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A quick definitional detour is in order here, as “geoengineering” means different things to different people. In fact, the term is so vague and all-encompassing as to have lost much meaning, despite still being in frequent use. The term “geoengineering” itself is largely an artefact and a result of the term’s frequent use in popular discourse. Experts are typically more precise, and for good reason.

Except for the book’s cover – mea culpa! – I do not use the term “geoengineering” in this book without further explanation, apart from in direct quotations. I instead use either “solar geoengineering” or “carbon removal.” The two are sometimes subsumed under the broad heading of “geoengineering,” but the two are, in fact, very different. Neither, in turn, is the only term used for either category of interventions.

Solar geoengineering is sometimes also called “solar radiation management” (SRM), “solar radiation modification” (conveniently, also abbreviated as SRM), or traditionally also “albedo modification.” It is a largescale, deliberate intervention to cool the planet by sending a small fraction of sunlight back into space, or by increasing the amount of solar radiation that escapes back into space. The plethora of terms here already indicates the problem. While those working on the topic would immediately recognize the abbreviation “SRM,” and I have used it myself in peer-reviewed papers and op-eds alike, I will eschew its use here in favor of “solar geoengineering.” The reason for this nomenclature is simple: the “solar” modifies the all-too-popular broader term. That doesn’t make “SRM” any less accurate. It’s just another term for the same idea.

Here it’s also useful to dissect the definition a bit further. One operative term is “largescale.” Wearing white in the summer does not count, nor does painting roofs or streets white in an attempt to cool cities – though they are all good illustrations of the broader point. Black absorbs heat, white reflects it.⁸ Even all of us in any one hemisphere wearing black winter coats or white summer shirts at once, however, does not alter the global climate. Aerosols in the stratosphere do. “Budyko’s blanket” – stratospheric aerosols – thus, is the most commonly discussed method, though by far not the only one. (See Part I for more in-depth discussions of different solar geoengineering methods.) More precisely then, I will often refer to stratospheric aerosols as the specific solar geoengineering method.

Sometimes I will also explicitly discuss another set of technologies that are often subsumed under the broader “geoengineering” heading but that are entirely different: a set of techniques typically called carbon removal, carbon dioxide removal (CDR), carbon geoengineering, or direct air capture. All of these technologies remove CO₂ from the atmosphere directly. Their big advantage: they address the root cause of climate change – excess atmospheric CO₂. Solar geoengineering does not. That makes carbon removal an important part of the world’s collective climate response, especially given where things stand today. Carbon removal also comes with its own set of important caveats. Many are entirely different from concerns about solar geoengineering. The one area where they do clearly overlap is vis-à-vis moral hazard considerations, their interaction with efforts to cut CO₂ emissions in the first place (see Chapter 7).

One carbon removal technology is planting trees, in turn sometimes subsumed under a broader umbrella of “natural climate solutions.” That is surely part of the overall solution, but it can indeed only be one part of it. Planting trees might sound more innocuous than building large industrial facilities to take CO₂ out of the atmosphere; however, it also comes with significant limitations. One of these is the time and space needed to plant the billions of trees needed to make a dent in atmospheric CO₂ concentrations. Another is permanence. Trees decay, releasing CO₂ in the process. In technical terms, trees help take CO₂ out of the atmosphere, but they keep the carbon in the biosphere instead of returning it to the geosphere. Other carbon removal techniques do, in fact, remove CO₂ from the biosphere entirely.

Meanwhile, even planting trees has now been used as a delaying tactic to avoid doing what’s necessary. U.S. Republicans under President Donald Trump, for example, have used their “One Trillion Trees” initiative as a way to detract from the need to cut CO₂ – moral hazard in action, or perhaps better: moral hazard inaction. None of this, of course, means that we should not be planting more trees. We should. However, we must not use it as an excuse to delay CO₂ emissions cuts.