Volcanic eruptions and weather

The basic mechanism

1. Large volcanic eruption ejects SO2 (sulfur dioxide) into stratosphere.
2. SO2 combines with water to form sulfate aerosol droplets.
3. Aerosols reflect solar radiation back to space.
4. Global surface cools.
5. Aerosols remain in stratosphere 1-3 years before falling out.
6. Cooling effect fades as aerosols clear.

What matters for climate

• Height of injection: stratosphere required (10+ km typically).
• Sulfur content: high-S eruptions cool most.
• Latitude: tropical eruptions distribute globally, high-latitude regionally.
• Total mass ejected.
• Ash vs sulfate: ash falls out fast, sulfate persists.

The Volcanic Explosivity Index (VEI)

The 1815 Tambora eruption

The largest documented volcanic eruption of modern era.

• Located in Indonesia.
• April 10-11, 1815.
• VEI 7 rating.
• ~150 km³ of material erupted.
• Immediate deaths from ash and pyroclastic flow: 71,000.
• Ejected 60+ Mt of SO2 into stratosphere.
• Caused "Year Without a Summer" — 1816.
• Global temperature dropped 1°F.
• Crop failures across Europe and North America.
• Famine deaths from cooling: 100,000+.

The Year Without a Summer

• Snow in June across New England.
• Frost in July and August.
• Crop failures worldwide.
• Grain prices tripled in Europe.
• Bread riots in France, England.
• Mass migration from New England to Midwest.
• Mary Shelley wrote Frankenstein during that cold summer at Lake Geneva.
• John Constable painted "sky studies" of tinted sunsets.

The 1883 Krakatoa eruption

• August 26-27, 1883.
• VEI 6 rating.
• Located in Sunda Strait, Indonesia.
• Cataclysmic explosion.
• Tsunami: 36,000 dead.
• Sound heard 3,000 miles away — loudest in modern history.
• Global temperature dropped 0.5°F for 5 years.
• Red sunsets for years — inspired Edvard Munch's "The Scream."
• Pyroclastic flows across Sunda Strait.

The 1991 Pinatubo eruption

The largest volcanic eruption of the 20th century.

• Philippines, June 15, 1991.
• VEI 6.
• 15 Mt of SO2 ejected into stratosphere.
• Global temperature dropped 0.9°F for 2-3 years.
• Warned in advance — evacuation saved lives.
• Direct deaths: 847.
• Property damage: $10 billion.
• Also disrupted El Niño patterns.
• Best-studied major eruption for climate impact.

Recent eruptions

The Hunga Tonga case

The 2022 Tonga eruption was unusual.

• Submarine volcano.
• Injected 146 Mt of water vapor into stratosphere.
• Highest volcanic plume ever measured (30+ miles).
• Warmed the stratosphere slightly (opposite of most eruptions).
• Small surface cooling of about 0.05°F possible.
• Long-term effects still being studied.

Volcanic forcing on climate

• Historical volcanic activity has been a major climate driver.
• Volcanic winters preceded fall of Roman Empire.
• Little Ice Age (1300-1850) partly driven by volcanic activity.
• Modern climate models must account for volcanic forcing.
• Regional impacts: acid rain, ozone effects, agricultural.

Volcanic geoengineering proposals

• Some researchers propose injecting stratospheric aerosols to counteract global warming.
• Would mimic volcanic cooling.
• Estimated cost: $10-20 billion per year for meaningful effect.
• Would need to continue indefinitely.
• Termination shock: sudden stop = rapid warming.
• Side effects: ozone depletion, altered precipitation.
• Governance issues: who controls?
• Highly controversial in climate policy.

The volcanic weather ripple effects

• Aviation: ash disrupts.
• Agriculture: growing seasons shortened.
• Fisheries: ocean chemistry changes.
• Species migration.
• Ozone layer: some eruptions damage.
• Solar power: reduced sunlight.
• Weather patterns: shifted for years.

Learn more

• Climate change and severe weather
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• Wildfire weather
• Weirdest weather events