Dot Earth, the influential New York Times science blog by Andrew Revkin carries an online discussion among climate experts, including UA's Jonathan Overpeck and Dan Sarewitz at ASU in response to a question about whether human-driven climate warming is just a 'side-show' compared to natural extremes we see in the geologic record.
An argument I've made before is that we've had major extinction events in the geologic past, wiping out perhaps 90% of all species on Earth. But that doesn't mean that humans are not wiping out species today or that we should just ignore human impacts because they may be smaller today than some non-human induced events of the past.