[mv37afnr]

Off the cuff, I would imagine that the sudden cooling would lead to an increased burning of fossil fuels for heating, thus exacerbating the problem.

Volcanoes also release a huge amount of greenhouse gas into the atmosphere (though any given volcano generates just a fraction of the CO2 that humans do in a year). The cooling effect of the ash is temporary, but the gases would remain in the atmosphere for quite a long time.