I'm not 100% sure this is the answer to your question, but it's worth beating in mind in any case:
Burning fossil fuel adds carbon to the ecosystem. At any one time some of that carbon exists as CO2, but much of it exists in other forms, e.g. in biomass. This is a cycle, but with a constant addition of carbon that had previously been locked away deep underground, essentially uninvolved. Even if certain specific molecules of CO2 only last however long, the overall carbon content, and thus the overall CO2 level, are going up.
Burning fossil fuel adds carbon to the ecosystem. At any one time some of that carbon exists as CO2, but much of it exists in other forms, e.g. in biomass. This is a cycle, but with a constant addition of carbon that had previously been locked away deep underground, essentially uninvolved. Even if certain specific molecules of CO2 only last however long, the overall carbon content, and thus the overall CO2 level, are going up.
(Which is utterly uncontroversial, see http://en.wikipedia.org/wiki/Keeling_Curve )
P.S. You mean only 20 billion per year.