The evidence that the Earth's "Big Five" extinction events were triggered by the enormous volcanic events that created the so-called Large Igneous Provinces (LIPs) continues to accumulate. The most catastrophic of these, the end Permian extinction, killed most of the species that existed at the time. It was long suspected that it was tied to the epic Siberian Traps volcanism (think many millions of Mount Saint Helens eruptions) and new geochronological evidence makes the evidence of simultaneity far more precise.
So why were these eruptions, which large as they were, were confined to relatively small portions of the globe, so catastrophic for the whole planet? The big culprit seems to have been carbon dioxide, with an assist from sulfur dioxide. The sulfur dioxide caused as sharp global cooling, lasting only a few years, and extreme acid rain. The CO2 triggered multi-millenium long global warming and ocean acidification, producing anoxic oceans that killed most sea life. The weather likely sucked too, perhaps including the enormous hypercanes (super powerful hurricanes).
The other big extinction events are also tied to the formation of large igneous provinces, though the timing is less precisely tied down. Even the end Cretaceous extinction event, usually associated with the impact that created the Chicxulub crater, is closely associated with the formation of the Deccan Traps LIP. The impact may have either accelerated the LIP or just provided another major aggravating factor. All of the other mass extinctions show the same big CO2 pulse.
Other evidence shows that warm periods in the Earth's history are associated with less spectacular instances of unusually widespread volcanism, again with CO2 as the executive agent.
At the risk of being tedious, one more sign that we are on a disastrous course with our own experiment in massive CO2 release.