At midday, as the sky turned gray, her father, Svante, brought lunch. Chickpeas and rice, which she ate, standing, alone in the crowd, in minute forkfuls, like a bird, before receiving a gaggle of third-graders in snowsuits. By the end of the afternoon, a full 7 hours standing outside in the cold, she was very tired. All she wanted to do, she said, was to go home and lie on the couch with her dogs.

Greta Thunberg is an unlikely, though not entirely accidental, activist.

The eldest of two girls, she grew up in Stockholm. She studied piano and ballet and theater. She did well in school. Like many children, she watched educational films about the melting Arctic and the fate of the polar bears and the marine mammals bloated with plastic. But unlike other children, she couldn’t let them go. “I became very affected. I began thinking about it all the time and I became very sad,” she said. “Those pictures were stuck in my head.”

Adolescence brought social pressures. She wasn’t into the things that many other kids were into. Mobile phones. Clothes. None of it interested her, her father recalled. “I think she was very isolated and very lonely,” Mr. Thunberg said.

By age 11, Greta had fallen into a deep funk. She stopped going to school. She stopped eating. She stopped growing. She spoke only to family, and, at school, only to one teacher, Anita von Berens.

“Before, my own world was very big,” she recalled. “I was all alone.”

Is the alone-world still there?