Managing Climate Overshoot: Where Solar Radiation Management Fits in the Climate Toolkit

Overview

Objectives

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

• Explain how solar radiation management (SRM) fits within the broader toolkit of climate responses — mitigation, adaptation, carbon dioxide removal (CDR), and SRM — and why it might be considered in a warming “overshoot” scenario
• Describe the mechanisms behind the two leading SRM methods, stratospheric aerosol injection (SAI) and marine cloud brightening (MCB), including the volcanic-eruption analogy and the Twomey effect
• Evaluate the trade-offs of SAI and MCB, including their differing costs, evidence bases, and side effects
• Discuss the risks and governance challenges of SRM, including termination shock, moral hazard, the “free driver” problem, and regional inequities, and explain why SRM is not a substitute for mitigation or carbon removal

Grade Level

Grades 9-12, Undergraduate

Standards

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

• HS-ESS3-1 — Construct an explanation based on evidence for how the availability of natural resources, occurrence of natural hazards, and changes in climate have influenced human activity.
• 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; Recognize perspectives
• 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 16 — Peace, Justice and Strong Institutions: Promote peaceful and inclusive societies for sustainable development, provide access to justice for all and build effective, accountable and inclusive institutions at all levels

Materials Required

• PowerPoint slideshow "ATMOS220_geoengineering.pptx" (Solar Radiation Management: How might we manage climate overshoot?, H. Hirasawa, ATMOS 220)
• Paper and pencil
• This discussion guide, since the lecture slides do not include built-in discussion prompts

Learning Sequence

Lesson Plan Introduction

As the world risks overshooting the Paris Agreement's warming targets, this lesson introduces solar radiation management as one possible tool in a broader climate response toolkit, and asks students to weigh its promise against its mechanisms, trade-offs, and governance risks.

Activity Details

1. Why Talk About Climate Intervention? (Slides 3–15)

• Introduce key terms (slides 4–5): geoengineering, solar radiation management (SRM), and how SRM differs from weather modification (large-scale, global, multi-year effects vs. local, short-term effects).
• Present the “emissions gap” (slides 6–7): current policies and pledges still leave the world on track to overshoot the 1.5°C target.
• Build up the climate-response framework piece by piece (slides 8–15): human society, carbon emissions, CO₂ concentrations, climate change, and climate impacts form a loop, and mitigation, adaptation, carbon dioxide removal (CDR), and SRM each interrupt this loop at a different point.
• Discussion: (Ask after slide 7) The world is currently on track to overshoot the 1.5°C warming target. What do you think makes it so difficult for countries to cut emissions fast enough to avoid this?
• Discussion: (Ask after slide 15) This framework shows four strategies — mitigation, adaptation, CDR, and SRM — each acting on a different part of the climate system. Which of these do you think is the most straightforward to put into practice, and which seems the most controversial? Why?

2. Stratospheric Aerosol Injection (Slides 16–21)

• Introduce SAI (slides 16–19): injecting reflective particles into the stratosphere, where they can remain aloft for multiple years, reducing the sunlight that reaches the surface.
• Show the volcanic-eruption analogy (slides 17–18): large eruptions naturally inject sulfur-containing compounds into the stratosphere and have been observed to cause multi-year global cooling.
• Present the pros and cons of SAI (slide 21): relatively little aerosol is needed to achieve substantial cooling, and the effect has real-world analogs and has been extensively studied — versus risks to the ozone layer and possible acid rain.
• Discussion: (Ask after slide 18) What historical evidence do scientists point to when arguing that injecting aerosols into the stratosphere could cool the planet?
• Discussion: (Ask after slide 21) Looking at SAI's pros and cons, which concern would worry you the most if this were deployed at a large scale, and why?

3. Marine Cloud Brightening (Slides 22–32)

• Introduce MCB (slides 22–23): spraying sea salt aerosol over ocean clouds to increase the number of cloud droplets (the “Twomey effect”), making clouds brighter and more reflective.
• Discuss the “Faustian Bargain” (slide 26): existing air pollution already brightens clouds and masks some greenhouse warming, so cleaning up that pollution — good for human health — could unmask hidden warming.
• Present MCB's pros and cons (slide 32): the sea salt aerosol used is less harmful than existing pollution and its effects wear off quickly if stopped, versus large uncertainties in aerosol-cloud interactions and the need for continual, sustained aerosol emissions to keep working.
• Discussion: (Ask after slide 23) Why would making cloud droplets smaller, using the same amount of water, make a cloud appear brighter and reflect more sunlight?
• Discussion: (Ask after slide 26) Why might reducing air pollution — something that's good for human health — accidentally speed up global warming? How does this connect to why some scientists are studying MCB?
• Discussion: (Ask after slide 32) Why might it be harder for scientists to confidently predict MCB's climate effects than SAI's?

4. Risks, Governance & Why SRM Isn't a Substitute (Slides 33–35)

• Present the disclaimer (slide 33): SRM is not a replacement for cutting emissions or removing CO₂ — it cannot address ocean acidification, and if suddenly stopped, could cause “termination shock,” a rapid burst of warming.
• Summarize the key points (slide 34): SRM is an emerging tool with real-world analogs suggesting it could cool the climate, but significant scientific, technological, and governance challenges remain.
• Introduce the discussion topics raised in the lecture (slide 35): governance challenges, moral hazard, termination shock, regional climate changes, the “free driver problem,” and unclear effects on ecosystems and agriculture.
• Discussion: (Ask after slide 33) Why can't SRM be used as a substitute for cutting emissions or removing CO₂ from the atmosphere? What is “termination shock,” and why is it dangerous?
• Discussion: (Ask after slide 35) If SRM could be deployed by a single country or company because it's relatively cheap (the “free driver problem”), what risks does that create? Who do you think should get to decide whether and how SRM is used?
• Discussion: (Ask after slide 35) Even if SRM cooled the global average temperature, could it still create regional winners and losers? What would that mean for fairness between countries?

5. Comparing the Full Climate Toolkit (Slide 36)

• Show the full landscape of climate interventions (slide 36): carbon dioxide removal (land- and ocean-based), solar radiation management (aerosol-based, space-based, and terrestrial radiation modification), and weather modification.
• Discussion: (Ask after slide 36) If you were advising a government on where to invest limited climate funding — mitigation, adaptation, CDR, or SRM — how would you prioritize these four strategies, and why?

Conclusion

Close with a values-based question: Given the risks and uncertainties covered in this lesson, do you think SRM research should continue? Under what conditions, if any, do you think SRM should actually be deployed?

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

Before the slides on the emissions gap, have students look up their own country's emissions and climate pledge, and where it sits in the gap.

Bring in other perspectives

When discussing SRM's risks and governance, ask groups to speak for a small island state, a large emitter and a country whose rainfall could change, and see whether they agree on the conditions for research.

Share beyond the classroom

Students can write a short recommendation on whether SRM research should continue, addressed to their country's delegation to the UN climate conference.

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: 25 of the 67 images in these slides have no alt text, and 11 slides (6–12, 14–16 and 36) have no title. Add them (PowerPoint's Accessibility Checker lists each one) before publishing the file.

  • Download PowerPoint: Lesson slides

• Stratospheric Aerosol Injection

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

  • Open Link: Stratospheric Aerosol Injection

• Marine Cloud Brightening

  Particle size, ship-based spraying, and the Twomey effect.

  • Open Link: Marine Cloud Brightening

• What Is Termination Shock?

  A plain-language explainer on the risk raised in slide 33.

  • Open Link: What Is Termination Shock?

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

  Background on the governance challenges raised in slide 35.

  • 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 National Academies report underlying much of the risk/governance discussion.

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

• UNEP Emissions Gap Report 2025

  The source cited on slide 7 for the current emissions gap.

  • Open Link: UNEP Emissions Gap Report 2025

Related lessons

• Solar Climate Intervention: Weighing Benefits, Risks, and Moral Hazard: another introduction to solar radiation modification, built around benefits, risks and moral hazard, with a graduate version on aerosol physics
• Reflecting Sunlight to Cool Earth: Evidence from Climate Models and Volcanic Eruptions: another introduction to solar radiation modification, built on climate model results and past volcanic eruptions
• Climate Change and Solar Radiation Modification: Science, Risks, and Who Decides: another introduction to solar radiation modification, from the state of the climate to governance, with a science–policy role-play version
• Climate Adaptation and Intervention: Wildfire Resilience and Solar Radiation Management: another introduction to solar radiation modification, framed around adapting to growing wildfire risk

Credit and contact