How Volcanic Eruptions Change the Weather Around the World
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In 1816 New England had snow in June, crops failed across Europe, and Mary Shelley, kept indoors by the rain in Switzerland, wrote Frankenstein. The cause was a volcano on the other side of the world that had erupted the previous April. Large eruptions reach the upper atmosphere, and what they put there can dim the sunlight over the entire planet for years. The mechanism is well understood now, and it is not the ash.
Ash falls, sulphur stays
An erupting volcano throws out ash, which is pulverised rock, and gases, mostly water vapour and carbon dioxide with a few percent of sulphur dioxide. The ash is the visible part and it does the local damage, burying fields and collapsing roofs, but it is heavy and falls out of the air within days to weeks. Its effect on the world's weather is small.
Sulphur dioxide is different. If the eruption is powerful enough to punch through the lower atmosphere into the stratosphere, above about fifteen kilometres, the gas reacts with water vapour there to form a haze of tiny sulphuric acid droplets. The stratosphere has no rain to wash them out, so the haze spreads around the globe on the high winds and lingers for a year or two, reflecting a fraction of the incoming sunlight back to space before it reaches the ground. The planet cools.
The year without a summer
Mount Tambora, on the Indonesian island of Sumbawa, erupted in April 1815 in the largest explosion in recorded history, about ten times the size of Krakatoa's in 1883. It killed tens of thousands locally and lofted an estimated 60 million tonnes of sulphur dioxide into the stratosphere. Global temperatures fell by around half a degree to a degree Celsius the following year, which sounds modest until it is spread across a growing season.
The summer of 1816 saw frost in every month in the northeastern United States, failed harvests from Ireland to Switzerland, food riots, and the last great subsistence crisis in the Western world. In Yunnan in China rice failed for three years. The spectacular red sunsets recorded by the painter Turner in those years were the haze. Nobody at the time connected any of it to a volcano nobody in Europe had heard of; the link was made only in the twentieth century.
Pinatubo, the measured eruption
The eruption of Mount Pinatubo in the Philippines in June 1991 was the first big one to happen under satellite observation, and it turned the theory into numbers. It injected about 20 million tonnes of sulphur dioxide into the stratosphere; the haze circled the Earth within three weeks and covered it within a year; and global average temperature dropped by about half a degree Celsius over the following fifteen months before recovering. Climate models that had predicted the cooling in advance got it right, which was an important test of the models themselves.
Pinatubo also showed the side effects. The haze absorbed sunlight as well as reflecting it, warming the stratosphere while the surface cooled, and it provided surfaces for the chemistry that destroys ozone, so the ozone layer thinned measurably for two years. Rainfall patterns shifted, with drier conditions across much of the tropics.
Why volcanoes do not cancel warming
The idea that volcanoes might offset human-caused warming comes up regularly and does not survive the arithmetic. All the world's volcanoes together emit somewhere between 100 and 300 million tonnes of carbon dioxide a year; human activity emits around 40 billion, more than a hundred times as much. Volcanic cooling, meanwhile, is temporary, lasting two or three years after a major eruption, while the carbon dioxide from burning fuel stays in the atmosphere for centuries.
The Pinatubo effect has inspired proposals for deliberately spraying sulphur into the stratosphere to cool the planet, a form of geoengineering. The eruption record shows it would work in the narrow sense and would have to be repeated indefinitely, since stopping would return all the deferred warming within years, and it would do nothing about ocean acidification. Whether that is a bargain worth taking is one of the harder questions in climate policy.
The eruptions that mattered
Not every big eruption cools the world; it has to be explosive enough to reach the stratosphere and rich enough in sulphur. Iceland's Laki fissure eruption of 1783 was not especially explosive but poured out sulphur for eight months, and the resulting haze killed a fifth of Iceland's population, poisoned livestock across Europe and is linked to a harsh winter and failed harvests that fed unrest in France. Some of the coldest decades of the last two thousand years line up with clusters of eruptions in ice cores. The largest of the historical period, with their effects:
- •Samalas, Indonesia, 1257: the largest eruption of the last millennium, followed by cold years and famine across Europe
- •Laki, Iceland, 1783: months of sulphurous haze, tens of thousands dead in Europe from the fumes and the winter
- •Tambora, Indonesia, 1815: the year without a summer
- •Krakatoa, Indonesia, 1883: global cooling of about a quarter of a degree, red skies for years
- •Pinatubo, Philippines, 1991: half a degree of cooling, measured from orbit
The takeaway
Big eruptions cool the planet not with ash but with sulphur dioxide that reaches the stratosphere and forms a reflective haze lasting a year or two. Tambora in 1815 produced the year without a summer and Pinatubo in 1991 cooled the Earth by half a degree under satellite watch. The effect is short-lived, and volcanic carbon dioxide is a rounding error next to human emissions.