The effects of the largest volcanic events can be even more dire than that.

The Permian-Triassic boundary extinction, the "Great Dying", is though to be related to the massive Siberian Traps large igneous province. This eruption had the unfortunate luck to be through one of the largest and oldest sedimentary basins on the planet. So, magma was introduced into this massive basin, cooking sedimentary docks including coal, organic-rich shales, and evaporites (salts).

The result was truly massive gas emission, producing over time hundreds of pipes up to ~1 km in diameter that ejected a nasty mix of gases, mostly steam and CO2 (lots of CO2), but including chlorinated organic compounds from the high temperature reaction of the salts and fossil carbon. The halocarbons would have been enough to collapse the ozone layer.

Afterwards, CO2 levels and temperatures stayed elevated for five million years (equatorial ocean temperature may have been too high for vertebrate life to survive there). The ordinary process by which CO2 is drawn back down (by absorption into the oceans and deposition of carbonates via Ca/Mg eroded off continents) was interrupted for some reason, perhaps because silica-utilizing ocean microorganisms had been killed off, causing those cations to instead form clays in a process called "reverse weathering".