More About the Airborne Fraction of CO2

After writing this post, I went looking around a bit to see what's in the literature.

I came across Canadell et al, PNAS (2007), which gives the results below saying that the global airborne fraction is slightly increasing with time, because the ocean is taking up less CO2:

At first I thought that calculating the ocean's uptake of CO2 might be relatively straightforward (which is not to say easy), but now I'm thinking probably not, as you'd need to know the ocean's temperature as a function of locale and depth, because Henry's constant varies with temperature. But it might make a nice back of the envelope calculation....