We know that the buildup in the atmosphere is anthropogenic by a variety of means, including changing carbon isotope ratios, corresponding decrease in atmospheric oxygen, and the fact that the buildup of CO2 is roughly half what we would expect from our emissions, showing that nature is a net carbon sink. Concerning that last part, increase CO2 uptake from the oceans and accompanying acidification is a crucial part of closing the carbon budget and figuring out where it all goes. 

That's not to say that underwater volcanic activity does not impact local pH levels near vents. In fact, looking near underwater vents is a useful tool for seeing how the rest of the ocean might respond to the observed changes: