Re: Natural release of CO2 from fosil fuels

I'm not sure I entirely understand your question, but let me start at the
end -- CO2 is not a commonly found natural by product.  

There ARE natural sources (and "sinks") of carbon dioxide, namely:

- Animal and plant respiration.  Conveniently, plant photosynthesis removes
CO2 from the atmosphere;
- Ocean-atmosphere exchange, where the oceans absorb, then release CO2 at
the sea surface; and
- Volcanic eruptions, which release carbon from rocks.  

Other than that, you'll be hard-pressed to find natural sources of CO2,
especially from fossil fuels.  

100 years from now?  If all fossil fuel use was stopped, right now, the CO2
from *those* emissions would likely have been scrubbed from the atmosphere.
 Animal, human and plant respiration, ocean-atmosphere exchange and
volcanic eruption carbon production (while minimal) would certainly produce
a considerable amount of CO2 for the atmosphere's balance -- our little
plants would have to scrub their little leaves out, especially if
populations continue to grow the way they are now.

I'm not a betting woman, so I wouldn't really assign percentages to these
things... 

Best of luck,

Marie-Helene
Mad Scientist