How Volcanic Eruptions Change the Weather Around the World

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.

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.

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.

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.

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

Most of the eruptions that shaped past climate happened before anyone measured temperature, so scientists reconstruct them from ice. Snow falling on Greenland and Antarctica carries a little of whatever is in the air, and after a big eruption that includes sulphate from the stratospheric haze. The snow compacts into yearly layers, and drilling a core lets researchers count back year by year and spot the sulphate spikes. When the same spike appears in both Greenland and Antarctica, the eruption was almost certainly in the tropics, because only a tropical plume spreads into both hemispheres.

Tree rings supply the other half of the evidence. A cold summer slows growth, so the ring for that year is narrow, and a sudden frost during the growing season can damage the forming cells and leave a visible scar known as a frost ring. Matching ring records from several continents against the ice core dates shows cold years following big sulphate spikes again and again. That is how the Samalas eruption was traced: a huge thirteenth-century sulphate signal was known for decades before geologists found the Indonesian volcano responsible for it.

Not every big eruption follows the Pinatubo pattern. In January 2022 the underwater volcano Hunga Tonga-Hunga Ha'apai in the South Pacific exploded with enormous force, sending a plume higher than any measured before, well past the stratosphere. Because the vent was beneath the sea, the blast vaporised a huge quantity of seawater and carried it upwards, while releasing comparatively little sulphur dioxide. Satellite instruments found that the stratosphere's water vapour content had risen by roughly a tenth, an unprecedented jump.

Water vapour behaves differently from a sulphate haze. Rather than reflecting sunlight away, it absorbs heat radiating up from the ground, so it acts as a greenhouse gas and pushes slightly towards warming instead of cooling. It also cools the stratosphere itself and can influence the chemistry of the ozone layer. Scientists are still working out how large and how lasting the net effect has been, since the eruption did produce some sulphate as well. The case shows that the type of material an eruption lofts, not just its size, decides what it does to the climate.

One of the starkest volcanic winters left its mark in written records long before anyone knew what caused it. In 536 the Byzantine historian Procopius wrote that the sun gave its light without brightness, like the moon, for the whole year, and the statesman Cassiodorus, writing in Italy, described a bluish sun and a summer without warmth. Chroniclers elsewhere recorded crop failures, summer frosts and hunger over the following years. For a long time historians wondered whether a comet or an asteroid might be to blame, since no particular volcano was known to have erupted at the time.

Ice cores settled much of the question. Precisely dated layers from Greenland and Antarctica show a large sulphate spike in 536 from an eruption in the Northern Hemisphere, followed by another from a tropical eruption around 540, and tree rings across Europe and Asia show some of the coldest summers of the last two thousand years in the decade that followed. Which volcanoes were responsible is still debated. Some historians link the hardship of these years to the Plague of Justinian, which reached the Mediterranean in 541, but how far a cold, hungry decade helped the disease spread remains an open question.

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.