This nice little overview of C-14 dating is from physicsforums:
http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/cardat.html
The half-life of an element is constant, and any fluctuation may be due to the experimental method.
The second problem is that the carbon ration fluctuates over time. That is another and different problem, and I was going to get around to that in the discussion with Andre where he cites a different half-life.
Plants absorb CO2 from the atmosphere and animals in turn consume plants. Carbon in plants is in the form of cellulose, sugars, nucleotides, etc, and in animals in proteins, sugars, fats, . . . . etc. When the plant or animal dies, the exchange of the forms of carbon from living processes stops.
Now in nature, carbon is largely C-12 (98.89%), C-13 (~1.11%), both of which are stable, and traces of C-14, which has a half-life of ~5730 years. During each half-life period, half of a radionuclide decays (transforms) into a different element, which in the case of the decay of C-14 is N-14. C-14 is actually produced by the collision of high energy neutron with N-14 nuclei (n,p reaction) high in the atmosphere, and the resulting C-14 diffuses into the atmosphere combining with oxygen to form CO2.
http://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Physics/carbondating.htm
Let's assume that the half-life of C-14 is well-known and constant, and let us assume that one can measure the C-14/C-12(C-13) ratio with reasonable accuracy. Well, there are two things that can affect the ratio in the earth's biosphere (atmosphere and oceans). One is the production rate of C-14 by cosmic radiation. If the reactions which produce the high energy neutrons, which produce the C-14, increase, then the ratio of C-14 to C-12 will increase. Conversely if the production of high energy neutrons decreases, then C-14 production will decrease. The other factor is the release of CO2 from volcanos or the exchange of carbon from carbonates or carbonic acid in the ground. If the volcano's CO2 or carbonates have been around for 10's millenia, or millions of years, they will have very little C-14 since it will have decayed awayed. After ~57300 yrs (10 half-lives), the C-14 will have been reduced by a factor of 210 or ~1000.
So one has to be careful regarding the environmental conditions in which organic fossils are found, as well as the environment in which the organism (plant or animal) existed, and how the environmental conditions might affect the C-14/C-12 ratio.
The C-14 half-life is experimentally determined quite independently of radiocarbon dating technique and those who use it.
THEREFORE: 99^ of carbon dioxide is C12 or C13, (the trivial C14 is NOT effective in global warming!) and man-made CO2 is NOT related to weight of the carbon and oxygen...