By looking backward at the course of great extinctions, a paleontologist sees what the future holds.More than 200 million years ago, a cataclysmic event known as the Permian extinction destroyed more than 90 percent of all species and nearly 97 percent of all living things. Its origins have long been a puzzle for paleontologists. During the 1990s and the early part of this century, a great battle was fought between those who thought that death had come from above and those who thought something more complicated was at work.Paleontologist Peter. D. Ward, fresh from helping prove that an asteroid had killed the dinosaurs, turned to the Permian problem, and he has come to a stunning conclusion. In his investigations of the fates of several groups of mollusks during that extinction and others, he discovered that the near-total devastation at the end of the Permian period was caused by rising levels of carbon dioxide leading to...