• Observational uncertainties
  • We have moved from three uncertainties to two. We have switched to grouping based on earth system components (i.e. the aerosols themselves versus everything else), and removed the ‘early deployment’ entry, since the observational requirements are closely related to those that would be needed as deployment scales up, and since the ‘baselining’ observations are common to both early and later stage deployments.
  • v1: Early Deployment Aerosol Observational Requirements, In-Situ Observational Infrastructure, and Satellite Observational Infrastructure
  • v2 (current): Climate Response Monitoring and Observations for Stratospheric Processes
• Extratropical Circulation
  • We merged Meridional Temperature Gradients into this uncertainty. The original Meridional Temperature Gradients uncertainty was confusingly worded, and is more usefully represented here and in other entries.
  • Old metric: ‘A detectable increase in extreme winter weather in mid-latitudes due to the extra-tropical circulation response to SAI, on a 50-year timescale at 0.5°C cooling, relative to a reference world with the same global mean temperature but neither higher CO2 nor SAI.'
  • New Metric: ‘A change in winter cold extremes in the mid-latitudes due to the circulation response to SAI, detectable on a 50-year timescale at 0.5°C cooling relative to a baseline climate state with the same global mean temperature.’
• Surface Climate Response to Stratospheric Heating
  • Old metric: ‘Including stratospheric heating in ESM simulations of SAI changes the root mean square error (RMSE) in temperature or precipitation relative to the baseline ('target') climate state by a factor of 2 or more'
  • New Metric: ‘Excluding stratospheric heating in ESM simulations of SAI with injection at 30N/S reduces the root mean square error (RMSE) in temperature or precipitation, over the full annual cycle, relative to the baseline ('target') climate state by a factor of two or more’

• Atlantic Meridional Overturning Circulation
  • Title expanded from ‘AMOC’ to ‘Atlantic Meridional Overturning Circulation’
  • Old metric: ‘AMOC strength at 0.5C cooling remains closer (weaker) to where it would have been without SAI, than it is to the (historical) level when global mean temperature was the same (as the 0.5°C cooler world).'
  • New Metric: ‘Under SAI used to hold global mean temperature at 1.5°C above pre-industrial, beginning in 2035, SAI offsets less than one third of the weakening under the background warming scenario, at 2100.’
  • Degree of uncertainty changed from Low to Medium

• Existing Aircraft Retrofitting:
  • Old metric: ‘Time to modify a fleet capable of delivering ~1Mt/yr payload to 14-15km is over 5 years from first large-scale funding.'
  • New Metric: ‘Time to achieve (acquire and modify) a fleet capable of delivering a ~1 Mt/year payload to 15 km is over 5 years from first large-scale funding.’
  • Degree of uncertainty changed from Medium to Low
• Material Dispersion Technology:
  • Old metric: ‘Time to design and verify the technology to quickly disperse the sulfate precursor, and ensure it is entrained into aircraft wake, is more than 5 years from first large-scale funding.'
  • New Metric: ‘Time to design and verify the technology to quickly disperse the sulfate precursor, and ensure it is entrained into aircraft wake, takes more than 2 years of well-funded work.’
• Physical Impacts of Deployment Interruption
  • We have removed this uncertainty as it is more a social rather than technical uncertainty, and therefore out of scope as noted in our methodology.
• Terrestrial Carbon Cycle:
  • Old metric: ‘Terrestrial carbon uptake decreases/remains the same under SAI relative to a no-SAI future.'
  • New Metric: ‘SAI at 0.5°C of cooling does not increase terrestrial carbon storage relative to the background warming scenario.’
• Terrestrial Vegetation:
  • Old metric: ‘An IPCC region's most produced crop (by total calories) has its yield reduced by more than 20% relative to a hypothetical production level associated with the future climate without SAI (i.e. the warmer world), and holding all else constant.'
  • New Metric: ‘SAI at 0.5°C of cooling decreases crop yield by more than 10% in any given IPCC region, except where this reduction represents a missed benefit to crop yield arising from warmer temperatures without SAI.’

As discussed in the prior changelog entry, we are updating the three uncertainties below. Note that none of the metrics or uncertainty/decision relevance quantifications changed for any of these.

• New Aircraft Development
• Material Dispersion Technology
• Ground Infrastructure

When we first released this database, we made it available to the sunlight reflection community as soon as we felt that it could be a beneficial resource. But assessing this broad set of uncertainties is a large task, so since this initial release, we have also begun a slower process of improvement and expansion of the content. This has been based in part on the invaluable feedback we have received on framing, design, and content. Thank you to all those who have contributed!

We are working through the uncertainty entries one-by-one, taking several weeks in each case to expand our literature reviews, make new figures and quantitative assessments, and talk with relevant experts. This process aims to produce: sharper decision-relevance and uncertainty justifications, more robust explanations of each uncertainty and its sources, and a more complete list of relevant literature.

We are publishing these updates in batches rather than waiting for all of them to be finished. The four uncertainties listed below now have updated versions live. Any changes to the metric or decision relevance or uncertainty quantifications are listed below. Please let us know what you think here.

• Aerosol Size Distribution:
  • Old metric: ‘Radiative forcing per unit injection is wrong by a factor of 2 relative to the multi-model mean, due to difference in size distribution.'
    New Metric: ‘The global aerosol effective radius of stratospheric sulfate is >0.5 μm, under 21km 30°N/S injection at 5Tg annual injection’
• Stratospheric Transport of Aerosols
  • Title changed from ‘Aerosol Spatial Distribution’ to ‘Stratospheric Transport of Aerosols’
  • Old metric: ‘The average spatial pattern obtained in releasing a hypothetical passive tracer is outside the multi-model range.’
    New Metric: ‘The mean lifetime of a passive tracer injected at 21km and 30°N/S is less than 2 years.’
• Surface Air Quality
  • Old metric: ‘Direct impacts of SAI cause a sufficient net increase in PM2.5 and surface ozone to increase global annual mortality by 10,000.’
    New metric: ‘There is a 2% increase in mortality from PM2.5 and surface ozone under SAI relative to the no-SAI future, at 0.5°C global cooling.'
  • Degree of uncertainty changed from Low to Medium
• Volcanic Eruptions During Deployment
  • Old metric: 'Direct impacts of SAI cause a sufficient net increase in PM2.5 and surface ozone to increase global annual mortality by 10,000.'
    New metric: 'There is a 2% increase in mortality from PM2.5 and surface ozone under SAI relative to the no-SAI future, at 0.5°C global cooling.'

Today we're launching SAI Uncertainties, a comprehensive resource for exploring and understanding research uncertainties related to stratospheric aerosol injection. This database represents months of work synthesizing expert knowledge, literature review, and structured assessment frameworks.

Our goal is to provide researchers, policymakers, and stakeholders with a clear picture of what we know, what we don't know, and where research investments could have the greatest impact on reducing uncertainty. Each entry includes detailed metrics, contextual notes, and references to relevant literature.

• Browse uncertainties by type (physical, technical, social, governance), probability level, and potential consequences
• Detailed uncertainty pages with assessment metrics, expert notes, and literature references
• Full-text search and multi-faceted filtering to find relevant uncertainties quickly
• Responsive design optimized for desktop and mobile viewing
• Accessibility features including keyboard navigation and screen reader support

This is just the beginning. We'll be continuously updating the site with new uncertainties, refining assessments as new research emerges, and adding features based on user feedback. Check back here for updates.