wo years ago, the crew of the Po- larstern, a German icebreaker frozen into Arctic sea ice, shot a green laser up into the night. The beam’s reflected light was meant to help researchers study icy winter clouds. Instead, the beam encountered something unexpected: a kilometers-thick layer of particles in the stratosphere, more than 7 kilometers up. The haze, the researchers later concluded, was smoke from enormous wildfires that had ripped through Siberia that summer.