particles into the upper atmosphere.3 Budyko’s proposal was translated into English in 1977. It was briefly known as “Budyko’s blanket,” but mentions of it in the scientific literature and especially public climate discourse soon disappeared.
A 1992 National Academies report picks up on the possibility,4 but it was not until the 2000s that the technology reemerged in broader scientific and climate conversations. After hearing vague mentions of solar geoengineering in the early 2000s, followed by quick dismissals, I first encountered solar geoengineering in earnest shortly after the late Nobel laureate Paul Crutzen wrote his now famous essay presenting stratospheric sulfur injections as a possible way “to resolve a policy dilemma.”5
The dilemma: Air pollution in the form of sulfur dioxide (SO2) kills millions each year; it also helps cool the planet. For example, Europe having begun to clean up its air pollution in the 1980s was clearly beneficial. Medieval cathedrals were no longer melting under acid rain. Forests – and people – are healthier. However, the Arctic is now around 0.5°C warmer as a direct result of decreased SO2 emissions.6 These are clear tradeoffs.
Crutzen, in his essay, presented this moral quandary. His essay was published jointly with one written by the late Ralph Cicerone, himself a famed atmospheric scientist and then the President of the U.S. National Academies of Sciences, who wrote in support of Crutzen’s controversial essay and of further research.7 While Crutzen and Cicerone’s essays did much to lift the self-imposed research moratorium, skepticism throughout the research and policy communities has remained to this day. I would hasten to add that much of that skepticism is, in fact, still healthy. Solar geoengineering is not a topic one should “embrace,” in any sense of the term. That goes for policymakers as much as for researchers “merely” trying to answer lingering scientific questions. To this day, much of the skepticism, in turn, can be explained by “moral hazard” worries, a topic we will discuss in depth in Chapter 7.
Narrowing down “geoengineering”
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.
A possible role for carbon removal and solar geoengineering
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
