Impact from the deep (2006) [pdf]

This is an excellent article from Scientific American about a proposed mechanism for the End-Permian mass extinction, the largest in Earth's history. The author describes how deoxygenation of the oceans may have allowed hydrogen sulphide, produced by anaerobic bacteria, to spread through the ocean and atmosphere, suffocating oxygen-breathing life and also compromising the ozone layer.

The whole article is thought provoking, but in particular the illustration on page 6 is not to be missed.

In the context of people asking what the worst case for global warming is, this is always what comes to my mind, rather than sea level rise or polar bears. The proposed mechanism is deoxygenation of the oceans due to rising temperatures and carbon dioxide levels, in turn caused by extreme volcanic activity.