The idea of geoengineering — tampering with the Earth’s climate to fit our needs — has been a favorite trope of science fiction since the 1920s. In the 1970s, Carl Sagan speculated that we could terraform Mars to make it into a second Earth. That inspired novelist Kim Stanley Robinson to write his Mars trilogy — Red Mars, Green Mars, and Blue Mars — in which he imagines how that scenario would play out. Robinson relied on actual science — and there’s plenty on this subject. As the dangers of climate change become imminently clear, some scientists believe that geoengineering’s time has come — not on Mars, but on Earth. Yale professor and atmospheric scientist Trude Storelvmo studies cirrus clouds, which tend to trap heat in the atmosphere. She analyzes what would happen if the clouds were seeded with ice crystals that would thin them. “If you don’t put enough of these seeding particles into the upper atmosphere, you would get no effect at all,” Storelvmo says. “But if you put too much,
