Q: Are volcanic eruptions good or bad for our climate? I know that they have short-term cooling impacts because they block out the sun, but they also give off CO2. So are there any long-term impacts on the climate, and which is more predominant - global warming or global cooling?
From Miss Katie Barker (October 2010)
Reply by Dr Marie Edmonds (University of Cambridge)
The short answer is that the cooling effect due to sulphate aerosols is most important, but it only lasts a few years after an eruption.
Volcanoes emit sulphur gases, which, during large explosive eruptions, form stratospheric aerosols when they combine with water. Sulphate aerosol both reflects solar radiation and absorbs it, resulting in stratospheric heating and tropospheric cooling. The cooling of the surface of the Earth was on the order of around 0.5 degrees C around the world after the 1991 eruption of Pinatubo in the Philippines, which emitted 20 million tons of sulphur dioxide gas. Aerosols have a relatively short lifetime in the atmosphere (because they sediment out), so even the largest eruptions only cause cooling which lasts a few years.
Volcanoes also emit carbon dioxide, but on a much smaller scale than emissions from human activity. Research suggests that the volcanic CO2 flux to the atmosphere is in the range 150 to 270 million metric tons per year. Anthropogenic CO2 flux (from burning fossil fuels, and land use changes) accounted for over 36,000 metric tons in 2008, according to an international study published in 2009 in a peer-reviewed journal, more than 130 times the volcanic CO2 output. It is beyond doubt that the contribution from volcanic CO2 to the current greenhouse warming is negligible.