Stratospheric aerosol injection research and existential risk

Publication by John Halstead
Published on 09 March 2018


  • It is uncertain whether Stratospheric Aerosol Injection(SAI) research is justifiable, but a tentative case be made for security-focused research.
  • SAI would eliminate the arguable environmental existential risks of climate change (<1% – 3.5%).
  • It is extremely unclear whether SAI would reduce willingness to mitigate, and extensive efforts should be made to reduce the risk of mitigation obstruction.
  • Termination shock risk is overstated.
  • The risk of unilateral deployment is overstated, but SAI introduces other serious security risks.


In the wake of the continued failure to mitigate greenhouse gases, researchers have explored the possibility of injecting aerosols into the stratosphere in order to cool global temperatures. This paper discusses whether Stratospheric Aerosol Injection (SAI) should be researched, on the controversial ethical assumption that reducing existential risk is overwhelmingly morally important. On the one hand, SAI could eliminate the environmental existential risks of climate change (arguably around a 1% chance of catastrophe), and reduce the risks of interstate conflict associated with extreme warming. Moreover, the risks of termination shock and unilateral deployment are overstated. On the other hand, SAI introduces risks of interstate conflict which are very difficult to quantify. Research into these security risks would be valuable, but also risks reducing willingness to mitigate. I conclude that the decision about whether to research SAI is one of ‘deep uncertainty’ or ‘complex cluelessness’, but that there is a tentative case for research initially primarily focused on the governance and security aspects of SAI.

This paper was originally presented at our 2016 Cambridge Conference on Catastrophic Risk, and was published in a Special Issue of Futures edited by Dr Adrian Currie.

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