- Uncertainty
- Medium
- Decision relevance
- Medium
- Resolvability scale
- Large-scale testing
There are a range of potential SAI impacts about which neither small- nor large-scale testing would provide meaningful information, but which we would learn about from the response to deployment over time (e.g. MacMartin & Kravitz, 2019). Adequate monitoring of these elements of the climate system ahead of deployment is required so that estimates of SAI’s impacts can be revised as soon as possible in light of incoming data. These impacts include surface climate responses and also non-aerosol responses in the stratosphere, such as stratospheric temperature, water vapor, and ozone. Determining the change due to a deployment requires a sufficiently long baseline of high-quality observations to define the state prior to deployment and its variability. Not all important impacts will be constrained by observations under SAI on policy relevant timescales, as some will have a low signal (i.e. forced response) relative to the noise of natural variability. For example, while a change in regional rainfall might be statistically significant in a large ensemble of model simulations (e.g. Ricke et al., 2023), it could still be too small to be detectable in the first few decades of a deployment given large natural variability in precipitation.
Metric
Monitoring of one or more critical aspects of the surface climate response to SAI is insufficient to support refinement of understanding of SAI’s impacts, more than 10 years from first large-scale funding
Uncertainty
The research activities required to define sufficient monitoring are well understood. They include modeling to understand the time-of-emergence of SAI signals for relevant climate systems, and how such signals would be used in a statistically principled way to narrow prediction ranges in light of evidence. The capabilities to monitor the relevant systems are available, and no new technology is required. Uncertainty here therefore arises from whether existing systems can be adequately maintained and how these observing capabilities shift in the presence of a significant aerosol burden. This final component motivates our medium uncertainty rating – it is not yet clear how remote sensing systems might need to change given a large sulfate aerosol burden under SAI.
Decision relevance
If monitoring of the climate response to SAI is insufficient, this could either delay deployment until sufficient monitoring is in place, or would mean that early deployment was insufficiently observed. In the latter case, this implies that possible improvement to our understanding of SAI’s impacts informed by observations would be missed. Perhaps more importantly, it also means any potential unanticipated effects might go un-noticed for significantly longer.
Further Information
There is new research required to show exactly which elements of the climate system must be observed, at what resolution, and for how long in advance of deployment. Likely candidates are features for which a substantial change under SAI is plausible, and which are themselves policy relevant or determinants of policy relevant climate impacts. In the stratosphere, these include stratospheric temperature, water vapour, and ozone. Because ozone can change for multiple reasons, measurements of trace gases related to ozone depleting substances are also necessary to determine the mechanism for any ozone change (WMO, 2022). At the surface, required measurements likely include temperature, precipitation, air quality, and radiation (particularly changes to UV and the diffuse/direct fraction), as well as metrics of polar change (particularly sea-ice extent) and ocean circulation (particularly AMOC).
For many, or perhaps all, of these elements, sufficient observing systems are in place today to adequately constrain the response to SAI. However, even in these cases, there can be risks to the future continuity of those systems (e.g. some critical ocean observations for tracking AMOC changes are currently at risk; Witze, 2026), and there is uncertainty over exactly what real-time monitoring and analysis would be required for maximally using observations to update our estimates of SAI’s effects, and for attribution of impacts. Furthermore, the observational capabilities that work at present may be different in the future. For example, observing ozone and water vapor through a significant stratospheric aerosol layer will require a particular instrument, even though these can be observed in many different ways currently. Detailed analysis of how the retrievals change with stratospheric aerosols will be necessary.
References
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