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The amount of CO2 emitted into the atmosphere from both natural and unnatural sources are still being absorbed by carbon sinks. IF you overload one side of a stock and flow system, in this case, the inflow will increase and, regardless of where that original stock came from, will continue outflowing at the same rate. This means that carbon sinks do not pick and choose what carbon dioxide to absorb based on it's point of origin as it is basically all the same outside of certain isotopic signatures.

http://www.planetseed.com/flash/science/...

http://www.watersfoundation.org/webed/mo...

When we look at a carbon cycle diagram, such as the one graphicconception posted, though old, we can see how the carbon cycle relates to the stock and flow animation above. You have your inflows of carbon, such as respiration and fossil fuels, and your outflows of carbon, such as weathering, for the atmosphere. If you will notice from the graphic he posted there is a, basically balanced, inflow and outflow from natural sources (This changes with changes in temperature which is why CO2 concentration has corresponded with temperature changes in the past) and humans are adding to that total through such things as burning fossil fuels, cement production and other things. This creates an imbalanced system where the inflow does not equal the outflow and, as a result, the stock, or carbon dioxide in the atmosphere, is increasing.

The point is, it does not matter where the source of additional stock is coming from as it all gets treated pretty much equally when it is part of the stock. You can;t look at the stock and come to the conclusions that this percentage came from fossil fuel use while this percentage did not. You can, however, measure the flows into the stock and come to the conclusion that the stock is increasing or decreasing based on those flows. I mean all of mans contributions to the carbon cycle may be being reabsorbed by carbon sinks but that means that there is still an increase in the stock because that additional carbon has been sent to the outflow instead of that from a natural source so the stock still increases. I'll give you some numbers on how much additional carbon and CO2 is being emitted by humans and how much the stock is increasing by.

CDIAC estimates of human emissions of carbon through cement manufacture and fossil fuel use: http://cdiac.ornl.gov/ftp/ndp030/global....

Multiplying the above 2010 total, 9167, by 3.667 to include the oxygen molecules gives us 33615 million metric tonnes or roughly 33.6 billion metric tonnes of carbon dioxide.

Measurements of atmospheric increase can be gained through the Scripps CO2 program: http://scrippsco2.ucsd.edu/data/in_situ_...

If we look at current trends of this data we see the stock is increasing at a rate of 2ppm per year.

http://www.esrl.noaa.gov/gmd/ccgg/trends...

This is equal too roughly 15.6 billion tonnes. 2*(1*2.13*3.667) = 15.62142

List of common conversion factors: http://cdiac.esd.ornl.gov/pns/convert.ht...

Conclusions are that the amount of stock is increasing at a rate less than half that of what the additional flow from human emissions have added. 33.6 is more than double 15.6. The outflow has had to increase as a result (One example is ocean absorption of CO2 has increased over the years) in order to maintain equilibrium between the two and keep the stock constant. However, natural changes to the outflow are slow which is why the stock is increasing. If we want to decrease human emissions of CO2 in the stock we will need to find an unnatural means of decreasing our contribution to that stock, whether that be stopping emissions and allowing natural processes to recover, a slower way, or using CO2 scrubbers or some form of geoengineering to decrease the stock, the faster and potentially more dangerous way. Basically, while the stock is large and the flow is small what matters is that flow rate and how it is changing. Increases from 280ppm, pre-indutrial revolution, to 400ppm today amounts to about 40%. Most of that increase can be attributable to man while some of it, particularly in the early phases of this time period, was natural.

There is a lot of confusion here at Yahoo Answers about stock versus flow. Much of it is deliberate and by anti-science posters.

If you get into a large bathtub, your climbing in will make the water level (stock) go up only very slightly. But if the tap is on (flow in) the water level will also rise (gradually). If the water level is already near the top, in a big, and you climb in, and the water splashes over the edge, is it correct to say that you didn't make it splash over, because you were small relative to the large amount water already in the tub?

Carbon in the air, oceans and plants is like water in a big tub. If you take carbon out of the ground (in form of fossil fuels like coal, oil, gas) and burn in it, the burned carbon goes into the air as carbon dioxide (and then later a bit of it goes from the air into the plants and ocean water). That is like someone climbing into a big tub. The amount of carbon humans put into the air from burning fossil fuels each year is small compared to what is already there. But the INCREASE over time, as we keep burning fossil fuels (and anti-science deniers keep pretending that this has no negative effects), is a result of human activity, and almost completely so.

http://en.wikipedia.org/wiki/Carbon_cycl...

Notice that the anti-science tricksters here will post their disinformation without links, or only with links to anti-science blogs like wattsup, not objective sources of information, like Wikipedia (usually, though not always) and certainly not to science sites such as realclimate.org

A little over 3%

http://www.manhattan-institute.org/energ...

You need to see the diagram here: