When scientists talk about "dimming the planet," they do not use analogies. Instead, they refer to solar geoengineering, which includes intentionally reflecting a portion of sunlight into space to cool the Earth, and has moved from science fiction to actual policy discussions. Proposals such as pumping aerosols into the stratosphere, brightening marine clouds, and installing reflecting particles in the high atmosphere are now receiving research funding in the United States, Europe, and China. Supporters argue that these tactics could temporarily lower global temperatures and buy time in the climate struggle....