As alarm over global warming spreads, a radical idea is gaining momentum. Forget cuts in greenhouse gas emissions, some scientists argue. Instead, bounce sunlight back into space by pumping reflective nanoparticles into the atmosphere. Launch mirrors into orbit around the Earth. Make clouds thicker and brighter to create a "planetary thermostat."

These ideas might sound like science fiction, but in fact they are part of a very old story. For more than a century, scientists, soldiers, and charlatans have tried to manipulate weather and climate, and like them, today's climate engineers...