Robert Boyd of McClatchy reports (Scientists close in on mass killer of life on earth) that some scientists think CO2 was implicated in the great extinction of 251 million years ago. Almost all life (95% of marine species, 85% of land species) died off:
Exactly what caused the ancient mass extinction is still unclear, but here's how many researchers think it may have unfolded:
Over a period of about a million years, an enormous quantity of lava from deep in the Earth's interior oozed up through giant cracks in Siberia's crust. The molten mass "froze" into step-like slabs of flood basalts, volcanic rocks known as the Siberian Traps.
Enough lava gushed out to cover an area almost as large as the continental U.S....
As is happening now, the Earth warmed more near the poles than it did at the Equator. The smaller temperature difference slowed the great ocean currents that keep the waters circulating. The oceans stagnated and lost most of their oxygen. Marine plants and animals suffocated.
What happened to snuff out life on land is still debated. Some researchers believe that bacteria in the ocean, living on sulfur instead of oxygen, churned out vast quantities of hydrogen sulfide (H2S), a lethal gas with a rotten-egg smell. As the hydrogen sulfide gas emerged from the sea, it choked half of all land creatures.
Atmospheric concentrations of CO2 reached 1,000 to 1,500 parts per million. Today's level is 385 parts per million and the concentration is currently rising at 2 parts per.
Boyd is interest in laying out this particular scenario and not in going into alternative ideas. He certainly doesn't assert that there is a scientific consensus behind this. This Wikipedia article lays out some ideas: Permian-Triassic extinction event.