There is an AGW site called RealClimate (I’m not going to link to them). They have a post up mocking  the use of volcano’s as a source of CO2/SO2 etc compared to human sources.

I did a post a while back looking at the amount of SO2 produced by one volcano.

Here is another news snippet from the same period.

“The sulfur dioxide (SO2) emitted from the Holuhraun eruption has reached up to 60,000 tons per day and averaged close to 20,000 tons since it began. For comparison, all the SO2 pollution in Europe, from industries, energy production, traffic and house heating, etc., amounts to 14,000 tons per day.”

Admittedly this is unusual. But there are 3,000,000 undersea volcanoes.

And they could have pumped out a lot of CO2 in the past.

The climate-driven rise and fall of sea level during the past million years matches up with valleys and ridges on the seafloor, suggesting ice ages influence underwater volcanic eruptions, two new studies reveal. And because volcanic chains suture some 37,000 miles (59,500 kilometers) of ocean floor, the eruptions could pump out enough carbon dioxide gas to shift planetary temperatures, the study authors suggest.