Okay, onon, let's make this simpler.

Let's start with your equation. You wrote:
Now, I may be wrong, but I believe you actually intended this equation:

Natural Contribution = CO2 Rise - Human Contribution. That's the only way you could've arrived at a negative number for Natural Contribution. Correct me if I'm wrong.

For brevity's sake I'll abbreviate this equation as: NC = CR - HC, if that's okay?

You claim to know the values for CR and HC. Although you've given several different values for these numbers, from several different timeframes, I'll use the numbers from here, and here; sources you provided. You've argued these statements:
and...
were made relative to "current" conditions, and the cited sources deal with "current" numbers. All discussion will stick with these assumptions, agreed?

CO2 Rise (CR)
Now, this is difficult since you've given several different numbers over several different time frames. If we want to stick to "current" conditions we'll need some idea of what the "current" rise in CO2 is. Along with a range of error. The best I could come up with, based upon your posts, is +2ppm/yr. I have to assume you mean +2ppmv/yr, which would equate to approximately 15.6Gt CO2/yr (based on your source). Now, despite your contention to the contrary, this number does have an error rate. Not only is an error rate implied in your wording ("the rise has been between about 1.5-2ppm"), it's also specifically stated on the website where you got the data.

"The estimated annual growth rates for Mauna Loa are close, but not identical, to the global growth rates. The standard deviation of the differences is 0.26 ppm."

Now, this actually implies two different error rates. The first, implied by you, indicates you took a range of dates, looked at the MLO data, and picked a maximum and a minimum. The second implies the data taken at the MLO reflects global data within a standard deviation of +/-.26ppmv. However, what is important to our discussion is how accurate are the measurements of atmospheric CO2 from the MLO, and how accurate are these measurements when interpolated into a global CO2 rise? The standard deviation of .26ppm gives us some indication as to how these measurements compare to a global reading, but we still need the error rate for the individual measurements. These measurements are not the "very accurate figure[s]" you claim them to be. As your source explains, these measurements are refined "depending on quality control checks of the measured data." In fact, "relatively large corrections are likely" for the MLO data from 1991 on.

The standard deviation you referenced in your last post ("Standard deviation given for any particular monthly average... is 0.10ppm (excluding the last 2 months which are always more - 0.17 ppm and 0.57 ppm") only deals with marine surface data. Are we talking about marine surface data, or atmospheric data?

Assuming you are talking about atmospheric data as provided by the MLO, I don't know which dates you looked at to arrive at your "between 1.5 and 2ppm". Since you've asserted you're talking about "current" data, it seems to me we should use the most current years, since HC is expressed in years. You've suggested 10 years, and I'm comfortable with that. I just need to know where you're getting the data, what the error rate is, and how you arrived at both. Now, I've read that the IR absorption method has an error rate of less than 1.0ppmv. This document explains that samples are taken in pairs. Pairs with a difference of less than, or equal to, .5ppm are rejected. That would imply that the measurements could be off as much as .5ppm. So, again, you pick. But recognize there will be an error range.

Human Contribution (HC)
From your source, we get 7.1GtC (+/-1.1GtC)/yr. That would equate to ~26.1Gt (+/-4.0) CO2/yr.Do you agree with this number?

I'm willing to accept it, but I think there's an interesting section of this source that really makes the above exercise moot. Just below the carbon dioxide sources for anthropogenic carbon fluxes is a section on "Partitioning among reservoirs". These numbers are:

Storage in the atmosphere 3.3 (3.1-3.5)
Oceanic uptake 2.0 (1.2-2.8)
Uptake by Northern Hemisphere forest
regrowth 0.5 (0.0-1.0)
Additional terrestrial sinks: CO2 fer-
tilization, nitrogen fertilization,
climatic effects 1.3 (-0.2-2.8)

Of course, these numbers balance to the anthropogenic CO2 emissions (3.3 + 2.0 + .5 + 1.3 = 7.1). But not if we look at the ranges. For example, if anthropogenic sources of CO2 were at the lower limit of 6.0, and the anthropogenic sinks were at the upper range of 10.1, we would have a net absorption of 4.1GtC/yr. Meaning net anthropogenic CO2 contribution would be negative, and the entire increase in CO2 would have to be natural (NC = CR - (-4.1)). In fact, we would have a net negative number on any combination of anthropogenic emissions up to 10GtC/yr.

I'm sure you'll dismiss this analysis out of hand, but you'll have to admit it is possible. Your source states, "The ranges in parentheses are 90 %-confidence intervals, meaning the authors estimate a 90 % chance that a given range encloses the true value of the respective flux." In other words, they are just as confident the numbers balance as they are the numbers equate to a net negative flux. There is no more reason to believe the amount of carbon emitted by Man exactly equals the amount of carbon sequestered by the planet than there is in believing the amount of carbon emitted by Man is entirely absorbed. Therefore, it is possible your statements are false. It is possible the net natural contribution to CO2 is positive, and CO2 levels in the atmosphere would continue to rise without human co2 emissions.