Relating volcanoes, humans and CO2 emissions:
volcanoes.usgs.gov/Hazards/What/VolGas/volgas.html
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Scientists have calculated that volcanoes emit between about 130-230 million tonnes (145-255 million tons) of CO2 into the atmosphere every year (Gerlach, 1999, 1991). This estimate includes both subaerial and submarine volcanoes, about in equal amounts. Emissions of CO2 by human activities, including fossil fuel burning, cement production, and gas flaring, amount to about 27 billion tonnes per year (30 billion tons) [ ( Marland, et al., 2006) - The reference gives the amount of released carbon (C), rather than CO2, through 2003.]. Human activities release more than 130 times the amount of CO2 emitted by volcanoes--the equivalent of more than 8,000 additional volcanoes like Kilauea (Kilauea emits about 3.3 million tonnes/year)! (Gerlach et. al., 2002)
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Even if we were to include more data from submarine volcanoes, I doubt they would make that much of a difference on the grand scheme of things, given the extremities over here. We do have data on most volcanoes and some submarine ones. And, of all the sources, it's seems pretty obvious that humans beat volcanoes in CO2 emissions by a great deal.
But keep in mind that many of the submarine ones are deep in the ocean, where the CO2 will usually stay and dissolve (or form compounds) rather than go up into the atmosphere. It's the same with methane, if you've ever read up on Methane Hydrates.
