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Fast means that solar geoengineering, fully deployed, could help lower global average temperatures within weeks and months – rather than the years and decades that it would take for CO₂ reductions. For example, Mt. Pinatubo’s eruption in June 1992 in the Philippines lowered global average temperatures by around 0.5°C within a year. A year later, temperatures were back to normal and have been rising ever since (see Chapter 2).

Cheap is relative, but most estimates put the direct engineering costs for deploying stratospheric aerosols at a scale somewhere in the single-digit billions of dollars per year. Think of several dozen newly designed planes with large fuselages and enormous wingspans flying missions into the stratosphere around the clock.¹³ That’s not exactly free, but it might as well be. The direct deployment costs are in the single-digit billions of dollars, compared to cutting CO₂ emissions or removing carbon ex post, both typically measured in trillions of dollars. It is cheap enough to ensure that the direct costs do not matter meaningfully in a deployment decision made by the world’s governments.

Imperfect is just that: solar geoengineering does not address the root cause of excess CO₂ in the atmosphere. It comes with plenty of potential risks. It might be a really bad idea to contemplate, and worse to actually go through with. Equally important, none of that might matter in light of the first two characteristics, all but pushing the world toward deploying solar geoengineering sooner than most of us might deem possible – or desirable – today.

The combination of fast and cheap puts solar geoengineering at the exact opposite end of the spectrum from cutting CO₂ emissions in the first place. Whereas cutting CO₂ is all about motivating more people, companies, and countries to do more, solar geoengineering governance is largely about stopping premature deployment – doing it too fast, too much, stupidly.