It is apparently not unusual among global warming skeptics to assert that volcanoes produce more carbon dioxide than human activities. However, a recent review of the literature in EOS, Transactions, American Geophysical Society show this not to be the case. Present day volcanoes emit about as much carbon dioxide annually as states like Florida, Michigan and Ohio.
Degassing by volcanoes forms part of the natural CO2 cycle, returning CO2 lost by weathering (turning CO2 into rock), formation of carbonate rocks and organic deposition (creatures dying). Rock that is subducted below plates eventually emerges in volcanoes which burp periodically (paroxysmal volcanic explosions) as well as the regular outgassing in sub-aerial and submarine volcanoes.
There is a range in annual emissions between 0.13 and 0.44 billion metric tons (gigtons) per year. Compare this to anthropogenic (human) sources in 2010 of 35 gigatons per year and you get the idea. These sorts of figures compare with countries like Pakistan (0.18 gigatons) or South Africa (0.44 gigatons). The most widely quoted and likely accurate study has the volcanic emissions at 0.26 gigatons per year. So what is the difference between anthropogenic and volcanic sources? A mere factor of 135!
The paroxysmal emissions sometimes match the human rate - so for example the 9 hour duration of Mt St Helens and Mt Pinatubo reached 0.001-0.006 gigtons per hour, compared to the 0.004 gigtons per hour from anthropogenic sources. While the rates are similar, the timescales are very different.
What is more, to blame current global emissions on volcanoes would require a startling amount of activity. The amount of magma that would be required to match the amount of carbon would be some 40 times the annual mid-ocean ridge amount of magma production. Another way of looking at it would to ask how many volcanoes would be required to account for 2010 CO2 emissions. The number is about 9500! One could deliver the 35 gigatons per year via paroxysmal eruptions, it would only take about 700 Mt Pinatubos per year.
Perhaps it really is easier to believe fossil fuel burning, land clearing etc are responsible.