From Karl J. Hansen:

Re: Did Native Americans contribute to global warming? (April 15): It is an interesting, but not surprising result. Although I have not read the actual study, I believe it is all sound science up to the point where they conclude that the added CO2 caused global warming. What basis could they probably have to conclude this?

We know today, that at current levels a 10% increase in CO2 is roughly causing (4, 7 ln(427/388) ) / 9 * 5 degree Celsius, namely about 0. 25 degree Celsius. However, this is a bit extreme and the question is if burning the forests in a controlled fashion would have caused a 10% CO2 increase, is plausible.

We have to remember that with all the human activity we have today, we still only contribute 5% of the CO2 according to the IPCC and a wild guess would suggest that we all-in-all produce more CO2 today, compared to native Americans a few thousand years ago.

It is important to browse the proportions: Yes, CO2 has the capability of causing warming, but the little we add to it, will be completely hidden in variations caused by other sources.