I haven't seen the movie, but I'm told that in
An Inconvenient Truth, Al Gore shows a chart that is supposed to prove that CO2 caused global warming in the past*, but it also happens to prove that global warming is a natural cycle, because all of the CO2 + high temp periods are before the Industrial Revolution...indeed, most of them are before we had civilizations.
*It doesn't prove it, it only proves that at times of high temp we also have high CO2. The high temps could be causing the high CO2 by causing it to outgas from the increasingly warmer oceans.