Two hundred years ago, Tambora, a volcano in Indonesia, blew its top in the most violent eruption in recent history. Damage from the volcano and an associated tsunami was immense; perhaps 100,000 people died immediately, or starved in the aftermath. The effects on the wider world, though, were even greater. Like all large eruptions, Tambora’s was to change the climate around much of the planet for years.
But just how do volcanoes change the climate?
Eruptions spew out not just lava and ash, but also gases—indeed it is these gases, trapped under great pressure in molten rock, that give an eruption its explosive power. For the climate, the key gas is sulphur dioxide. Once it gets into the stratosphere, sulphur dioxide from a volcano mingles with water, forming tiny sulphate particles.
These particles reflect some sunlight back into space, and the surface below cools. They also absorb some sunlight, warming up the stratosphere. These temperature changes have big knock-on effects. A cooler surface means less evaporation, and thus less rainfall. A warmer stratosphere means stronger jet streams. In the year after Tambora’s eruption, scientists estimate the stratosphere's sulphate veil caused a three percent drop in rainfall and cooled the planet by one degree Celsius. That is a temperature drop in one year twice as large as the long-term warming the Earth has seen over the past half-century. The climate upheaval caused a hiatus of the Indian monsoon, drought in southern Africa and widespread crop failures in Europe, where it was known as the year without a summer.
No one can say when an eruption large enough to have such drastic effects will happen next. That one will happen, though, is a certainty.