Every time a volcano erupts, the average temperature of the Earth drops a little.

The dense ash plume spewing out of the top contains all manner of nasty stuff — pulverized rock, minerals and volcanic glass, as well as water, carbon dioxide, sulphur dioxide and hydrogen sulphide and a selection of trace gases.

The ash particles themselves are small — from about a millimetre down to about a thousand times tinier, depending on the composition of the magma in the volcano. These are the particles that famously interfered with air travel across Europe during the eruption of Eyjafjallajökull in 2o10. But they drop out of the sky within a few weeks under their own weight.

Instead, it’s another type of particle that cools the planet — known as stratospheric sulphur aerosols. These are ten times smaller than the smallest ash particles, so they stay in the air far longer. They’re formed when the suphur dioxide that comes out of the volcano interacts with water vapour and other gases.

These particles are the perfect size to interfere with sunlight, which changes the appearance of the sky. At sunrise and sunset it causes glorious red colours as the particles scatter more blue light out than usual.

During the daytime, however, the sky gets ‘hazier’ as more sunlight is bounced back into space before it can reach the surface of the Earth. This reflection is what causes the atmosphere to cool following volcanic eruptions — the bigger the eruption, the bigger the effect. For example, in 1991, the explosion of Mount Pinatubo in the Philippines caused global temperatures to drop for two to three years.

The eruotion of Mount Pinatubo caused global temperatures to drop for two to three years // USGS

Historically, this effect has played havoc with human civilisation. The five-year eruption of Mount Tarawera in New Zealand between 1315 and 1317 is thought to have cooled the planet enough to cause famine in Europe as harvests failed.

In 1783, Iceland’s Laki volcano caused a famine that spread across Europe, Africa and India, contributing to the deaths of more than six million people. However the extreme weather associated with this eruption provided the spark that eventually led amateur meteorologist Luke Howard to name the clouds.

Further back, a massive eruption of the supervolcano at Lake Toba in Indonesia about 70,000 years ago may have destroyed enough of primitive human society to nearly wipe us out. It’s been suggested that the ‘volcanic winter’ caused by the eruption may have left behind only around 3,000 to 10,000 individuals, causing a genetic bottleneck in human evolution.