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Potential risk of Stratospheric Aerosol Injection (SAI): termination shock

Summary​

Stratospheric Aerosol Injection (SAI) carries significant risks of "termination shock" - a rapid temperature increase if deployment stops suddenly. This risk arises because sulfate particles must be replenished every 1 – 3 years to maintain cooling effects. Potential triggers include infrastructure failure, political instability, or deliberate shutdown. Termination shock could cause faster warming than current climate change projections, with impacts lasting generations. Existing fossil fuel emissions already create a similar risk through atmospheric aerosols that mask warming. Recent shipping fuel regulations demonstrate real-world examples of abrupt aerosol reduction leading to temperature spikes. Effective climate strategies must pair any SAI use with aggressive CO₂ emission reductions to minimize these risks.

A key risk of SAI is its potential to cause "termination shock". With regard to SAI, termination shock is defined as the rapid and substantial rise in global temperatures following the cessation of SAI deployment 1. An analogy for termination shock is the action of going "cold turkey" from a drug or alcohol dependency, where the dependency is analogous to the use of SAI, and the withdrawal symptoms are the rapid and substantial rise in global temperatures. The effects of the cessation of SAI (the withdrawal symptoms) are felt stronger when the cessation is rapid or immediate, compared to a more gradual cessation.

SAI achieves its global cooling effects by reducing the portion of sunlight reaching the Earth's surface via the physical principle of light scattering. In order to achieve this cooling effect, a layer of light-scattering particles such as sulfate aerosols would be injected into the stratosphere, where they remain suspended for 1 – 3 years. As the particles only have a suspension time of up to three years after which they will settle back down to the Earth's surface, they would need to be constantly monitored and replaced to ensure that the cooling effect is maintained 1 2.

This unavoidable necessity for constant replacement and monitoring of the SAI layer opens up the very real possibility of termination shock occurring. The cause for a termination shock event could be triggered in multiple ways such as unintended destruction of the SAI injecting infrastructure, an inability of states or global organizations to maintain constant SAI injection due to economic or political reasons, and the deliberate termination of SAI by an actor who may evaluate the regional negative implications of the program as too damaging 1 3. There are also other concerns regarding the ability to rapidly adapt to changing global atmospheric conditions such as a large volcanic eruption 3.

The specific consequences of a termination shock event are hard to predict and carry a lot of uncertainty. However, it is mostly agreed upon that a termination shock event will cause a sharp rise in global temperatures, with some estimates finding that the increases would exceed the predicted rises caused by climate change 2. However, the degree of this temperature increase is highly dependent on the levels of cooling that are trying to be achieved 1. This in turn supports the idea that SAI cannot be the "plan B" or only solution to climate change without a robust plan for a rapid and substantial reduction in CO₂ emissions, as an adaptation plan which solely relied on SAI would require an enormous negative thermal forcing which greatly increases the risk of termination shock 1.

Another concern with the risk of termination shock is the intergenerational risk that it poses. There is an inherent intergenerational burden passed on to future generations (in the same way as climate change is an intergenerational burden) which involves questions of intergenerational equity 2. Future generations may be required to bear the burden of a failure of SAI, or at the very least, they may have to live with the constant fear that failure to maintain SAI could cause a catastrophic termination shock event.

Termination shock from anthropogenic atmospheric aerosols is not just a hypothesized risk related to SAI. The usage of fossil fuels already contributes to a net cooling effect on global temperatures caused by light scattering from low altitude aerosols. If an immediate cessation of fossil fuels was to occur, the net cooling effect would disappear in a rapid timescale as the aerosols are quickly washed out of the atmosphere by rain, while the net heating effect from the carbon dioxide emissions would remain for many decades 4. This in turn could cause its own termination shock event. And finally, termination shock is being witnessed today through the recent international maritime organization regulation to reduce sulfur content in shipping fuels, which is now being analyzed as an explanation for the rapid global average temperature rises seen since 2021.

Sources​

Footnotes​

  1. Parker, A., & Irvine, P. J. (2018). The risk of termination shock from solar geoengineering. Earth's Future, 6(3), 456–467. https://doi.org/10.1002/2017EF000735 ↩ ↩2 ↩3 ↩4 ↩5

  2. 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

  3. Tang, A., & Kemp, L. (2021). A fate worse than warming? Stratospheric aerosol injection and global catastrophic risk. Frontiers in Climate, 3, Article 720312. https://doi.org/10.3389/fclim.2021.720312 ↩ ↩2

  4. Hansen, J., Kharecha, P., & Sato, M. (2013). Climate forcing growth rates: Doubling down on our Faustian bargain. Environmental Research Letters, 8(1), 011006. https://doi.org/10.1088/1748-9326/8/1/011006 ↩