First off, SO2 breaks down quickly on geologic timescales. The cooling effect is short-lived. CO2 takes longer to move out of the atmosphere. Next, modern day

First off, SO2 breaks down quickly on geologic timescales. The cooling effect is short-lived. CO2 takes longer to move out of the atmosphere. Next, modern day volcanic emissions aren't major players in current climatic changes, but that's not the case for the past. Large igneous provinces can dramatically alter global climate. CAMP magmatism was extremely widespread and prolific. You don't see the bulk of it at the surface, but large swaths of the eastern US coast and NW African coast are sitting on top of gigantic piles of basalt. However, it's not _only_ CO2 for these large magmatic events. You're also dramatically altering ocean basins. Large igneous provinces are most frequently produced as continents break up. When that happens, you get a new ocean basin, change circulation, split things apart, introduce new species, raise/lower global sea level (ocean basin shape changes), and generally do a whole lot to shake things up. Continents can break up without much magmatic activity as well, but when you have mantle plumes involved in continental breakup, you tend to get the double-whammy of changes in ocean shape + lots of volcanic activity over huge areas. The extinction at ~200Ma (end of the Triassic, though the round number is a concidence) is a great example of magmatic activity having a large effect, but don't forget the impact that changing ocean basin shape/volume has as well. Mass extinctions usually aren't things with single causes. Instead, they're generally the result of a lot of things combining. Folks focus on the impact as the single cause of the K-T extinction, and while there's not doubt that it's what pushed things over the edge, other changes that had been ongoing for millions of years before had already stressed many different global ecosystems. You had major changes in volcanism (another large igneous province occurring during continental breakup, though it's smaller), reconfiguration of ocean basins, and other things occurring at the same time. The timing makes it clear that the impact was the sudden shock, but things were already heavily shaken up before that.