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Most importantly, we must stop burning fossil fuels and putting CO₂ into the atmosphere. Nothing else will do. There are indeed other, even more potent, and thus important greenhouse gases. Methane (CH₄), for example, might be more important than CO₂ for the rate of global warming – something solar geoengineering, too, has a direct role in affecting (see Chapter 2).⁹ Nitrous oxide (N₂O) is similarly more potent than CO₂, around 300 times so on a 100-year timescale. And yes, technically water (H₂O) is the most important greenhouse gas of them all. However, human CO₂ emissions stand alone in their long-term influence on the changing climate.

Cutting CO₂, even to zero, will only stop the further increase in climate impacts. It won’t stop them altogether. That immediately leads to another important step: coping with what’s already in store. Not unlike both carbon removal and especially solar geoengineering today, mentioning climate adaptation was once considered taboo among many committed environmentalists, and for similar reasons. “Let’s stop climate change first,” the refrain went in the 1990s, “only then can we start talking about adapting to warming already in store.” Even Vice President Al Gore believed as much at the time, considering adaptation a mere distraction. He has long since publicly changed his mind on the topic.¹⁰

Adaptation, of course, can only go so far. For one, there are the usual endemic inequalities. It’s the rich who adapt. The poor suffer. Then there are limits to adaptation. Building a seawall to protect against extreme storm surges is one thing; adapting to one or two meters of sea-level rise by century’s end by moving entire cities to higher land within decades is quite another. Parts of Miami are flooding today, on sunny days.¹¹

Enter carbon removal, taking excess CO₂ out of the atmosphere and, ideally, putting it back underground, into the geosphere. Carbon removal, meanwhile, comes with important caveats of its own, not least the same kind of moral hazard that beset earlier adaptation conversations. Equally important, much like cutting CO₂ emissions in the first place, removing it from the atmosphere is both slow and, for the most part, relatively expensive.

Solar geoengineering, by contrast, is fast, cheap, and imperfect.¹² These three characteristics make solar geoengineering unique among possible climate policy interventions. They also go to the heart of the solar geoengineering gamble. Little is fully known and, thus, certain. Lots depends on details yet to be worked out, and some may never be known for sure. Governance is key. Each of the three core characteristics figures in this assessment.