Tambora and the Year Without a Summer

The eruption

Tambora, on the island of Sumbawa, had been rumbling for three years. On 10 April 1815 it erupted catastrophically — a VEI 7, ejecting roughly 100 km³ of rock and ash and cutting about 1,400 m off the summit, leaving a caldera 6 km wide. Ash fell over an area larger than France. Pyroclastic flows and the tsunamis they triggered killed on the order of 10,000 people outright; the famine and disease that followed across the region raised the total, by most estimates, to somewhere between 70,000 and over 100,000.

How a volcano cools the planet

The key ingredient is sulfur, not ash. A large explosive eruption injects sulfur dioxide gas directly into the stratosphere, above the weather. There it reacts with water to form a haze of tiny sulfuric-acid droplets that spreads worldwide and lingers for one to three years, reflecting a fraction of incoming sunlight back to space.

The effect is measurable and temporary. Tambora is estimated to have cooled global average surface temperature by roughly 0.4–0.7 °C for a year or two. Pinatubo in 1991, a smaller VEI 6 but unusually sulfur-rich, cooled the planet by about 0.5 °C — an effect clearly visible in the satellite temperature record.

1816: the year without a summer

A fraction of a degree in the global average translates into much larger swings in specific places and seasons. The summer of 1816 in western Europe and north-eastern North America was cold, wet and dark. There was frost and even snow in June in New England; harvests failed across Europe; food prices doubled or worse; and there were bread riots in several countries. It was, in much of the North Atlantic world, the coldest summer in centuries.

The cultural footprint is oddly large. Trapped indoors by the miserable weather at a villa on Lake Geneva, a house party of writers set themselves a ghost-story challenge; the results included Mary Shelley’s Frankenstein and the first modern vampire story. The strange yellow skies of the period show up in J. M. W. Turner’s sunsets.

Could it happen again?

Yes. VEI 7 eruptions occur roughly once or twice per thousand years; the previous one before Tambora was Samalas, also in Indonesia, in 1257. A repeat today would be forecast days to weeks in advance by monitoring, so the local death toll would likely be far lower — but the global climate and agricultural disruption would hit a far larger, more interconnected population. It is one of the natural hazards that most concerns risk analysts precisely because it is rare, global, and largely unmitigated.