While the above ground volcanic activity would bring a cooling as you state, the underwater activity is now being found to do the direct opposite. Many of these vents were not even known to exist in the numbers they have found today as well, and the activity although known was never documented as to it cycles and frequency. The volume was also underestimated if I am reading correct, (from many recent studies on this).  


I believed just as you do, but now with much more coming to light via research and new understandings, I don't think it can so easily be dismissed anymore as it was in the past. Man has an impact, but what is his impact if there are much greater events taking place in nature that were misunderstood in the past, and may today be of much greater magnitude than man's effect on the climate? 



Glad you're here, always enjoyed reading your input, respectfully GREENLEAF, I omit the "2" out of respect, there's only one GREENLEAF! Ever lose that other impostor? Going to eat!