As capitalist society remains incapable of addressing climate breakdown, one measure is waiting in the wings: solar geoengineering. No other technology can cut global temperatures immediately. It would alleviate the symptoms of the crisis, not its causes. But might it be combined with radical emissions cuts? This essay, the first instalment of two, scrutinises the rationalist-optimist case for geoengineering: the idea that soot planes in the sky can shield the Earth from the worst heat while society rids itself of fossil fuels. A more likely outcome is that they encourage business-as-usual to continue, while negative side-effects from geoengineering itself pile up. The logic of the enterprise points in the direction of a catastrophic termination shock. A subsequent, second instalment will subject geoengineering to a materialist psychoanalysis and argue that it represents a fantasy of repression, setting itself up for a dreadful return of the repressed.