Do volcanoes contribute to a heating climate?

Why I'm asking 

Over on Twitter, someone complained that I shouldn't be moaning about farmers contributing to climate change. I should be worrying about increased volcanism, because this was a much bigger problem for global warming. So I thought I'd check my facts.

I put the question to my Microsoft friend, Bing and it pointed me to the US Geological Service, which had this to say.

In a Nutshell

The USGS provided some interesting facts:

• Volcanoes do have an influence on climate change.
• Ash from volcanoes has little impact on climate change because they stay in the atmosphere for a short time; days to weeks.
• Volcanic gasses such as sulphur dioxide can cause global cooling.
• Volcanic carbon dioxide can potentially contribute to global warming.

So, we can discount ash as a problem for climate either way, but what about sulphur dioxide and carbon dioxide?

Sulphur Dioxide

Sulphur dioxide has the biggest impact on climate because it is quickly converted to sulphuric acid, which then condenses into sulphate aerosols in the stratosphere. These fine particles are reflective, resulting in more of the Sun's heat being reflected back into space and resulting in cooling of the atmosphere beneath.

In 1991 Mount Pinatubo in the Phillipines erupted, putting out the biggest sulphur dioxide cloud in living memory. This lowered the global temperature for three years, by up to about half a degree Celsius.

Carbon Dioxide

Volcanoes can indeed emit significant amounts of carbon dioxide. Mount St Helens in the USA emitted about 10,000,000 tonnes of carbon dioxide in 9 hours when it erupted in 1980. This seems like a lot, but it takes mankind only 2.5 hours to do the same. 

While large scale eruptions are noteworthy, they are intermittent, whereas our carbon dioxide emissions go on continuously. Scientists estimated that, in one year, mankind would cause carbon dioxide emissions 80 to 270 times greater than all the Earth's volcanoes combined.

Conclusion

I think you know where this is heading.

• Throughout history, mankind has only observed widespread cooling as a result of vulcanism.
• The mechanism for this is sulphate aerosols in the stratosphere reflecting the Sun's heat back into space.
• Volcanic carbon dioxide is a greenhouse gas, but its effect is overridden by the stronger effects of the sulphur dioxide.
• The amount of carbon dioxide emitted by volcanoes is very small compared to that emitted by human activities.

So no, volcanoes do not contribute significantly to a heating climate. Current evidence suggests the reverse. Volcanism generally results in a short term cooling effect.

Other reading:

Eos, Vol. 92, No. 24, 14 June 2011 - Volcanic Versus Anthropogenic Carbon Dioxide