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Editor’s Note: This is the second in a two-part series on the case for researching the taboo concept of solar geoengineering as a potential bridge to achieving net zero emissions. Part one summarizes some concerns raised about solar engineering. This piece analyzes the use of stratospheric aerosol injection and what we need to research further if we want to know how to effectively deploy and regulate it.
Achieving net-zero emissions remains far off, and ambitions for limiting warming to less than 1.5 degrees Celsius have continued to dim, as evidenced by the most recent U.N. climate conference. As a result, interest continues to grow in exploring geoengineering as a potential way to mitigate climate impacts. For reasons we explain below, however, it is likely that a decade or more of further research is needed before geoengineering deployment could be realistically considered. Moreover, this research could show that the technology is not worth pursuing at all.
As we explained in our previous article, solar geoengineering remains taboo and, until recently, has received relatively little attention as a serious policy option. Research funding has been scant. The result is that we know relatively little about how to effectively deploy geoengineering and even less about its potential impacts, including the distributional consequences of using these technologies.
Greenhouse gases (GHGs) warm the planet by preventing energy from escaping Earth’s atmosphere. In theory, one way to counteract this effect is to limit the solar energy that enters the climate system in the first place. “Solar geoengineering” encompasses a group of proposed technologies, all of which would reflect a small amount of incoming sunlight back into space in order to reduce the total amount of energy in the atmosphere and thereby cool the atmosphere. These technologies range from altering clouds to make them more reflective to putting space mirrors into orbit around Earth. The most researched and best understood technology is known as stratospheric aerosol injection (SAI). SAI would involve deploying a “veil” of reflective aerosol particles into the stratosphere and then reapplying it every year or two, as needed, to replace the aerosols that inevitably fall back to Earth.
We know, with fairly high confidence, that SAI can reduce global average temperatures because scientists have observed that aerosols injected into the atmosphere by volcanic eruptions have a measurable, but temporary, impact on global average temperatures. (Ironically, aerosols emitted from burning fossil fuels also have a similar cooling impact and have kept Earth cooler than it would be otherwise given the increase in GHG concentrations in the atmosphere.)
The current state of knowledge about solar geoengineering was summarized in a 2021 report by the National Academies of Science (NAS). As that report lays out, SAI is likely to have various side effects, and those side effects could increase with the magnitude of the cooling effect. For instance, scientists are somewhat confident that SAI would alter rainfall patterns, decreasing precipitation in some areas and increasing it in others—though it’s unclear how these changes might manifest regionally, or whether such risks would be even greater in a warmer world without SAI. The other effects of SAI on the climate system, in terms of kind and scope, remain even more speculative but are nonetheless concerning. For instance, scientists believe that SAI may lead to a somewhat stronger polar vortex over Europe and Asia, compared to climate scenarios without SAI. It could further damage the ozone layer. And, by changing the amount of diffuse (versus direct) sunlight that reaches Earth’s surface, SAI could adversely affect plant life, including agricultural crops.
And we know that SAI would not restore the preindustrial climate. The NAS report explains that, relative to a preindustrial climate, SAI could cause relatively cooler summers and relatively warmer winters, “over-cool” the tropics while “under-cooling” high latitudes, and result in a somewhat drier climate. It is possible that this variability could be limited through deployment choices, although the effectiveness of such tailoring will remain unknown without further research.
Relatedly, because SAI is not a perfect substitute for the preindustrial climate, solar geoengineering would not address all aspects of carbon dioxide pollution, including some of the most harmful impacts of climate change. It wouldn’t halt the ocean processes that are destabilizing marine ice sheets from underneath, stop coral bleaching, or reverse ocean acidification.
Beyond these conclusions, however, it is difficult to overstate our ignorance. As the NAS report politely put it, “[O]ur ability to estimate climate responses and the downstream impacts of those responses is currently very limited” (emphasis added).
For instance, we lack basic information on the mechanics of cooling and therefore are unsure how to optimize cooling and minimize adverse effects. The NAS report explained that scientists are still debating what kinds of aerosol particles to use to reflect sunlight, the volume of such aerosols necessary for a given level of cooling, where to release these aerosols (latitude and altitude), the optimal timing for injecting the particles (for example, during which season), how often to reapply these aerosols, and the best method for delivery (for example, new aircraft would need to be designed to reach the necessary altitudes and deploy material). These are not questions at the margins, but serious uncertainties about how to deploy SAI to get the desired cooling outcome. In short, despite the growing interest SAI has received, we know shockingly little about how to effectively deploy aerosols to achieve a desired climate effect.
We also don’t fully understand the consequences of deployment on the climate. We outlined some “known unknowns” above, for instance, regarding the impact on plant life, but there may be “unknown unknowns” as well. Adding another layer of uncertainty, impacts (both known and unknown) are unlikely to be evenly distributed geographically. How changes to precipitation, plant life, and temperature would manifest regionally is likely to be uneven, and we cannot yet predict which regions will bear which impacts. While there might be ways to mitigate some of SAI’s negative effects depending on deployment choices, it is unlikely that all of them could be mitigated. In other words, trade-offs would be inherent to deployment choices; getting a little more (beneficial) precipitation in one region could result in adverse impacts in another. In worst-case scenarios, the regional impacts could be catastrophic, for instance, by significantly disrupting the Indian and Asian monsoons.
The nature and scope of our limited knowledge about the consequences of SAI deployment make it too early to draw conclusions about the desirability or inevitability of geoengineering deployment—which also means it is premature to make decisions in the fraught debate over the governance structures necessary for that deployment. (As a related matter, these uncertainties also make it unlikely that a rogue actor or state will deploy SAI on a planetary scale in the near future, limiting geopolitical concerns for the moment.) We do not know much, and certainly not enough, to evaluate whether SAI is a worthwhile policy choice, and what we do know suggests that it is far from obvious whether geoengineering will ever make sense, whether assessed as a scientific, political, or moral matter.
Yet we suspect that, despite these uncertainties, SAI will look increasingly attractive as the climate crisis worsens. All the more reason to start serious research on SAI now, before anyone becomes desperate enough to make an uninformed choice.