Another common argument is that climate is always changing, and that Earth has had higher levels of CO2 in the past… and was much warmer. Richard Alley in the National Ice Core Lab in Denver CO, shows the hard (and very cold!) evidence that today’s levels of CO2 are unlike anything seen in the whole of human civilization, for much more than 400,000 years. And, by the way, when Earth was much, much hotter it was fine for dinosaurs. We might not like it quite so much.

Earth: The Operators’ Manual