In today’s Britain, a rare public figure can bring together Brexiteers and Remainers, Conservatives and Labour. Yet the teenage climate activist Greta Thunberg did just that on a visit to London in April, when she was feted by British politicians from across the political spectrum. In an address to Parliament, Thunberg said she spoke for the children who had been betrayed by politicians and voters who had failed to prevent climate change. She also claimed to speak for the unborn billions of people who will bear the brunt of a rapidly warming world. “I am 16 years old,” she said....