It began with a solitary, 15-year-old girl camped out in front of Sweden's parliament next to a hand-written sign: "SCHOOL STRIKE FOR CLIMATE".
Barely six months later, Friday's youth-led strike -- promising 1,000 actions spread over 100 countries -- could be a "milestone moment" in a grassroots campaign to goad world leaders into confronting the threat of global warming, activists and experts say.
"We are only seeing the beginning," tweeted Greta Thunberg, who has turned 16. "I think that change is on the horizon and the people will stand up for their future."
So far, the weekly walk-outs have seen tens of thousands of mostly high school students spill out into the streets in Germany, Belgium, Britain and France, with a smattering of actions in half-a-dozen other countries.
But today, classrooms are set to empty in cities across the globe, from Boston to Bogota, Montreal to Melbourne, Dhaka to Durban, Lagos to London.
Some local and national leaders have tried to cajole or threaten students, but such efforts have mostly backfired.
Climate change impacts vary from one region to the next, but teens everywhere confronted with a climate-addled future have decided where to direct their ire.
"Dear adults, use your power!", has become an unofficial slogan for the #FridaysForFuture movement.
In France, the message scrawled on posters in the street is more pointed: "In 2050 you will be dead, not us."
The crescendo of impatience expressed by youth toward their elders is not hard to understand.
Despite 30 years of warnings about dire impacts, carbon dioxide (CO2) emissions hit record levels in 2017 and again last year. Loading the atmosphere with greenhouse gases at current rates, scientists agree, will eventually lead to an unlivable hothouse planet.
The problem, in other words, continues to outstrip efforts to rein it in.
Along with Greta's movement other groups have sprung up in the last six months, a new generation of climate activists has adopted a long-haul strategy of civil disobedience.
"We are the last generation to have a realistic chance to prevent a climate catastrophe," said Global Strike for the Future, one such group, spokesperson Linus Steinmetz.
“If adults won't comply with the rules, we won't either."
Despite facing obstacles in other countries, Greta Thunberg found support from MPs of her country. Norwegian lawmakers have nominated the fiery 15-year-old for Nobel prize.
Meanwhile, four environmental NGOs yesterday sued the French government for failing to uphold its commitments on fighting climate change, the latest in a series of high-profile cases that aim to force governments around the world into action.
Over two million people have signed a petition backing the move, which follows similar cases lodged in the Netherlands, Pakistan and Colombia that yielded historic wins for campaigners.