Solar geoengineering solves---their ev doesn’t assume that different technologies will be combined to minimize the drawbacks of each

*GHG = Greenhouse Gas, SAI = Solar Aerosol Injection, SG = Solar Geoengineering, MCB = Marine Cloud Brightening, CCT = Cirrus Cloud Thinning

As discussed in Chapter 1, current scientific understanding makes clear that the changes in climate are being driven by the rapid rate at which greenhouse gas (GHG) concentrations are increasing in the atmosphere. GHGs are relatively transparent to incoming solar radiation, but they absorb (and reemit) infrared (IR) radiation emitted from Earth’s surface. As GHGs accumulate, energy is retained longer in the global climate system. This raises temperatures and causes many other changes within the Earth system. Aggressive action to stabilize and reduce atmospheric GHG concentrations can address this problem directly. However, given the enormous risks that climate change poses now and in the future, a variety of complementary strategies— including strategies based on increasing the amount of sunlight reflected back into space—are being considered as possible options to help stabilize the climate and protect human safety worldwide. There are two main approaches considered herein for increasing how much incoming solar energy is reflected back to space: stratospheric aerosol injection (SAI) and marine cloud brightening (MCB). In addition, this report considers cirrus cloud thinning (CCT), which differs from the other strategies in that it focuses on increasing outgoing longwave radiation and thus is not technically “solar” geoengineering. Each of these three approaches would affect Earth’s radiation balance in different ways, and they are described briefly below and illustrated in Figure 2.1. SAI is the most studied and best understood of the SG approaches proposed to date. It is based on increasing the number of liquid or solid particles in the stratosphere, where they can reflect sunlight. Unlike the highly turbulent troposphere, the stratosphere is relatively stable, and the aerosols in this region of the atmosphere can remain for 1 year or more before being transported to the troposphere and eventually removed by sedimentation and precipitation. Large volcanic eruptions (e.g., the 1991 eruption of Mt. Pinatubo) add significant amounts of hydrogen sulfide (H2S) and sulfur dioxide (SO2) into the stratosphere, where they are oxidized to form sulfuric acid (H2SO4), which then forms reflective sulfate aerosols. The Mt. Pinatubo eruption is estimated to have cooled global mean temperatures by approximately 0.5°C for 1 year or more (IPCC, 2013). A similar effect could, in principle, be achieved deliberately, either by adding SO2 or H2S gas, adding sulfate particles directly, or adding some solid particles such as calcite. While sulfate has the advantage that it is more directly analogous to material expelled by volcanoes, it also absorbs IR radiation and can pose undesirable effects on atmospheric chemistry (discussed later in this chapter); these concerns have motivated some exploration of other aerosol choices. Because aerosols spread relatively uniformly in longitude and are transported broadly poleward in latitude during their lifetime in the stratosphere, the cooling effects of SAI would be inherently global. Regional-scale climate impacts may vary considerably, however, and the details of impacts at this scale are highly uncertain and will depend on how SAI is deployed (e.g., choices such as the latitude at which it is injected and the aerosol material used). MCB is based on the idea of cooling Earth by increasing the reflectivity of low clouds over certain parts of the ocean. As an analogue, under the right conditions, the aerosol pollution from ships leaves behind a “ship track” caused by the emitted aerosols acting as additional cloud condensation nuclei. For the same total cloud water content, more droplets (from more nuclei) result in higher surface area and a more reflective cloud. It has been proposed that the same effect could be achieved by spraying a fine mist of salt water into the marine atmosphere. NRC (2015) provides a detailed review of the decades of research on aerosol and marine cloud interactions, including ship track studies. Yet despite this large research base, many uncertainties remain regarding MCB strategies, including limited understanding of aerosol-cloud interactions and how these interactions affect a cloud’s total water content and lifespan. This understanding needs to be improved in order to reliably project where, when, and by how much cloud albedo could be modified. The expectation is that MCB would be implemented at the regional level, potentially allowing more targeted interventions (e.g., to protect a specific coral reef ecosystem). However, if the MCB forcing could only be applied over a relatively small fraction of Earth’s surface, actually reducing global mean temperature would require a relatively larger radiative forcing (RF) to be applied over that smaller area, likely inducing more spatially heterogeneous climate responses than SAI. It is also important to recognize that for MCB the albedo modification effect is localized, but the resulting cooling effects are not, since the atmosphere will transport the changes in heating to other areas. Cirrus clouds—thin wispy clouds composed primarily of ice crystals that form in the upper troposphere—warm the planet (particularly at higher latitudes) because they reduce outgoing longwave radiation more than they reflect incoming sunlight. Reducing cirrus cover would thus produce a net cooling. It has been hypothesized that in the right conditions, it may be possible to seed cirrus with ice nuclei that would lead to fewer, larger ice crystals, with higher fall velocities, thus decreasing lifetime and hence cirrus cover. This approach would only work in locations where cirrus clouds form through homogeneous freezing—that is, where there are not currently enough ice nuclei to allow heterogeneous freezing. If there are already sufficient ice nuclei in some regions, then adding more could have the opposite effect of leading to more, smaller ice particles with longer lifetimes—and hence a warming effect. CCT is the least well understood of the three methods considered herein. If CCT were feasible, it has the advantage that it works by increasing the outgoing longwave radiation and thus more directly compensates for the radiative effects of increased atmospheric GHG concentrations. The maximum RF achievable with this method would be limited by the amount of cirrus cover currently formed through homogeneous nucleation. It should be noted that applying combinations of SAI, MCB, and CCT strategies could offer possible ways to leverage the benefits and reduce the negative consequences of each approach individually. But there has to date been little to no focused study of these combined strategies.