Where does the majority of CO2 turnover take place?

Question

What is it's median life span for this local?

Answer 1

cars, factories...  

Answer 2

The oceans.  The life span is tricky, it depends on ocean currents bringing up CO2 from the depths.  But "years" would get you in the ballpark.

Note though, that isotopic ratios prove the major increase in CO2 seen recently is mostly due to burning fossil fuels.  The ocean is more balanced with regard to CO2 in being roughly the same as CO2 out.