What I would like to see is a comparative study of the CO2 levels in Antarctic ice samples correlated with events from the same time period. For example, in those past ages where the ice samples show elevated CO2 levels were there large volcanic eruptions, or massive fires, etc.? Then we could compare the current CO2 levels with modern similar events and see if there is any correlation. If it turns out that we have elevated CO2 levels without the corresponding CO2 producing events then that certainly would bolster the position that man-made CO2 is having an effect. Unfortunately, the hard part is that we would only be able to make a correlation since recorded history. There may be a few events that we could correlate with like Krakatoa in 1883 and there may be some others.
I suppose in either event, the world has enough garbage thrown into the air with forest fires and volcanic eruptions that we might say if we have a choice of whether to generate more CO2 or not, why not, not?
Here is a quote from John Hofmeister's book, "Why We Hate the Oil Companies". Hofmeister is former President of Shell Oil North America.
“As the debate rages on, we’re throwing ton after ton of gaseous effluent into the atmosphere and doing little or nothing about it. That’s wrong. It’s wrong for today, tomorrow, and forever. We breathe what we put into the air. It is time to come to grips with gaseous waste in much the same manner that we have tackled other waste problems. If we did not deal with physical waste in the modern age, we’d be suffocated by our own trash. If we did not deal with liquid waste in the modern age, we’d be poisoned by our own filthy water. The fact that we are not dealing adequately with gaseous waste is wrong, wrong, wrong. How strongly can I say it?” (Pg. 65)