This story was originally published by The Guardian and is reproduced here as part of the Climate Desk collaboration.

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 criticized 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 March 15, 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 6 am to prepare for school and heading back home at 3 pm. “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 paralyzing depression, which it once was.

At about the age of 8, 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.