One Friday in September, Swedish teenager Greta Thunberg set herself up outside the Swedish Parliament with handmade signs and a message: Climate change is here, it’s threatening the future, and the grown-ups in charge aren't taking it seriously—so now I, Greta, will go on strike for the climate. Someone had to do something about it, she told reporters later, so why not her?

Since then, her movement has swept around the world. From Poland to Colombia to Australia, thousands of young people have spent their Fridays striking in the name of climate change, signposting their activism with the hashtag #FridaysforFuture. Some step outside their school buildings for just a moment; others throng together, marching through the streets by the thousands. But