Skip to main content

Proposed methods of Stratospheric Aerosol Injection (SAI): Various methods have been proposed for Stratospheric Aerosol Injection (SAI) deployment including: aircraft, balloons, ballistics, and permanently tethered structures

Summary​

Stratospheric Aerosol Injection (SAI) deployment methods face many technical challenges, with proposed approaches including aircraft, balloons, ballistic systems, and tethered structures. Aircraft remain the most viable option despite requiring custom designs to reach 65,000 feet altitudes, with estimates suggesting 1 Mt of sulfur annually could achieve 1 W/m² radiative forcing. For perspective, the global air freight capacity in was 52.2 Mt. Alternative methods like high-altitude balloons face cost barriers, while theoretical concepts like tethered structures require major engineering breakthroughs.​

The technology and methods of deployment for SAI, let alone the research and experimentation of the technology, are considered to still be in the embryonic phase 1. The philosophical and material arguments against SAI, as well as the divergence of opinions on the appropriate degree of intervention required without running the risk of 'termination shock', make it difficult to know what deployment capabilities will be required.

Aircraft deployment​

The most likely form of SAI deployment would be through the use of aircraft as they are an existing technology, are a known quantity, and are seen to be the cheapest option 1. As runways and refueling stations are already heavily scattered throughout the globe, low degrees of auxiliary infrastructure would need to be built. A key challenge for SAI deployment via aircraft is the high altitude that these aircraft would need to fly. A trans-pacific passenger airliner has a cruising altitude of between 30,000 – 40,000 feet (9,144 – 12,192 meters), whereas there is broad agreement that SAI would need to be deployed at an altitude of at least 65,000 feet (20km) for effective cooling 1. 20km is just a minimum altitude, as higher altitudes would further increase the cooling efficiency by increasing the aerosol retention time, thus requiring less material to be injected 1. The requirement for high altitude aircraft is a non-trivial challenge, as the majority of existing aircraft are not optimized for this type of flight 2. It is for this reason that many researchers have proposed the creation of custom-built SAI aircraft specifically for high altitude operations and large payload delivery 2 3.

The payload size and delivery for aircraft SAI also need to be considered. Under the assumption that sulfate aerosols would be used via the precursor gas sulfur dioxide (SO₂), a relatively low thermal forcing of 1.00 W/m² would require 1 Mt of sulfur each year; however, this value is highly dependent on considerations such as location, altitude, and ramping 2 3. To put this value into perspective, the estimated combined mass of global air freight in 2015 was 52.2 Mt 4. It should also be noted that sulfur dioxide is a gas and would need to be compressed and cooled into a liquid or solid form for delivery. Studies have also explored the possibility of transporting sulfate aerosols directly or combusting sulfur in situ (on the aircraft), but the benefits of these methods are not yet clear 1.

Airships and balloons​

Airships have been proposed as a potential delivery method as they have the potential for high payload capacity and the potential for long endurance times resulting from low fuel usage 2. Airships run into the same issues as aircraft in that current technology for high altitude flight with a large payload is not yet available and are seen as more costly than aircraft 1.

Guns (ballistic, pneumatically, and magnetically propelled)​

Guns have also been proposed as an SAI delivery method. The main advantage of this propulsion is the ability to release aerosol at significantly higher altitudes of above 100,000 feet (30km) where many of the potential risks and uncertainties of SAI could be minimized 1. It is assumed that the gun technology required for SAI deployment will not be available for some time.

Permanent tethered structures​

Seen more as a theoretical concept, a permanent tethered structure floating in the stratosphere while remaining connected to the ground has been proposed 2. The proposed structure would contain a pipe which can pump a slurry or gas up to the platform to be dispersed into the stratosphere. This system is viewed as highly unlikely to occur as it would require advancements in multiple areas of science and engineering to be realized 2.

Sources​

Footnotes​

  1. National Academies of Sciences, Engineering, and Medicine. (2021). Reflecting sunlight: Recommendations for solar geoengineering research and research governance. National Academies Press. https://doi.org/10.17226/25762 ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7

  2. McClellan, J., Keith, D. W., & Apt, J. (2012). Cost analysis of stratospheric albedo modification delivery systems. Environmental Research Letters, 7(3), 034019. https://doi.org/10.1088/1748-9326/7/3/034019 ↩ ↩2 ↩3 ↩4 ↩5 ↩6

  3. Smith, W., & Wagner, G. (2018). Stratospheric aerosol injection tactics and costs in the first 15 years of deployment. Environmental Research Letters, 13(12), 124001. https://doi.org/10.1088/1748-9326/aae98d ↩ ↩2

  4. Shepherd, B., Shingal, A., & Raj, A. (2016). Value of air cargo: Air transport and global value chains. International Air Transport Association. ↩