There is irrefutable scientific evidence of five global mass extinctions in the history of this planet independent of a flood, loose morals, or a mythical, Puck like god flitting through the bushes playing tricks on the gullible humans to make it seem like the earth has been around long enough to be overdue at the cosmic library. Elizabeth Kolbert has written a very readable account of these extinctions and represented contemporary evidence that suggests a sixth extinction is in progress and may well be inevitable no matter how much profit Exxon or BP made last year.
Kolbert makes it clear that what causes an extinction can be catastrophic or gradual but the bottom line is that the existing species on the planet are in trouble when something alters the environment, even a little bit. So when you hear that Global Warming is nothing to worry about—it has happened throughout the life of the planet—it’s good to remember that compared to the dominance of most animals on this Earth, humans are little more than a speck on the timeline. And those other dominate species are all extinct now, their deaths greatly the result of environmental changes much like Global Warming.