There is a terrifying article by Peter Ward in this month's Scientific American. There may be strong evidence that several of the last few great extinctions may be due in part to global warming. There is a clear geochemical signature that the most recent one 65 million years ago that wiped out the dinosaurs was caused by an asteroid strike in the Yucatan peninsula but the Great Dying at the end of the Permian 250 million years ago for example (see my previous post here) looks completely different.
As I wrote before, this extinction was marked by anoxia in the oceans. What I didn't write was that biomarkers such as certain lipids have been found in the ancient strata that indicate the presence of lots of photosynthetic green sulfur bacteria. For energy, they oxidize hydrogen sulfide (H2S) and convert it into sulfur. This means that the oceans were enriched with H2S at that time. If the oxygen level is sufficiently high then the H2S can be confined to the deep ocean by oxygen diffusing downwards. However, if the oxygen level drops enough the H2S will bubble to the surface. In addition to being foul smelling and poisonous, the H2S can also destroy the ozone layer and increasing UV radiation.
The circumstances that lead to this outcome could have been triggered by global warming from
massive volcanic activity that spewed tons of C02 into the atmosphere. This heated the oceans and made it harder to absorb oxygen. The extinction would begin in the ocean and then spread to land. A less intense version of this scenario may have taken place as recently as 54 million years ago at the end of the Paleocene era. During that time the concentration of C02 was about 1000 parts per million. We are currently at 385 and at current rates could reach 1000 by the end of the next century. So, if you start smelling rotten eggs on your stroll along the beach...