Introduction to Marine Cloud Brightening

Overview

Objectives

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

• Explain why reflecting a small fraction of incoming sunlight could offset a large change in greenhouse gases, using the example that doubling CO₂ adds about 3.7 W/m², roughly 1% of the sunlight Earth receives
• Describe how adding particles to a cloud produces more, smaller droplets, and why smaller droplets make a cloud more reflective
• Use ship tracks and recent limits on sulfur pollution from ships as real-world evidence about how aerosols affect clouds
• Identify the open research questions about MCB, including competing effects on cloud lifetime and cloud cover, costs, and side effects
• Evaluate the technical feasibility of using MCB regionally to protect Arctic sea ice

Grade Level

Needs review: The draft suggests Grades 9-12 and undergraduate for this short lecture; confirm the range with Matthew Henry.

Grades 9-12, Undergraduate

Standards

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

• HS-ESS2-4 — Use a model to describe how variations in the flow of energy into and out of Earth’s systems result in changes in climate.
• 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
• 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 14 — Life Below Water: Conserve and sustainably use the oceans, seas and marine resources for sustainable development

Materials Required

• PDF slideshow “HP_London_Henry_March24.pdf” (Introduction to Marine Cloud Brightening, M. Henry)
• Optional: PowerPoint slideshow “MCB_intro_slides_for_Ben.pptx” (Marine Cloud Brightening, J. Zhang), for a closer look at aerosol–cloud interactions, shipping pollution and research checkpoints
• Paper and pencil
• This discussion guide, since the lecture slides do not include built-in discussion prompts

Learning Sequence

Lesson Plan Introduction

Low clouds over the ocean already reflect a great deal of sunlight. This lesson asks whether people could deliberately make them a little brighter, what the evidence says so far, and what would have to be true for marine cloud brightening to make a measurable difference to the climate.

Activity Details

1. Why Reflecting Sunlight Could Matter (Slides 1–5)

• Introduce the family of sunlight reflection methods (slides 2–3).
• Compare the energy involved (slides 4–5): Earth receives about 340 W/m² of sunlight on average, and doubling CO₂ adds a forcing of about 3.7 W/m², roughly 1% of that. Reflecting a small extra fraction of sunlight could therefore offset a large increase in greenhouse gases.
• Discussion question: If the effect of doubling CO₂ is so small compared with incoming sunlight, why does it cause so much warming?

2. How Cloud Brightening Works (Slides 6–9)

• Explain what a cloud is, and how adding dust or similar particles produces more and smaller cloud droplets (slides 6–7).
• Show why smaller droplets are more reflective, using the comparison of 4 mm and 40 micron glass balls (slide 8).
• Present the proposal, credited to Stephen Salter, to brighten clouds by adding sea salt (slide 9).
• Discussion question: Why would the same amount of water reflect more sunlight as many small droplets than as fewer large ones?

3. Evidence from Ships (Slides 10–11)

• Show ship tracks: bright lines in clouds where ship exhaust has added particles (slide 10).
• Discuss how International Maritime Organization rules that cut sulfur pollution from ships led to a small amount of extra warming (slide 11).
• To go further, use Jianhao Zhang’s slides 5 and 8: ships emit about 10 Tg of sulfur dioxide a year, about one-eighth of all SO₂ emissions, and the resulting particles have been brightening clouds and masking part of greenhouse warming without anyone intending it. Ship tracks show the effect directly.
• Discussion question: What does the effect of cleaner ship fuel tell us about whether MCB might work, and what can’t it tell us?

4. Open Research Questions (Slides 12–16)

• Smaller droplets make clouds brighter, but they also reduce rain, so clouds may last longer, and they increase evaporation, which may reduce cloud cover. Which effect wins out? (slides 12–14)
• Consider the energy and infrastructure needed to produce enough droplets of the right size to have a measurable effect (slide 15), and possible side effects of adding sea salt to the atmosphere, including on surface ozone pollution and the lifetime of methane (slide 16).
• To go further, use Zhang’s slides 6, 7 and 9. Slides 6 and 7 name the three cloud responses: the Twomey effect (for the same amount of water, more and smaller droplets reflect more sunlight), the Albrecht effect (smaller droplets suppress rain, so clouds may last longer), and cloud water adjustments (smaller droplets increase drying at cloud top, so clouds can lose water). Slide 9 shows a simulation in which seeding a field of open-cell clouds with sea salt closes the cells and reflects more sunlight.
• Present the physical science checkpoints of Diamond et al. (2022) on Zhang’s slide 4: research would stop if, for example, particles of the right size can’t be generated and delivered, the signal couldn’t be detected, or risks to marine ecosystems or large-scale circulation outweigh those of unmitigated warming.
• Discussion question: Which of these questions would you want answered first before any real-world test, and how could scientists study it safely? Which checkpoint do you think would be hardest to pass?

5. Could MCB Protect Arctic Sea Ice? (Slides 17–20)

• Present modeling work in preparation with Haruki Hirasawa: sea salt can only be emitted over open ocean, not land or sea ice (slide 17), and emitting about 100 Tg of sea salt a year in the model restores Arctic sea ice compared with a scenario without MCB (slides 18–19).
• Discuss technical feasibility (slide 20): at 100 liters of seawater a minute per sprayer, about 200,000 sprayers would be needed, although the 100 Tg a year figure is very uncertain and efficiency gains may be possible.
• Discussion question: Even if MCB could restore Arctic sea ice, who should decide whether to use it, and what would they need to know first?

Conclusion

Close with a values-based question: Given what we know and don’t yet know about marine cloud brightening, should research continue toward small outdoor experiments? What conditions, if any, would need to be met first?

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 whether students live near a coast or a shipping lane, and use ship tracks, sea fog or low clouds they have seen as everyday examples of how particles change clouds.

Bring in other perspectives

Discuss who lives near the ocean regions where MCB might be tested, including fishing and island communities, and how they could be consulted before any outdoor experiment.

Share beyond the classroom

Groups can prepare a two-minute explanation of the open research questions for a local audience, such as a coastal community group or another class.

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: This PDF was exported from Google Slides without tags, so screen readers can't follow its structure or read it in order. Remediate it (for example in Equidox) before publishing the file.

  • Download PDF: Lesson slides

• Supplementary slides on ship tracks and cloud responses

  Needs review: Claude added alt text to the 31 images that lacked it (and replaced the placeholder "Fig. 1"), and made the existing heading on slides 1-7 and 9 each slide's title. Ask Jianhao Zhang to check the alt text, especially the descriptions of figures from his own research on slide 9.

  Jianhao Zhang's slides on how ship pollution changes clouds, the three cloud responses, and the research checkpoints for MCB; used in steps 3 and 4.

  • Download PowerPoint: Supplementary slides on ship tracks and cloud responses

• Marine Cloud Brightening

  SRM360’s explainer on particle size, ship-based spraying, and MCB’s regional cooling trade-offs.

  • Open Link: Marine Cloud Brightening

• Physical Science of Marine Cloud Brightening: Knowledge and Gaps

  Jianhao Zhang and Graham Feingold's article in the October 2024 newsletter of the American Physical Society's Topical Group on the Physics of Climate.

  • Open PDF: Physical Science of Marine Cloud Brightening: Knowledge and Gaps

• To assess marine cloud brightening's technical feasibility, we need to know what to study—and when to stop

  Needs review: The DOI redirects to PNAS, which blocks automated link checks, so it hasn't been verified. Open the link to confirm it works.

  Diamond et al. (2022), PNAS: the physical science checkpoints shown in Zhang's slides.

  • Open Link: To assess marine cloud brightening's technical feasibility, we need to know what to study—and when to stop

• Physical science research needed to evaluate the viability and risks of marine cloud brightening

  Needs review: The DOI redirects to Science Advances, which blocks automated link checks, so it hasn't been verified. Open the link to confirm it works.

  Feingold et al. (2024), Science Advances: the source of the cloud-process diagrams in Zhang's slides.

  • Open Link: Physical science research needed to evaluate the viability and risks of marine cloud brightening

Related lessons

• Can Stratospheric Aerosol Injection Refreeze the Arctic?: the same question for stratospheric aerosol injection: could it refreeze the Arctic?
• Cloud in a Bottle Experiment: a hands-on experiment that shows how more aerosol makes a brighter cloud

Credit and contact