Coyote writes:
Right now, we don't know the exact percentages from natural and human contributions.
Yes and no. We have multiple glaciation cycles we can look at, and CO2 seems to top out at around 300 ppm during the warm interglacial periods.
We were also at that maximum prior to the Industrial Revolution. In just 150 years we have gone from 300 ppm to nearly 400 ppm, a level of atmospheric carbon dioxide never seen in the ice cores. We have also seen a change in the carbon isotope makeup of that atmospheric CO2, and it matches the ratios found in fossil fuels.
I would call that extremely strong evidence that humans are the cause for the spike in atmospheric CO2 over the last 150 years. Is all of that extra CO2 from the burning of fossil fuels? Probably not. There are also feedbacks that would release other naturally occuring resevoirs, such as an initial increase of CO2 causing the oceans to warm and release more CO2. Nonetheless, our role seems to be obvious to me.