Nothing really was happening in my life,” she recalls. “I have always been that girl in the back who doesn’t say anything. I thought I couldn’t make a difference because I was too small.”

She was never quite like the other kids. Her mother, Malena Ernman, is one of Sweden’s most celebrated opera singers. Her father, Svante Thunberg, is an actor and author (named after Svante Arrhenius, the Nobel prize-winning scientist who in 1896 first calculated how carbon dioxide emissions could lead to the greenhouse effect). Greta was exceptionally bright. Four years ago, she was diagnosed with Asperger’s.

“I overthink. Some people can just let things go, but I can’t, especially if there’s something that worries me or makes me sad. I remember when I was younger, and in school, our teachers showed us films of plastic in the ocean, starving polar bears and so on. I cried through all the movies. My classmates were concerned when they watched the film, but when it stopped, they started thinking about other things. I couldn’t do that. Those pictures were stuck in my head.”

She has come to accept this as part of who she is – and made it a motivating force instead of a source of paralysing depression, which it once was.

At about the age of eight, when she first learned about climate change, she was shocked that adults did not appear to be taking the issue seriously. It was not the only reason she became depressed a few years later, but it was a significant factor.

Facebook Twitter Pinterest Thunberg on strike last August. Photograph: Michael Campanella/The Guardian

“I kept thinking about it and I just wondered if I am going to have a future. And I kept that to myself because I’m not very much of a talker, and that wasn’t healthy. I became very depressed and stopped going to school. When I was home, my parents took care of me, and we started talking because we had nothing else to do. And then I told them about my worries and concerns about the climate crisis and the environment. And it felt good to just get that off my chest.

“They just told me everything will be all right. That didn’t help, of course, but it was good to talk. And then I kept on going, talking about this all the time and showing my parents pictures, graphs and films, articles and reports. And, after a while, they started listening to what I actually said. That’s when I kind of realised I could make a difference. And how I got out of that depression was that I thought: it is just a waste of time feeling this way because I can do so much good with my life. I am trying to do that still now.”

Her parents were the guinea pigs. She discovered she had remarkable powers of persuasion, and her mother gave up flying, which had a severe impact on her career. Her father became a vegetarian. As well as feeling relieved by the transformation of their formerly quiet and morose daughter, they say they were persuaded by her reasoning. “Over the years, I ran out of arguments,” says her father. “She kept showing us documentaries, and we read books together. Before that, I really didn’t have a clue. I thought we had the climate issue sorted,” he says. “She changed us and now she is changing a great many other people. There was no hint of this in her childhood. It’s unbelievable. If this can happen, anything can happen.”

The climate strike was inspired by students from the Parkland school in Florida, who walked out of classes in protest against the US gun laws that enabled the massacre on their campus. Greta was part of a group that wanted to do something similar to raise awareness about climate change, but they couldn’t agree what. Last summer, after a record heatwave in northern Europe and forest fires that ravaged swathes of Swedish land up to the Arctic, Thunberg decided to go it alone. Day one was 20 August 2018.

“I painted the sign on a piece of wood and, for the flyers, wrote down some facts I thought everyone should know. And then I took my bike to the parliament and just sat there,” she recalls. “The first day, I sat alone from about 8.30am to 3pm – the regular schoolday. And then on the second day, people started joining me. After that, there were people there all the time.”

She kept her promise to strike every day until the Swedish national elections. Afterwards, she agreed to make a speech in front of thousands of people at a People’s Climate March rally. Her parents were reluctant. Knowing Thunberg had been so reticent that she had previously been diagnosed with selective mutism, they tried to talk her out of it. But the teenager was determined. “In some cases where I am really passionate, I will not change my mind,” she says. Despite her family’s concerns, she delivered the address in nearly flawless English, and invited the crowd to film her on their mobile phones and spread the message through social media. “I cried,” says her proud dad.

Facebook Twitter Pinterest Thunberg … ‘They are still not doing anything.’ Photograph: Michael Campanella/The Guardian

People with selective mutism have a tendency to worry more than others. Thunberg has since weaponised this in meetings with political leaders, and with billionaire entrepreneurs in Davos. “I don’t want you to be hopeful. I want you to panic. I want you to feel the fear I feel every day. And then I want you to act,” she told them.

Such tongue-lashings have gone down well. Many politicians laud her candidness. In return, she listens to their claims that stronger climate policies are unrealistic unless the public make the issue more of a priority. She is unconvinced. “They are still not doing anything. So I don’t know really why they are supporting us because we are criticising them. It’s kind of weird.” She has also been withering about leaders in the US, UK and Australia who either ignore the strikers or admonish them for skipping classes. “They are desperately trying to change the subject whenever the school strikes come up. They know they can’t win this fight because they haven’t done anything.”

Play Video 2:55 'I want you to panic': 16-year-old issues climate warning at Davos – video

Such blunt talk has found a broad audience among people jaded by empty promises and eager to find a climate leader willing to ramp up ambition. Thunberg’s rise coincides with growing scientific concern. A slew of recent reports has warned oceans are heating and the poles melting faster than expected. Last year’s UN Intergovernmental Panel on Climate Change spelled out the dangers of surpassing 1.5C of global warming. To have any chance of avoiding that outcome, it said, emissions must fall rapidly by 2030. That will require far more pressure on politicians – and nobody has proved more effective at that over the past eight months than Thunberg.

The girl who once slipped into despair is now a beacon of hope. One after another, veteran campaigners and grizzled scientists have described her as the best news for the climate movement in decades. She has been lauded at the UN, met the French president, Emmanuel Macron, shared a podium with the European commission president Jean-Claude Juncker and has been endorsed by the German chancellor, Angela Merkel.

No, I am not more hopeful than when I started. The emissions are increasing

You may think this would put the weight of the world on the 16-year-old’s shoulders, but she claims to feel no pressure. If “people are so desperate for hope”, she says, that is not her or the other strikers’ responsibility.

“I don’t care if what I’m doing – what we’re doing – is hopeful. We need to do it anyway. Even if there’s no hope left and everything is hopeless, we must do what we can.”

In this regard, her family see her Asperger’s as a blessing. She is someone who strips away social distractions and focuses with black-and-white clarity on the issues. “It’s nothing that I want to change about me,” she says. “It’s just who I am. If I had been just like everyone else and been social, then I would have just tried to start an organisation. But I couldn’t do that. I’m not very good with people, so I did something myself instead.”

While she has little time for chit-chat, she gets satisfaction from speaking to a big audience about climate change. Regardless of the size of the crowd, she says she does not feel the least bit nervous.

She seems incapable of the cognitive dissonance that allows other people to lament what is happening to the climate one minute, then tuck into a steak, buy a car or fly off for a weekend break the next. Although Thunberg believes political action far outweighs individual changes to consumer habits, she lives her values. She is a vegan, and only travels abroad by train.

Play Video 1:57 Teen climate activist Greta Thunberg speaks at four school strikes in a week – video

At its best, this sharpness can slice through the Gordian knot of the climate debate. It can also sting. There are no comfortable reassurances in her speech, just a steady frankness. Asked whether she has become more optimistic because the climate issue has risen up the political agenda and politicians in the US and Europe are considering green New Deals that would ramp up the transition to renewable energy, her reply is brutally honest. “No, I am not more hopeful than when I started. The emissions are increasing and that is the only thing that matters. I think that needs to be our focus. We cannot talk about anything else.”

Some people consider this a threat. A handful of fossil fuel lobbyists, politicians and journalists have argued Thunberg is not what she seems; that she was propelled into prominence by environmental groups and sustainable business interests. They say the entrepreneur who first tweeted about the climate strike, Ingmar Rentzhog, used Thunberg’s name to raise investment for his company, but her father says the connection was overblown. Greta, he says, initiated the strike before anyone in the family had heard of Rentzhog. As soon as she found he had used her name without her permission, she cut all links with the company, and has since vowed never to be associated with commercial interests. Her family says she has never been paid for her activities. In a recent interview, Rentzhog defended his actions, denied exploiting Greta and said that climate change, not profit, was his motive.

On social media, there have been other crude attacks on Thunberg’s reputation and appearance. Already familiar with bullying from school, she appears unfazed. “I expected when I started that if this is going to become big, then there will be a lot of hate,” she says. “It’s a positive sign. I think that must be because they see us as a threat. That means that something has changed in the debate, and we are making a difference.”

She intends to strike outside parliament every Friday until the Swedish government’s policies are in line with the Paris climate agreement. This has led to what she calls “strange contrasts”: balancing her maths homework with her fight to save the planet; listening attentively to teachers and decrying the immaturity of world leaders; weighing up the existential threat of climate change alongside the agonising choice of what subjects to study in high school.

It can be gruelling. She still gets up at 6am to get ready for school. Interviews and writing speeches can leave her working 12- to 15-hour days. “Of course, it takes a lot of energy. I don’t have much spare time. But I just keep reminding myself why I am doing this, and then I just try to do as much as I can.” So far, this does not appear to have affected her academic performance. She keeps up with homework and is in the top five in her class, according to her father.

And now that she is active on climate, she is no longer lonely, no longer silent, no longer so depressed. She is too busy trying to make a difference. And enjoying herself.

This Friday, when she takes her usual spot outside the Swedish parliament, she will be joined by classmates and students from other schools. “It’s going to be very, very big internationally, with hundreds of thousands of children going to strike from school to say that we aren’t going to accept this any more,” she says. “I think we are only seeing the beginning. I think that change is on the horizon and the people will stand up for their future.”

And then the activist slips back into being a teenager. “I’m looking forward to it and to see all the pictures the day afterwards. It’s going to be fun.”

• This article was amended on 11 March 2019 to revise an explanation of mutism. An earlier version of the piece said: “People with selective mutism typically do not suffer from an inability to talk; rather, they choose not to engage in conversations they do not consider worthwhile.” This is not the accepted definition. The National Health Service, for example, notes: “Selective mutism is a severe anxiety disorder where a person is unable to speak in certain social situations , such as with classmates at school or to relatives they don’t see very often … A child or adult with selective mutism doesn’t refuse or choose not to speak, they’re literally unable to speak.”