The mean residence time (τ\tauτ) of an element in a biogeochemical reservoir is calculated by dividing the total mass of the element in the reservoir by the sum of its outflow (removal) fluxes.

Based on the simplified global carbon cycle model shown above, what is the mean residence time of carbon in the atmosphere?
2.0 years2.0\text{ years}2.0 years
4.0 years4.0\text{ years}4.0 years
7.0 years7.0\text{ years}7.0 years
9.3 years9.3\text{ years}9.3 years