Solar radiation management

DEFINITION: Subfield of geoengineering concerned with the alteration of Earth’s climate through changes in the interaction between sunlight and the planet

A small but growing number of scientists have suggested that strategies to limit global warming through reduction of greenhouse gas emissions may not work, either because of lack of willingness to limit emissions or because global warming may have already progressed to the point that it cannot easily be reversed. Solar radiation management has been proposed as a possible countermeasure in the event that limiting greenhouse gas emissions fails to halt global warming.

Most strategies for combating global warming involve reducing the amount of greenhouse gases in Earth’s atmosphere. The strategy of solar management, in contrast, involves either limiting the amount of sunlight reaching Earth’s surface (solar mitigation) or reflecting some of the sunlight that does reach the surface back into space (albedo modification). Both approaches would result in less of sunlight and thus less solar heating of the planet. Theoretically, less solar heating could counter the increased atmospheric heating linked with greenhouse gases.

Albedo Modification

One way of limiting how much sunlight is absorbed to heat Earth is simply to reflect the light back into space. The fraction of sunlight striking a planet that is reflected is called the albedo of the planet. The higher the albedo, the more light is reflected. Variations in albedo occur naturally on local scales. On cloudy days, sunlight is reflected into space by clouds, causing the temperature to be lower on the ground. One suggested approach to solar radiation management is to increase Earth’s albedo artificially to lower the planet’s temperature by reflecting sunlight into space so that it cannot be absorbed.

Several strategies for making modifications to Earth’s albedo have been suggested. One idea is to mimic the effect of cloudy days by making more clouds or making existing clouds thicker. Cloud seeding can be accomplished through the injection of into the atmosphere to form nucleation sites for water droplets. Other suggestions include using ships with powerful pumps to inject a seawater mist into clouds, making them thicker and more reflective.

Very large volcanic eruptions can send sulfur dioxide and other sulfur contaminants high into the stratosphere. These sulfur contaminants can be reflective enough to increase Earth’s albedo by several percent. Since the sunlight is reflected in the stratosphere, it does not reach the ground or the lower atmosphere, and the surface temperature of Earth drops. Historically, massive volcanic eruptions have often been followed by years of cooler weather. One idea for increasing Earth’s albedo involves injecting large amounts of similar sulfur contaminants into the to cause conditions that would mimic those that lead to the temperature reductions that sometimes follow massive volcanic eruptions.

Albedo modifications can be made on land, too. Some scientists have suggested that simply painting roads white and making the roofs of buildings reflective would have a local effect on weather, particularly near large urban areas. It is unclear whether making human construction artifacts more reflective would have a sufficient impact on global climate to compensate for increased greenhouse gases if that technique is not used in conjunction with other measures to limit global warming. More radical proposals have included the suggestion that reflective paint could be applied to deserts, to mimic the increase in albedo of snowfall. Painting the ground white near where ice has melted is one possible strategy to limit the positive feedback of melting polar ice caps.

Solar Irradiation Mitigation

Some volcanic eruptions inject more light-absorbing than reflective particles into the stratosphere. This ash, by absorbing light, reduces the sunlight that reaches Earth’s surface, in much the same way as reflecting light would do, causing a cooling of the planet’s surface. Some scientists have suggested that the injection of or even dirty airplane exhaust into the stratosphere could act to cool Earth’s surface. This strategy has many drawbacks, however. Particles absorbing solar energy in the stratosphere, although reducing the absorption of solar energy at the ground, still result in solar energy being absorbed somewhere in the atmosphere, possibly resulting in an eventual change in atmospheric structure. This could eventually lead to temporary reduction in the temperature on the ground at the expense of much larger climate disruption years later.

One very ambitious proposal calls for deploying a cloud of mirrors in space between Earth and the sun. Through the deployment of enough objects to reflect less than 2 percent of the sunlight reaching Earth, a thermal balance could be reached, offsetting the effect of increased atmospheric greenhouse gases. Once launched, such a space-based system would require far less effort to maintain than other solar radiation management plans. Reflectors in space are passive systems, so they continue to operate even if they are not actively maintained. The only maintenance required would be to make sure that they stay in stable orbits. This would be achievable if they could be placed in high orbit around Earth, but there they would interfere with spacecraft and the activities of communication satellites. Another possible location for a space-based solar shield would be at the point between Earth and the sun known as the L1 Lagrange point. A gravitational balance between Earth and sun causes objects at the L1 point to tend to stay near that point.

A space-based shield would not raise the concerns associated with the injection of pollutants into the atmosphere to alter Earth’s albedo. Furthermore, because such a shield would make no modifications to the atmosphere or to land on Earth, it would not have the possible negative environmental impacts that other approaches to solar radiation management would have. A major difficulty with implementing any plan to create a space-based shield, however, is that it is extremely expensive to launch objects into space.

Criticisms

Numerous criticisms have been leveled against the suggestions that have been made by scientists examining the possibility of solar radiation management. Critics point out that any approach to solar radiation management on a global scale would be exceedingly expensive, would require a great deal of international cooperation, and would require significant investments of time and effort. Even proponents of geoengineering admit that implementation of any of these plans on a global scale would be an extremely difficult and expensive undertaking; they assert, however, that solar radiation management may be the only way to check otherwise unstoppable climate change. There is no agreement among climate scientists on how much climate change is unstoppable without such drastic measures.

Solar radiation management is often seen as only a temporary measure—it may have an effect opposite to that of greenhouse gases, but it does not address the root problem of excessive concentrations of atmospheric greenhouse gases. If solar radiation management programs were to be deployed successfully and then ever stopped for some reason—for example, because of war or the economic collapse of the nations supporting the programs—then the high levels of greenhouse gases in the atmosphere could potentially result in a catastrophic rise in global temperatures. Even proponents of solar radiation management thus often suggest that it be only one part of a multipronged approach to controlling climate change.

A further criticism of solar radiation management is rooted in the fact that solar heating drives many aspects of Earth’s ecology. Interfering with natural solar radiation could potentially result in such unintended consequences as changes in the character and frequency of storms, which could result in drought and flooding in diverse areas of Earth. Altering the climate in such a way may also change the acidity of the oceans, resulting in wide-scale extinctions.

Environmentalists often oppose atmospheric modifications such as injecting materials into the stratosphere or into clouds because, they argue, these actions amount to intentional of the planet; the materials that scientists have proposed injecting into the atmosphere could have deleterious effects on the health of humans and other species. Proponents of these plans argue that despite the risks of possible negative health effects, these effects would ultimately be less damaging than the effects of unchecked global warming. Space-based solar radiation management plans would have the fewest potential negative impacts on Earth’s environment, but they would also be among the most expensive to implement.

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