She stopped talking, stopped eating, and slipped into a depression that left both her and her terrified parents feeling helpless.

A diagnosis of Asperger’s would help Greta to understand why she felt different to other children and why the transient worries that came and left their lives clung to her with an ever-tightening grip.

But Asperger’s wasn’t the problem. Her worries were the problem, and those worries were real. Greta’s world — our world — was heading for calamity, courtesy of climate breakdown and society’s failure to acknowledge the extent of the emergency.

Greta didn’t believe it would all be OK, and from that refusal to be placated grew a global movement that this week brought hundreds of thousands of children and young people out of their schools and onto the streets in more than 70 countries, all echoing her demand for an end to platitudes and the start of real action.

She also got nominated for the Nobel Peace Prize this week. Not bad for a girl who has just turned 16.

Greta Thunberg was born in Stockholm, the capital of Sweden, in 2003, the first of two daughters of opera singer Malena Ernman and actor, producer, and writer Svante Thunberg.

Her mother, a classically trained mezzo soprano, who more recently has a mixed classic and contemporary repertoire, performed around the world and represented Sweden with a pop-opera number, ‘The Voice’, when Greta was six.

But Greta’s life, by her own accounts, was ordinary. She was “the girl in the back who doesn’t say anything” as she told The Guardian this week. She was, she said, introverted and not much of a talker.

“I could not make a difference because I was too small,” she said of how insignificant she felt at the time.

When she was around seven or eight, however, the issue of climate change crept into her consciousness. At school, she and her classmates watched documentaries on the disappearing Arctic ice and starving polar bears and cried.

Then the others would wipe away their tears, tuck the troubling facts away and get back to being children. Greta couldn’t find anywhere to tidy away her thoughts. They stayed with her, making her question, doubt and fear.

“I couldn’t let go,” she said.

By the age of 10, the Thunberg family was in crisis. Greta hardly ate and she was both physically and mentally ill. She lost a year of school and her parents realised that they needed to make a dramatic change to their lifestyles to make their daughter believe all was not lost.

They became vegetarian and later vegan. Malena stopped flying — with dramatic effect on her international career — and they began trying to live as sustainably as possible.

Greta appreciated their efforts, but she continued to study the causes and effects of climate change and her frustration with the comparative ignorance of adults grew.

At the UN’s COP24 climate change convention in Poland last December, she criticised both politicians and journalists for “not even knowing about the Keeling curve and Albedo effect” (two basic tools for understanding the growth in carbon emissions and escalation of global warming). It wasn’t clear from the expressions of those around her if they knew either.

The Albedo effect, for all the urgency it should arouse, wasn’t going to grab society’s attention, however. That took the Greta effect.

It was the students of Stoneman Douglas High School in Parkland, Florida who got her thinking. Seventeen of their classmates and teachers were murdered by a student in February last year and they went on strike for gun control.

Greta and her classmates talked about doing something similar for climate action but in the end, it was Greta alone who took a homemade banner to the gates of the Swedish parliament and sat there from 8.30am to 3pm one Friday last August, pledging to do the same every week until those in power started working to save her planet.

She did the same the following Friday, and the next, until gradually others joined her and her story spread and Fridays for Future became a global phenomenon.

Critics call her a publicity stunt and claimed she is manipulated by organisations with ulterior motives but in her interviews and presentations, she has never looked anything but in complete control of her own opinions and actions.

At the COP24 session, her father became briefly tongue-tied, explaining how Greta had exposed her parents’ hypocrisy in supporting human rights causes while ignoring the greatest threat to humanity.

They had learned they must “live as we spreech”, he said, before excusing himself and trying again. On the third attempt, Greta leaned over, bent his microphone to her and helped him out.

“Practice as we preach,” she corrected. “Practice as we preach.”

That fixed, she allowed herself the tiniest of smiles that lit up her habitually intense expression. Next to fix the world.