Last year, the planet was plagued by powerful hurricanes, blistering fires, and temperatures that ranked as some of the hottest on record—ratcheting up concern that we’re already knee-deep in climate change. To stave off the heat, some scientists have proposed blanketing Earth in a sheet of sunlight-reflecting particles called aerosols. This solar shield could cool the planet and buy us time, but a new study suggests that if politicians turned off the hypothetical cloud, they could plunge the planet into a sudden ecological Armageddon. “Gradual climate change, which is happening now, is bad enough, but if we do something that...