Greta Thunberg was detained last month in Germany, during a sit-in at a munitions factory that sells weapons to Israel. She was wearing a Yalla Intifada T-shirt. Perhaps you thought Thunberg was a climate activist. She used to be. Thunberg gained fame by warning that greenhouse-gas emissions posed a threat to human life so urgent and existential that the world must put aside trifling concerns to solve it immediately. “I want you to panic. I want you to feel the fear I feel every day. And then I want you to act. I want you to act as you would...