There's a terrific article by T. Gerlach in last week's issue of AGU's EOS newspaper [PDF]. Gerlach is apparently a volcanologist, not a climate scientist. The article compares estimates of CO2 emission by volcanic sources to human-made emissions. There is a common misconception (I'm not sure how common) about the relative contributions of volcanos and humans to the emission of CO2.
So, which do you think is the larger source of CO2, global volcanos or human-made sources (includes land-use change, cement production, transport, energy, etc)?
The answer is that humans produce (as of 2010) about 135 times more CO2 than all the volcanoes in the world. The numbers are something like humans emitting about 35 billion tons versus volcanoes emitting 260 million tons. There doesn't appear to be any tricky business going on, and all kinds of volcanoes (including underwater volcanoes) are included. In fact, Gerlach goes on to make lots of interesting comparisons: one good one is that human activities produce the same amount of CO2 emissions as the global volcanic annual source every 12.5 hours.
Gerlach also accounts for the explosive volcanic eruptions like Mount St. Helens and Pinatubo, which are called paroxysms. These can suddenly emit a lot of CO2, but they are rare. While these paroxysms are happening, their emission rate might be as large as all of humanity's. The catch is that they only last a few hours, and wind up contributing very little to the global CO2 emissions.
Really nailing the coffin shut, Gerlach also considers what it would mean for volcanism if the emission rate did match the human-made CO2 emissions of about 135 Gigatons per year. It sounds like it would mean that we would need one (or more) supereruptions per year. Supereruptions are huge volcanic eruptions, like when Yellowstone blew up 2 million years ago, and they are rare even for geologists, happening only every 100,000-200,000 years or so.
The only weakness of the article is that Gerlach forgets to address the obvious climate change denial argument that humans are not emitting as much CO2 as claimed. There are hints of how to deal with this, however, as Gerlach mentions that the total volcanic emission is equal to that of about two dozen large coal-fired power plants. The volcanic number is so small that we can easily eliminate huge amounts of human-made emissions and still have more human-made emissions than volcanic emissions by more than an order of magnitude. So I think this presentation is a great place to point to really come to terms with this comparison, and the vast difference in the volcanic versus human-made emissions means that there can be no dispute.
Also see another Gerlach article at EARTH Magazine [LINK].