[palmer]

Due to the isotopic signature of fossil fuel CO2, the contribution of human activities to the total CO2 content of the atmosphere can be estimated with high accuracy

There's no quantitative analysis with any accuracy showing how much CO2 comes from fossil sources. The big thing that is missing is accounting for natural fluctuations in the isotope ratios. There are certainly qualitative analysis that show "much" of the CO2 comes from fossil sources and the simple fact is the excess CO2 (regardless of isotopes) can't have come from natural sources (e.g. oceans) in sufficient quantity to form today's CO2 spike.

[cogitator]

There's no quantitative analysis with any accuracy showing how much CO2 comes from fossil sources.

Making me do a literature search, huh? The best quantitation is from flux measurements. Oceanographers do atmospheric and in-water pCO2; very simple math gives you the flux at that point. Land scientists can do similar either with flux chambers or soil moisture CO2 measurements. That's how you get a schematic like this:

Now, as for quantifying the Suess effect, try this one. It's a Powerpoint presentation; several interesting references in it.

http://ecology.botany.ufl.edu/radiocarbon07/Downloads/2006%20Lectures/2006%20Atmosphere.ppt

Look at slides 18 and 52. Those plots imply to me that there's a model (more than one, actually) estimating CO2 fluxes and deriving the Suess effect numbers for comparison with observations.

If the models match the observations as well as shown, then that's quantitation in my book. (In case anyone despairs models, this is how it's done. You construct a model that predicts measurable quantities and then examine the predictions of the measurable quantities against the actual measurements.)