The eruption of the Eyjafjallajöekull volcano in Iceland in 2010 was the latest in a series of volcanic events across the globe. But what effect do volcanoes have on our climate?
We know volcanoes emit two gases which can have an impact on global temperatures — sulphur dioxide and carbon dioxide (CO2). They each have very different effects and work on different timescales:
Sulphur dioxide — when this gas is emitted to high altitudes (about 50,000 ft or above) it enters the stratosphere. Here it can form acid droplets that partially scatter and reflect sunlight away from the Earth, which cools the surface. The droplets have a fairly immediate impact and, in enough, concentrations may cool the climate for a few months — or even a year or two — but then the droplets fall out of the stratosphere and things return to normal.
CO2 — we know this is a greenhouse gas, so when it is emitted in large enough quantities it could have a warming impact on our climate. CO2 lasts in the atmosphere for about 100 years, so any impact will be felt over a long timescale.
The effect of volcanoes on our climate has led some to doubt human impact on climate change. For example, it has been suggested volcanoes put out more CO2 than human activity, but this is not the case.
In an average year, volcanoes put out an estimated 100—130 million tonnes of CO2 globally. Estimates suggest human activity puts out more than 200 times as much — about 26,000 million tonnes of CO2 per year.
Volcanic activity has been fairly consistent over a long period of time. While there are some major eruptions which cause brief changes in climate, the amount of CO2 in the atmosphere has balanced out fairly quickly due to natural cycles.
The 35% increase in CO2 which has been observed over the last century is believed to be unprecedented, and the only plausible explanation for that increase is human activity. All the evidence suggests it is this increase in CO2 which is magnifying the greenhouse effect and causing our climate to change.
Recent history has given us a good case study of the impact of a large volcanic eruption. Mt Pinatubo in the Philippines erupted in 1991, marking one of the biggest volcanic events of the 20th century.
It put about 20 million tonnes of sulphur dioxide into the stratosphere. As we'd expect, this impacted global temperatures. The average for the following year was reduced by between 0.1 °C and 0.2 °C, although temperatures quickly recovered the year after.
A large amount of CO2 was also emitted, with an estimated 250 million tonnes being put into the atmosphere. This is a significant amount, but is still much smaller than produced from human activity, such as burning fossil fuels and changing land use, which is estimated to put out about 26,000 million tonnes of CO2 a year.
Indications are that the eruption in Iceland won't have a significant impact on our climate. Compared to an eruption such as Mt Pinatubo, Eyjafjallajöekull has been very small. The quantities of both sulphur dioxide and CO2 emitted are far smaller, and are unlikely to have a discernable impact.