The bottom line? Annual anthropogenic CO2 emissions exceed annual volcanic CO2 by two orders of magnitude, and probably exceed the CO2 output of one or more super-eruptions.  Super-eruptions are extremely rare, with recurrence intervals of 100,000–200,000 years; none have occurred historically, the most recent examples being Indonesia’s Toba volcano, which erupted 74,000 years ago, and the United States’ Yellowstone caldera, which erupted 2 million years ago.

At issue for the denialists, is fatalism. "There's nothing you can do; give up!". I would like to imagine that there might be something that one could do, to lessen bad effects.