Reflecting Sunlight to Cool Earth: Evidence from Climate Models and Volcanic Eruptions

Overview

Objectives

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

• Place solar radiation modification among the other responses to global warming: conservation, efficiency, low-carbon energy, carbon dioxide removal, and adaptation
• Weigh the potential benefits of stratospheric geoengineering against its risks and concerns, and explain why each needs to be quantified
• Describe what climate model studies, including the Geoengineering Model Intercomparison Project (GeoMIP), show about regional effects on precipitation, ecosystems, and crops
• Use past volcanic eruptions, such as Krakatau in 1883 and Pinatubo in 1991, as natural analogs for the benefits and side effects of stratospheric geoengineering
• Discuss whether geoengineering could itself become “dangerous anthropogenic interference with the climate system”

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.
• 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 2: current climate change as caused by human greenhouse gas emissions
  • 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 2 — Zero Hunger: End hunger, achieve food security and improved nutrition and promote sustainable agriculture
• SDG 13 — Climate Action: Take urgent action to combat climate change and its impacts

Materials Required

Needs review: The Futurama clip (BlottingOutSun.flv) is copyrighted television, so it isn't posted here or on YouTube or Kaltura; teachers show the scene from a licensed copy of "Crimes of the Hot" (2002). The clip is kept only in the local materials mirror. Confirm this approach, and the episode name, with Alan Robock.

• PowerPoint slideshow “Climate Intervention as a Response to Global Warming” (A. Robock, Rutgers University), 33 slides
• Optional: the “blotting out the sun” scene from the Futurama episode “Crimes of the Hot” (2002), about two minutes long, from a licensed copy of the episode
• Paper and pencil
• This discussion guide, since the lecture slides do not include built-in discussion prompts

Learning Sequence

Lesson Plan Introduction

Some scientists and policymakers have suggested reflecting part of the sunlight that reaches Earth to offset global warming. This lesson, built from a talk by Alan Robock, asks what such an intervention could and could not do, what climate models and past volcanic eruptions tell us about its side effects, and who should decide whether it is ever used.

Activity Details

1. Responses to Global Warming (Slides 1–4)

• Introduce the talk (slide 1) and Ken Caldeira’s diagram of responses to global warming (slide 2): conservation, efficiency, low-carbon energy, solar radiation modification, carbon dioxide removal, adaptation, and suffering.
• Define geoengineering as “deliberate large-scale manipulation of the planetary environment to counteract anthropogenic climate change” (Royal Society, 2009), now more often called climate intervention (slide 3).
• Optional video: after slide 3, show the “blotting out the sun” scene from the Futurama episode “Crimes of the Hot” (2002), in which a giant shade placed between the Sun and Earth instead focuses sunlight into a beam that sets the city on fire.
• Discussion question: What went wrong in the Futurama scheme, and does it resemble any real concern about reflecting sunlight?
• Present the observed temperature record and what has driven it: recovery from volcanic eruptions, masking of warming by pollution aerosols, and the dominance of greenhouse gases (slide 4).

2. Benefits and Risks (Slides 5–6)

• Present Robock’s list of the benefits and risks or concerns of stratospheric geoengineering (slides 5–6), from reducing warming, sea ice loss, and sea level rise to drought in Africa and Asia and whiter skies.
• Have each group choose one benefit and one risk, and decide what evidence would be needed to quantify it, since “each of these needs to be quantified so that society can make informed decisions.”

3. What Climate Models Show (Slides 7–16)

• Explain what climate models are (slide 7), then compare model results for sulfur dioxide injected in the Arctic and in the tropics, including their different effects on regional precipitation (slides 8–10).
• Present the Geoengineering Model Intercomparison Project (GeoMIP), which runs the same experiments across many climate models to test whether results are robust, for example for Asian monsoon rainfall and ozone (slide 11).
• Discuss research on impacts on ecosystems and agriculture (slides 12–13), the “napkin diagram” of how intervention could fit with emissions cuts and carbon removal (slide 14), open questions about targets, starting, and stopping (slide 15), and how the best scenario for crop production varies by nation (slide 16).
• Discussion question: If the best intervention scenario for crops differs from country to country, how could the world agree on one?

4. Lessons from Volcanic Eruptions (Slides 17–26)

• Revisit the benefits and risks (slide 17), then show how the 1991 Pinatubo eruption affected the global water cycle (slide 18).
• Explain how volcanic aerosol clouds produce red and yellow skies after sunset (slides 19–20), illustrated by William Ascroft’s watercolors after Krakatau in 1883 and Edvard Munch’s “The Scream” (slides 21–22).
• Show how diffuse light after Pinatubo made the sky whiter (slides 23–24), then summarize what volcanoes tell us: stratospheric geoengineering could cool the surface, but it could also reduce summer monsoon rain, destroy ozone, cause rapid warming if stopped, make the sky white, and reduce solar power (slides 25–26).
• Discussion question: Which of the side effects in slide 25 do you think matters most, and to whom?

5. Feasibility and Governance (Slides 27–33)

• Discuss whether it is even technically possible to create a stratospheric aerosol cloud with the desired particle sizes (slide 27).
• Present the objective of the UN Framework Convention on Climate Change, to prevent “dangerous anthropogenic interference with the climate system,” and Robock’s argument that this pledge must now include geoengineering (slides 28–29).
• Ask whether SRM makes that interference more dangerous (slide 30), then close with the images of the Krakatau sunsets and time for questions (slides 31–33).

Conclusion

Close with a values-based question: Would deliberately reflecting sunlight count as “dangerous anthropogenic interference with the climate system”? Should research on it continue, and who should decide whether it is ever used?

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

The optional Futurama clip may be unfamiliar outside the U.S. Instead, open with a news story about a recent volcanic eruption or heat wave that students know, then ask what could go wrong with blocking sunlight.

Bring in other perspectives

The talk's sponsor slide lists U.S. agencies. Pair it with research or commentary from scientists in the Global South, for example through the Degrees Initiative, and compare their concerns with Robock's list of risks.

Share beyond the classroom

Students can present their answer to the question of “dangerous interference” to another class or to families, and compare how different audiences respond.

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: In these slides, 12 of the 27 images have no alt text and none of the 33 slides has a title. Slide 29 also includes a New York Times illustration (Henning Wagenbreth, 2007); confirm it can be shared publicly. Add alt text and titles (PowerPoint's Accessibility Checker lists each one) before publishing the file.

  The 33 slides of Robock's talk, taken from his full lecture deck.

  • Download PowerPoint: Lesson slides

• 20 reasons why geoengineering may be a bad idea

  Robock’s 2008 article in the Bulletin of the Atomic Scientists, the basis for the benefits and risks slides.

  • Open Link: 20 reasons why geoengineering may be a bad idea

• Potential ecological impacts of climate intervention by reflecting sunlight to cool Earth

  Zarnetske et al. (2021), Proceedings of the National Academy of Sciences, cited on slide 13.

  • Open Link: Potential ecological impacts of climate intervention by reflecting sunlight to cool Earth

• Geoengineering: Whiter skies?

  Kravitz, MacMartin, and Caldeira (2012), Geophysical Research Letters, shown on slide 24.

  • Open Link: Geoengineering: Whiter skies?

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
• Managing Climate Overshoot: Where Solar Radiation Management Fits in the Climate Toolkit: another introduction to solar radiation modification, comparing it with emissions cuts, carbon removal and adaptation
• 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