The amount of cosmic rays penetrating the Earth's atmosphere affects the amount of 14C produced and therefore dating the system. The amount of cosmic rays reaching the Earth varies with the sun's activity, and with the Earth's passage through magnetic clouds as the solar system travels around the Milky Way galaxy.

The strength of the Earth's magnetic field affects the amount of cosmic rays entering the atmosphere. A stronger magnetic field deflects more cosmic rays away from the Earth. Overall, the energy of the Earth's magnetic field has been decreasing,so more 14C is being produced now than in the past. This will make old things look older than they really are.

Also, the Genesis flood would have greatly upset the carbon balance. The flood buried a huge amount of carbon, which became coal, oil, etc., lowering the total 12C in the biosphere (including the atmosphere—plants regrowing after the flood absorb CO2, which is not replaced by the decay of the buried vegetation). Total 14C is also proportionately lowered at this time, but whereas no terrestrial process generates any more 12C, 14C is continually being produced, and at a rate which does not depend on carbon levels (it comes from nitrogen). Therefore, the 14C/12C ratio in plants/animals/the atmosphere before the flood had to be lower than what it is now.

Unless this effect (which is additional to the magnetic field issue just discussed) were corrected for, carbon dating of fossils formed in the flood would give ages much older than the true ages.

Creationist researchers have suggested that dates of 35,000 - 45,000 years should be re-calibrated to the biblical date of the flood. Such a re-calibration makes sense of anomalous data from carbon dating—for example, very discordant “dates” for different parts of a frozen musk ox carcass from Alaska and an inordinately slow rate of accumulation of ground sloth dung pellets in the older layers of a cave where the layers were carbon dated.

Also, volcanoes emit much CO2 depleted in 14C. Since the flood was accompanied by much volcanism (see Noah's Flood…, How did animals get from the Ark to isolated places?, and What About Continental Drift?), fossils formed in the early post-flood period would give radiocarbon ages older than they really are.

In summary, the carbon-14 method, when corrected for the effects of the flood, can give useful results, but needs to be applied carefully. It does not give dates of millions of years and when corrected properly fits well with the biblical flood.