Accurate estimate of CO2 flux to the ocean

Would appreciate if anyone can spare some time to help solve this problem?

1. Assume the present concentration of CO2 in the atmosphere is 360ppmv (mL/m3), and the average concentration in the surface layer of the ocean is 0.868 mg/Lof CO2(aq). Calculate the flux rate in Gt C/yr(1 gigaton = 1015g) to the ocean from the atmosphere. Can you tell why it is so difficult to get accurate estimate of CO2 flux to the ocean?

A= 3.6×1018cm2 ocean area
H= Henryˈs constant = 0.77(mg/Lair)/( (mg/LH2O) at 15˚C
KL= overall mass transfer coefficient = 20.0 cm/h