Solar geoengineering

Solar geoengineering, or solar radiation management (SRM), refers to the use of various technologies and techniques to reflect a fraction of sunlight back into space or increase the amount of solar radiation that escapes back into space to help cool the planet and mitigate the effects of rising temperatures and climate change. These techniques stem from emerging technologies known as geoengineering and aim to break the link from concentrations of temperatures and reduce some potential climate damages. There are several proposed geoengineering technologies, including marine cloud brightening, cirrus cloud thinning, stratospheric aerosol scattering, and space-based technologies.

Geoengineering is a set of technologies intended to manipulate the environment to partially offset some of the impacts of climate change. Geoengineering is conventionally split into two broad categories: geoengineering, or carbon dioxide removal, and solar geoengineering, also referred to as solar radiation management.

One issue looming over all solar geoengineering approaches is how they will be implemented. These approaches are largely considered to be supplemental to mitigation techniques, in that the approaches can help return the planet to pre-industrial conditions or slow the progress and damages of increasing temperatures across the globe and offer more time for mitigation techniques to address the underlying issues of climate change. However, there remain concerns that some could see solar geoengineering as a "quick fix" to climate change, allowing those to backpedal on projects to cut greenhouse gas emissions or create other efficiencies. Similarly, solar geoengineering cannot address certain effects of climate change, such as ocean acidification, as it does not remove carbon dioxide from the atmosphere.

Further, concerns around the potential impacts of the technology—especially unforeseen impacts on other climates or the potential of weaponizing the technology to create unfavorable climate conditions, such as drought conditions, and increase pressure on an adversarial country—have led to many calls for the technology to be regulated globally. This potential has led for calls for regulations and laws around launching largely untested and potentially harmful technology into the Earth's stratosphere. And that governments should come to agreements around the use of the technology and transparency around the use of the technology to ensure other countries cannot use them as part of an overall aggressor strategy.

Others would want governance to ensure that any experiments or implementations of solar geoengineering be properly scrutinized and tested to ensure confidence in the technology and reduce as many unintended side effects. This type of governance would require, according to some, transparent scientific research with open scrutiny and debate for a full understanding of the potential harms and effects of any community expected to bear the brunt of these changes. They would also require mechanisms for oversight and ensure that any conditions of outdoor experimentation are understood and allowed to take place.

The maintenance of deployed solar geoengineering technologies is also of increasing concern. The concern centers around two major points: how easy or expensive the approach is to maintain, and how possible it is to stop the approach in the case of unintended consequences. For example, the cost of maintaining approaches such as space-based sunshades or ocean mirrors could make the attempts prohibitive, while the carbon emissions of other effects could create a net negative effect overall.

Meanwhile, if solar geoengineering proved capable and masked significant warming effects of the carbon dioxide in the atmosphere, and that technology was stopped abruptly and not resumed, this could result in rapid climate warming and consequences more severe than a gradual rise in temperature of the same magnitude. Known as "termination shock," this effect, in the opinion of some researchers, could be relatively easy to prevent as any state would resume the terminated deployment.

Others have noted that once starting solar geoengineering, it would be basically impossible to stop. This is true in technical terms for some approaches—such as stratospheric aerosol injection, which, once injected, would be increasingly difficult to remove the particles from the atmosphere. However, in short-term strategies—which tend to be approaches that have a natural lifespan of a decade or less—this runaway solar geoengineering is less of a concern. Rather, the reliance on solar geoengineering to maintain a climate equilibrium could reduce incentives to change habits and, therefore, would be impossible to stop because stopping would return to the concerns over termination shock.

Example of a stratospheric balloon injecting reflective particles in the stratosphere to constrict sunlight.

Stratospheric aerosol injection (SAI) is the injection of tiny reflective particles, or aerosols, into the upper atmosphere. These particles are intended to reflect sunlight and thereby restrict the amount of solar radiation reaching Earth and cooling the planet. This is a simulation of what occurs during volcanic eruptions, in which small particles enter the upper atmosphere and lead to a cooling period while they remain in the stratosphere. In the case of volcanoes, this period may be a few years after injection. In the case of SAI, mimicking the effects of a volcanic eruption to affect global temperatures is the goal, but the duration of those particles in the atmosphere could be difficult to guess, and attempting to achieve or reverse the procedure has been considered even more difficult. Furthermore, some researchers have warned of several unknown effects, pointing to the potential of altering precipitation patterns across the planet in unforeseen ways.

Example of techniques and effects of marine cloud brightening.

Marine cloud brightening (MCB) is another solar geoengineering approach, which uses sea salt to stimulate cloud formation over the ocean to help reflect sunlight in the region. This would involve spraying salt into low-lying marine clouds to increase their reflectivity and increase regional-scale cooling. This could, in theory, involve the use of ships to spray saltwater into the clouds and is assumed to be capable of being used over a very specific area, unlike SAI. This could allow MCB to cool areas over a coral reef or in a particular region off the Gulf of Mexico that could reduce storm strength through an area. A few studies on MCB have looked at the potential impact, and the modeling has suggested that MCB could be used to restore temperature and ice cover, address coral bleaching, and have mixed impacts on global crop yields.

However, MCB has concerns, specifically over its effect on other aspects of a climate system, especially a localized climate system, which would have a greater effect than something like SAI, which has a smaller impact over a wider area. In MCB, there is a large and localized effect that could result in regional variation in the climate response and could result in unfavorable weather in specific parts of the world.

Example of low and high albedo roofs.

Another proposed technique is to increase the albedo of buildings in order to reflect more sunlight. This would involve, put simply, making rooftops and walls brighter by painting them white or increasing their overall reflectivity. This technique is less controversial and less likely to make a large difference to global temperatures. It could be used in cities to counteract heatwaves and potentially reduce the highest temperatures and the health problems populations experience during these heatwaves. White buildings are already common in areas such as the Mediterranean.

The technique has also been suggested to be applied to crops, potentially by introducing genes that could give plants a waxy sheen. Research into this potential has remained theoretical, with the theory maintaining that more reflective crop species could be developed to decrease local and regional temperatures and help with heatwave management. However, this technique has potential negative side effects, such as reducing crop productivity and overall food production. Interest in high-albedo crops has come from the potential to use the technique on a smaller scale and reduces the risk of cross-boundary conflict or termination shock.

Another well-known approach to solar geoengineering is the use of ocean mirrors. Rather than large mirrors placed on the oceans, ocean mirror refers to the use of sea vessels to churn up millions of tiny microbubbles on the ocean surface. The sea-foam offers greater reflectivity than the ocean's surface, allowing it to reflect away sunlight and offer a cooling effect. This sea-foam has shown a reflectance ten times higher than the ocean itself, and if used to occupy a large part of the ocean (which itself occupies 71 percent of the planet's surface), could be used to reflect more sunlight. Further, ocean mirrors meet a requirement of climate intervention in that they can be shut down quickly if things start to go wrong, as the sea-foam is shut down when the bubbles burst.

While the possible cooling effect of microbubbles is quite large, there are potential drawbacks. One study found that the use of microbubbles could reduce the amount of sunlight reaching below the ocean's surface and negatively impact marine ecosystems, with an especially negative impact on marine plants and their ability to carry out photosynthesis, and therefore lead to a drop in the number of marine plants and unsettle the overall food chain.

Another study, using modeling to research the impacts of using microbubbles to address global warming, found the technique would not have the impacts on ocean productivity as previously believed but would require large amounts of energy expenditure to properly affect and maintain the bubbles for several days or weeks once created.

Another option suggested for reducing the effects of solar radiation is the removal or thinning of cirrus clouds from the atmosphere. Cirrus clouds are thin, wispy clouds made of ice crystals that form at high altitudes. While these clouds reflect some sunlight, they also absorb large amounts of long-wave radiation, which means they overall warm the planet. The heat-trapping effect of cirrus clouds is considered to be so large that it exceeds that of human-released carbon dioxide. This means the removal or thinning of cirrus clouds could more than offset the warming caused by carbon dioxide.

Theoretically, it is considered possible to thin cirrus clouds through aerial vehicles delivering and seeding the clouds with solid aerosol particles, such as desert dust or pollen. This seeding would cause the clouds to dissipate and lessen their overall effect. However, overseeding could lead to the formation of thicker and more persistent cirrus clouds, which could lead to additional warming rather than the intended cooling. Further, questions remain regarding how cirrus cloud thinning could affect other aspects of the climate system.

Example of a sunshade used to reduce the amount of sunlight reaching Earth.

Another often-considered approach to solar geoengineering involves sending a giant mirror, or fleet of mirrors, into orbit in order to reflect sunlight away from the Earth (or, in some cases, using solar panels if the problem of sending the captured electricity is solved). The size of the mirror would determine how much sunlight it could reflect back toward space and, therefore, its cooling effect. Some estimates suggest that approximately a 2 percent reduction in sunlight from a sunshade could be sufficient to offset the warming from a doubling of carbon dioxide from its pre-industrial level.

However, introducing a space mirror into orbit, either around the Earth or the sun, is a large technological challenge that would need to be constantly altered based on changes in atmospheric carbon dioxide. On the other side, sunshades implemented in space are not environmentally disruptive but would have prohibitively expensive technology when compared to other, relatively simpler, solar geoengineering techniques.