Volcanic eruptions can influence climate through the same mechanism as SAI would – by producing a layer of reflective sulfate aerosols in the stratosphere. Large sulfur-rich eruptions can cause substantial cooling for several years, and the largest eruptions can cause major, temporary disruptions to regional climate, including drought caused by short-term ITCZ movement (Ridley et al., 2015). If SAI were deployed for many decades or even centuries, as is implied by peak-shaving scenarios with significant overshoots (Baur et al., 2023), then it is likely that at least one large sulfur-rich volcanic eruption will occur during deployment. The Pinatubo eruption of 1991, with a Volcanic Explosivity Index of 6, has a return time of order 100 years (Deligne et al., 2010).
Metric
A Pinutabo-sized sulfur-rich eruption at any latitude would cause larger negative impacts under a 1°C-cooling SAI scenario, than without SAI.
Uncertainty
At present, there are no clear mechanisms for such increased risk, other than via disruption to deployment, but such disruption would have to be extensive and prolonged. In fact, evidence suggests the reverse is true, and that the largest risks from a Pinatubo-like eruption (rapid cooling and regional rainfall disruption) could be reduced by strategically altering SAI injections in response.
Decision relevance
The climate impacts of Pinatubo-sized eruptions are large, so an increase in risk would be a material change in overall side-effects from SAI. Additionally, the need to account for eruptions in design of ground infrastructure and injection strategy, as well as the need to design response strategies and rapid decision-making frameworks for those responses (MacMartin et al., 2019), makes this a decision-relevant uncertainty. While the metric here does not account for rarer, halogen-rich eruptions, the consequences on the ozone layer of these eruptions under SAI are likely significant and not yet well studied.
Further Information
There is reason to believe SAI reduces some volcanic risks by providing the capacity to offset the eruption’s impacts through changes in the injection strategy (Quaglia et al., 2024). The magnitude of climate forcing from eruptions is also expected to be reduced when there is a background aerosol layer from SAI, owing to faster growth and removal of sulfate aerosols (Laakso et al., 2016). Eruptions can disrupt air traffic, posing risks to the maintenance of the SAI deployment strategy which need to be accounted for in planning ground infrastructure and injection strategies.
Volcanic eruptions can also add large amounts of water vapour (e.g. the Hunga Tonga eruption; Millan et al., 2022) and halogens to the stratosphere, which would interact with the sulfate aerosol layer under SAI. Halogen injection would deplete stratospheric ozone and potentially alter lifetime and size distribution (and thus forcing efficacy) of sulfate aerosols (Staunton-Sykes et al., 2021).
Decision relevance and state of understanding
Several potential risks have been suggested from a large sulfur-rich eruption occurring during SAI deployment. These include harmful excessive cooling from the combination of SAI and a very large eruption. That is, the risk that a very large eruption which would cause harm through global cooling would cause more harm under a prior cooling from SAI (e.g. Robock et al., 2008). However, while the absolute global temperature would be lower under a background SAI deployment, the change in temperature with the eruption would actually be reduced under SAI for two reasons: first, the background aerosol layer from SAI would cause faster growth and removal of volcanic sulfate aerosols (Laakso et al., 2016); and, second, SAI deployment could be paused to compensate for the eruption (MacMartin et al., 2019; Quaglia et al., 2024). Similarly, Quaglia et al. (2024) show that SAI could have the potential to reduce risks from disruption to regional climate via interhemispheric imbalances in forcing from eruptions if the SAI injection strategy were used to offset these imbalances through rapid changes to injection location or magnitude.
A separate risk arises from the potential for eruptions to disrupt air traffic, potentially causing interruption to deployment at one or more locations. An interruption to deployment would have to last for more than several months to have any appreciable impact (Parker & Irvine, 2018). The largest recent disruption to global air traffic lasted around 10 days over most of European airspace, after the eruption of Eyjafjallajökull in Iceland in 2010.
Finally, a halogen-rich eruption could cause significant ozone depletion (Staunton-Sykes et al., 2021), which could add to depletion due to SAI, increasing potential risks to human health and ecology from UV exposure, but there has been little research on this risk under SAI.
Future research directions
Future research could investigate the impacts of water vapour and halogen-rich eruptions on stratospheric chemistry and the resulting climate impacts and ozone changes under background SAI conditions. Additionally, more research is needed to consider the design of strategies for risk mitigation by varying injection strategy across different eruption parameters as well as risk mitigation strategies for logistics and infrastructure to prevent disruption to deployment after eruptions.
References
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