Modeling the Potential Impacts of Stratospheric Aerosol Injection: Knowns & Unknowns

Overview

Objectives

By participating fully in this activity, students will learn how to:

• Explain how climate models simulate the transformation of injected SO₂ into a stratospheric sulfate aerosol cloud, and how particle size affects that cloud's ability to reflect sunlight
• Distinguish between scenario-related uncertainties (choices about when, where, what, and how much to inject) and model-related uncertainties (aerosol microphysics, dynamics, chemistry) in SAI research
• Interpret multi-model comparison (GeoMIP) results to evaluate the level of confidence — or lack thereof — in specific quantitative predictions of SAI's climate effects
• Evaluate what scientific and modeling advances would be needed to move toward a robust, decision-relevant assessment of SAI's risks and benefits

Grade Level

Grades 11-12, Undergraduate

Standards

These are Next Generation Science Standards (NGSS) performance expectations.

• HS-ESS3-5 — Analyze geoscience data and the results from global climate models to make an evidence-based forecast of the current rate of global or regional climate change and associated future impacts to Earth systems.
• HS-ESS3-4 — Evaluate or refine a technological solution that reduces impacts of human activities on natural systems, with the clarification statement explicitly naming large-scale geoengineering design solutions (such as altering global temperatures by making large changes to the atmosphere or ocean) as an example.
• HS-ETS1-3 — Evaluate a solution to a complex real-world problem based on prioritized criteria and trade-offs that account for a range of constraints, including cost, safety, reliability, and aesthetics, as well as possible social, cultural, and environmental impacts.

This lesson also supports these international frameworks:

• Global competence (Asia Society and OECD): Investigate the world
• UNESCO learning objectives for SDG 13 (Climate Action):
  • Cognitive 5: prevention, mitigation and adaptation strategies
  • Behavioural 3: assessing the impact of decisions on other people and world regions

United Nations Sustainable Development Goals

• SDG 13 — Climate Action: Take urgent action to combat climate change and its impacts
• SDG 9 — Industry, Innovation and Infrastructure: Build resilient infrastructure, promote inclusive and sustainable industrialization, and foster innovation — including enhancing scientific research and upgrading technological capabilities

Materials Required

• PowerPoint slideshow "Guest_class.pptx" (Modeling the Potential Impacts of Stratospheric Aerosol Injections: Knowns & Unknowns, D. Visioni, Cornell University)
• Paper and pencil
• This discussion guide, since the lecture slides do not include built-in discussion prompts

Learning Sequence

Lesson Plan Introduction

As climate models increasingly attempt to simulate the potential effects of Stratospheric Aerosol Injection, this lesson examines how those simulations work, why they disagree with one another, and what scientific advances are needed before SAI's effects could be assessed with real confidence.

Activity Details

1. Why Climate Intervention, and What Is SAI? (Slides 2–12)

• Present the “you are here” framing (slides 2–9): under a business-as-usual emissions path, climate impacts continue to escalate; cutting emissions aggressively substantially reduces — but does not eliminate — those future impacts, motivating CO₂ removal (CDR) and Solar Radiation Modification (SRM) as additional tools.
• Define SAI (slides 11–12) as a form of SRM: the deliberate injection of sulfate aerosol precursors into the stratosphere to reflect incoming solar radiation, mimicking the effect of a large, sulfur-rich explosive volcanic eruption.
• Discussion: (Ask after slide 9) Looking at the impacts trajectory even under aggressive emissions cuts, why might emissions reductions alone not be enough to avoid significant climate impacts — and what role might CDR and SRM play as a result?
• Discussion: (Ask after slide 12) How is SAI similar to, and different from, a large volcanic eruption?

2. From SO₂ to the Aerosol Cloud (Slides 13–19)

• Walk through the chemical pathway (SO₂ → H₂SO₄ gas → sulfate aerosol particles, roughly 0.2–0.7 μm) and how particle size affects the aerosol's ability to scatter shortwave radiation versus absorb longwave radiation.
• Discuss the physical processes that spread the aerosol cloud once formed: self-lofting from localized heating, large-scale stratospheric circulation and mixing, and gravitational settling.
• Discussion: (Ask after slide 19) Why does the size of the resulting aerosol particles matter for how effectively they cool the climate? What might happen if the particles grow too large?

3. Uncertainties in SAI Research & Multi-Model Simulations (Slides 20–29)

• Present the central research questions (slides 20–25): Would SAI reduce global temperatures? What side effects would it produce? What would the regional surface impacts be? Are there ways to do it that can minimize harm? — plus societal/political uncertainties.
• Discussion: (Ask after slide 25) Which of these questions do you think is hardest to answer with scientific confidence, and why? Which do you think is most important to answer before any real-world deployment decision?
• Show results from GeoMIP (slides 26–29), where multiple independent climate models simulate the same SAI scenario, yet disagree substantially on quantities like the resulting stratospheric aerosol optical depth and the SO₂ needed to hit the same cooling target.
• Discussion: (Ask after slide 29) What does it mean for our confidence in SAI's predicted effects when independent models, given the exact same instructions, still produce different results? How should this shape how we communicate SAI's likely impacts to the public or policymakers?

4. Surface Impacts and the Broader Uncertainty Chain (Slides 30–36)

• Show maps of regional surface impacts under different SAI strategies (slides 30–33), emphasizing that these differences depend heavily on the baseline being compared against.
• Discussion: (Ask after slide 33) Why does the choice of “compared to what” matter so much when evaluating whether SAI's regional impacts are, on net, positive or negative?
• Trace the full uncertainty chain (slides 34–36): from what's injected into the aerosol layer, to how the aerosol layer affects the radiative balance, to how that translates into surface climate changes.
• Discussion: (Ask after slide 36) Where in this chain — aerosol behavior, radiative response, or surface climate response — do you think the biggest uncertainties lie? Why might uncertainty tend to compound as you move further down the chain?

5. Narrowing Uncertainties, Robust Assessment & Conclusions (Slides 44–51)

• Discuss why narrowing stratospheric uncertainties may require simulating very small spatial scales (slides 44–47, e.g. a 100 km × 100 km × 1 km model domain) to resolve the initial aerosol plume.
• Discussion: (Ask after slide 47) Why might a coarse, global-scale climate model struggle to accurately capture what happens in the first hours or days after aerosols are injected? What would be gained — and what would it cost — to run a much higher-resolution simulation instead?
• Present the idea (slide 48) of designing SAI research scenarios around specific policy-relevant questions, combined with contingency planning, as a path toward robust, decision-relevant assessments.
• Discussion: (Ask after slide 48) What do you think a “robust” scientific assessment of SAI would need to include before it could meaningfully inform a real policy decision?
• Summarize the conclusions (slides 49–51): scenario-related uncertainties (our choices) versus model-related uncertainties (limits of our models), and the closing point that stabilizing CO₂ concentrations would likely present fewer physical risks than SAI — but only if the world can achieve that.
• Discussion: (Ask after slide 51) Which type of uncertainty — scenario-related or model-related — do you think is more addressable through further scientific research, and which is more a matter of political or societal choice?

Optional extension (Slides 52–56)

• If time allows, briefly show the supplementary slides on climate extremes, precipitation changes, cloud coverage, air quality effects, and the scale of SO₂ emissions needed to offset all warming from a high-emissions scenario.

Conclusion

Close with a values-based question: Given the scientific uncertainties presented in this lecture, do you think there is currently a sufficient basis to move forward with real-world SAI deployment, or should more research be done first? What would need to be true for you to feel confident either way?

Global Connections

Needs review: Claude drafted the Global Connections section, and the global competence and UNESCO SDG 13 entries under Standards, from the lesson materials. Review and edit them.

Start where you teach

Ask students what a climate model would need to get right for their own place, such as monsoon rainfall, heat waves or snowpack, and use that to frame the regional uncertainties.

Bring in other perspectives

The GeoMIP results show that regional effects differ. Have each group take a different part of the world and report how confident the models are about its rainfall and temperature under SAI.

Share beyond the classroom

Students can write a plain-language summary of what is and isn't known, for decision makers where they live.

Lesson materials

Download the files below to teach this lesson. PDFs are accessible versions; PowerPoint and Excel files can be edited for your class.

Teacher resources

• Lesson slides

  Needs review: 112 of the 115 images in these slides have no alt text, and none of the 56 slides has a title. These slides need substantial remediation before the file is published.

  • Download PowerPoint: Lesson slides

• Stratospheric Aerosol Injection

  How SAI works, the Pinatubo analogy, and the stratosphere's role.

  • Open Link: Stratospheric Aerosol Injection

• SRM360: The Case for a Solar Geoengineering Research Governance Platform

  Accessible overview of why governance is a gap and what's being proposed to fill it.

  • Open Link: SRM360: The Case for a Solar Geoengineering Research Governance Platform

• NASEM (2021), Reflecting Sunlight: Recommendations for Solar Geoengineering Research and Research Governance

  The source of the moral hazard discussion, including the finding that deterrence evidence is equivocal.

  • Open Link: NASEM (2021), Reflecting Sunlight: Recommendations for Solar Geoengineering Research and Research Governance

• GeoMIP: The Geoengineering Model Intercomparison Project

  Background on the multi-model comparison project referenced in the lecture.

  • Open Link: GeoMIP: The Geoengineering Model Intercomparison Project

• An Overview of the Geoengineering Model Intercomparison Project (GeoMIP)

  Kravitz et al., the foundational GeoMIP paper, for a more technical overview.

  • Open Link: An Overview of the Geoengineering Model Intercomparison Project (GeoMIP)

• The Science of Solar Radiation Modification: Stratospheric Aerosol Injections and Marine Cloud Brightening

  Background reading: a chapter by Daniele Visioni, Pornapai Narenpitak, and Matthias Honegger in Elsevier's Reference Module in Earth Systems and Environmental Sciences (2024). Access may require a library subscription.

  • Open Link: The Science of Solar Radiation Modification: Stratospheric Aerosol Injections and Marine Cloud Brightening

Related lessons

• Can Stratospheric Aerosol Injection Refreeze the Arctic?: a regional case study: could stratospheric aerosol injection cool the Arctic, and why would it still affect the whole planet?

Credit and contact