Yawn.<p>The relevant question is "what do we do?". That depends (somewhat) on "how much change?". (We do know that the earth has been warmer and cooler, and we also know that it has had significantly more atmospheric CO2 so the "runaway" folks are wrong.)<p>Note that "how much change" does not depend on "how much did we do". For example, suppose that global warming was caused by a recent trend of volcanos spewing CO2. We might still need to do something.<p>In other words "natural causes good, artificial causes bad" is wrong.<p>I do find it interesting that people think that any deviation from 1972 or so is bad. The 70s were nice and all, but I don't see why they're the only acceptable temperature.