This Science Brief discusses Solar Radiation Modification (SRM), or solar geoengineering, which are technologies aimed at cooling the planet by
Solar radiation management or solar geoengineering is a large category of diverse climate engineering approaches that mitigate or reverse Global Warming by reflecting sunlight (i.e., solar radiation/shortwave radiation) into space before it is absorbed by the environment and converted into heat (i.e., transformed solar radiation, thermal radiation, thermal motion of particles, vibrational energy or longwave radiation). Solar radiation management also has approaches that try to move heat away from the Earth’s surface and/or outside our atmosphere (into space). Solar radiation management approaches protect the planet emitted wavelengths of light from the sun. The solar spectrum (the solar radiation that hits the Earth’s upper atmosphere) includes infrared (52-55%), visible light (42-43%) and ultraviolet (3-5%) (Figure 2). In the model, 30% (atmosphere (6%) + clouds (20%) + Earth’s surface (4%)) of the incoming solar radiation (shortwave radiation) is reflected back into space before it is converted to heat (thermal radiation or longwave radiation).
Nevertheless, James Hansen, director of the Program on Climate Science, Awareness and Solutions at Columbia’s Climate School, who first warned Congress about climate change risks in 1988, and a group of over 60 scientists are calling for more research into solar geoengineering. Most research into solar geoengineering strategies is currently focused on stratospheric aerosol injection (SAI, also called solar radiation management or SRM) and marine cloud brightening; other strategies include cirrus cloud thinning and the use of mirrors or sunshades. According to Gernot Wagner, co-founder of Harvard’s Solar Geoengineering Research Program and currently a climate economist at Columbia, the most important and determinative modeling variables are how high up in the stratosphere and where specifically SAI is deployed. In 2011, David Keith, Harvard’s Solar Geoengineering Research Program co-founder who is now at the University of Chicago, and atmospheric scientist Ken Caldeira estimated that to reverse 10% of the warming caused by a doubling of CO2 levels compared to the pre-industrial era, several hundred thousand tons of sulfur dioxide would have to be injected annually over a decade.
Stratospheric aerosol injection (SAI) involves the intentional introduction of aerosols into the stratosphere to create a cooling effect through global dimming
In this chapter, we briefly review what is known about proposed solar radiation management (SRM) approaches and related governance and ethical issues and conclude with a discussion of the research needed to better understand SRM. For use of SRM as a potential “backstop option” in the case of an emerging “climate emergency,” improved observations and understanding of climate system thresholds, reversibility, and abrupt changes (see Chapter 6)—for example, observations to let us know when an ice sheet or methane hydrate field may become unstable (e.g., Khvorostyanov et al., 2008; Shakhova et al., 2010)—could inform societal debate and decision making about needs for deployment of a climate intervention system. There is, however, additional research that would be needed to support full evaluation of SRM approaches (just as there is with other options for limiting the magnitude of future climate change), including a variety of social, ecological, and physical sciences (see Chapter 4).
The most-researched SRM method is stratospheric aerosol injection (SAI), in which small reflective particles would be introduced into the upper atmosphere to
**Solar Radiation Modification (SRM) refers to deliberate, large-scale actions intended to decrease global average surface temperatures by increasing the reflection of sunlight away from the Earth.** Proposed SRM methods involve the use of aerosols (small particles) or other materials to increase the reflectivity of the atmosphere, clouds, or Earth’s surface. **Long-term protection of Earth’s climate and oceans requires substantial reductions in emissions and atmospheric concentrations of CO2 and other GHGs. SRM is not considered a substitute for climate mitigation efforts, which include decarbonization and GHG emission cuts.** SRM research is being conducted as a response to growing concerns that the pace of CO2 emissions reductions and CDR technology development is not sufficient to avoid severe impacts of climate change in the next decades. **Many of the processes most important for understanding SRM approaches—such as those that control the formation of clouds and aerosols—are among the most uncertain components of the climate system.** Climate models differ in simulating large-scale aerosol climate effects, including on surface temperatures, due to variations in how aerosol processes, atmospheric transport and mixing, and physics are represented.
A lot of these proposed solutions almost sound like they’re out of a science fiction novel and you may have heard of them as “geoengineering.” This can include reflecting the sun’s energy back into space, filtering the solar energy reaching the Earth’s surface or capturing greenhouse gases from the atmosphere. The oversupply of heat already in the deep ocean, paired with changes in ocean surface temperature caused by SRM, could influence weather patterns, ocean currents and the supply of nutrients essential to ocean plant and animal life in ways that are difficult to predict. Changes in the amount or quality of solar radiation reaching the ocean because of SRM could alter photosynthesis by marine algae and other plants, with unknown follow-on effects for the marine food web and for the carbon cycle. Decisions about SRM will involve both atmospheric and oceanic governance. The NASEM study highlights just how much we need to learn still about climate intervention methods like SRM.