Slate: When, in late 1965, the problem of climate change first arrived on the desk of the president of the United States, Lyndon Johnson’s advisers presented him with only one possible solution: geoengineering. They suggested that scattering small reflective particles over millions of square miles of ocean might reflect enough sunlight away from Earth to moderate the warming. After lying dormant for more than a generation, geoengineering is back with a vengeance. A rush of new books and …