How do you measure how big a volcanic eruption is?

Volcanologists use a scale called the Volcanic Explosivity Index, or VEI. It was devised in 1982 by Christopher Newhall of the US Geological Survey and Stephen Self, and it rates an explosive eruption by how much material is thrown out, to what height, and how long the eruption lasts.

The scale is open-ended in principle, but the biggest eruptions in history get an 8. A 0 is a non-explosive eruption that throws out less than 10,000 cubic metres of ash and rock. An 8 can eject a trillion cubic metres and send a cloud more than 20 km high. From VEI 2 upward every step means about ten times more ejecta, which is why the numbers feel small and the eruptions are not. Tambora in 1815 was a 7, the most recent confirmed eruption of that size. Pinatubo in 1991 was a 6, the second-largest of the 20th century.

Most eruptions are modest. The Smithsonian's Global Volcanism Program has put a VEI on 7,742 eruptions of the last 11,700 years, and about 49 percent rate 2 or lower, 90 percent 3 or lower. Giants are rare: at least 10 eruptions of VEI 7 in that time, and about 40 of VEI 8 in 132 million years.

Here is the catch. In 1783 the Laki fissure in Iceland rated only a 4, and yet it spread a haze over Europe, which people called the Laki haze, and it counts among the most climatically and socially significant natural events of its millennium. The VEI is good at ranking how explosive an eruption was. It is not as good as sulfur dioxide emissions at telling you what an eruption will do to the climate.