Santer’s work is called “fingerprinting”—because natural climate variation leaves different patterns and traces than warming caused by greenhouse gases. Santer looks for these fingerprints. The most important one involves two parts of our atmosphere: the troposphere, the warm blanket closest to the Earth’s surface, and the stratosphere, the thinner, colder part above it. Physics tells us that if the Sun were causing global warming—as some skeptics continue to insist—we’d expect both the troposphere and the stratosphere to warm, as heat comes into the atmosphere from outer space. But if the warming is caused by greenhouse gases emitted at the surface and largely trapped in the lower atmosphere, then we expect the troposphere to warm, but the stratosphere to cool.