Additional info from the U.S. Geological Survey
Do the Earth’s volcanoes emit more CO2 than human activities? Research findings indicate that the answer to this frequently asked question is a clear and unequivocal “No.” Human activities, responsible for about 35 billion metric tons (gigatons) of CO2 emissions in 2010, release an amount of CO2 that dwarfs the annual CO2 emissions of all the world’s degassing subaerial and submarine volcanoes.
The anthropogenic CO2 emissions for 2010 are 135 times larger than the most-commonly accepted maximum global volcanic CO2 estimate of 0.26 gigatons per year. About 70 volcanoes are normally active each year on the Earth’s subaerial terrain. One of these is Kīlauea Volcano in Hawaii, which has an annual baseline CO2 output of about 0.0031 gigatons per year. It would take a huge addition of volcanoes to the subaerial landscape — the equivalent of an extra 11,200 Kīlauea volcanoes — to scale up the global volcanic CO2 emission rate to the anthropogenic CO2 emission rate. Similarly, scaling up the volcanic rate to the current anthropogenic rate by adding more submarine volcanoes would require an addition of about 360 more mid-ocean ridge systems to the sea floor, based on mid-ocean ridge CO2 estimates of Marty and Tolstikhin.
There continue to be efforts to reduce uncertainties and improve estimates of present-day global volcanic CO2 emissions, but there is little doubt among volcanic gas scientists that the anthropogenic CO2 emissions dwarf global volcanic CO2 emissions.
For additional information about this subject, please read the American Geophysical Union's Eos article "Volcanic Versus Anthropogenic Carbon Dioxide", written by USGS scientist Terrence M. Gerlach.