The chart on the left (view in full) shows how rising carbon dioxide levels are associated with mass extinctions and that based on projections we could be heading into another such period. The graphic appears in Peter Ward's article, Impact from the Deep(1) in the October issue of Scientific American.

1. http://www.sciam.com/article.cfm?articleID=00037A5D-A938-150E-A93883414B7F0000&pageNumber=1&catID=2