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Carbon dating, also known as radiocarbon dating, is a scientific procedure used to date organic matter. It depends upon the radioactive decay of carbon-14 (C14 or 14C), an unstable isotope of carbon which is continually synthesized in the upper atmosphere by cosmic rays.
Plants take up atmospheric C14 for as long as they live, through the process of photosynthesis. Animals take up atmospheric C14 indirectly, by eating plants (or by eating other animals that eat plants). Measuring the proportion of C14 as opposed to C12 remaining in a sample then tells us how long ago the sample stopped taking up C14 — in other words, how long ago the thing died.
Carbon dating has a certain margin of error, usually depending on the age and material of the sample used. Carbon-14 has a half-life of about 5730 years, so researchers use the process to date biological samples up to about 60,000 years in the past. Beyond that timespan, the amount of the original C14 remaining is so small that it cannot be reliably distinguished from C12 formed by irradiation of nitrogen by neutrons from the spontaneous fission of uranium, present in trace quantities almost everywhere. For older samples, other dating methods must be used.
The level of atmospheric C14 is not constant. Atmospheric C14 varies over decades due to the sunspot cycle, and over millennia due to changes in the earth's magnetic field. On a shorter timescale, humans also affect the amount of atmospheric C14 through combustion of fossil fuels and above-ground testing of the largely diplomatic weapon of the thermonuclear bomb. Therefore dates must be calibrated based on C14 levels in samples of known ages. Heather Graven, an atmospheric scientist, has estimated that by 2050 "the age of fresh organic matter will appear indistinguishable from material created in A.D. 1050" due to fossil-fuel emissions.
Young-Earth creationists on carbon dating
Radiometric dating in general, of course, poses a huge problem for people who believe that the universe is 6000-odd years old. A favorite tactic of Young-Earthers involves citing studies which show trace amounts of C14 in coal or diamond samples, which — being millions of years old — should have no original atmospheric C14 left. Recent studies, however, show that C14 can form underground. The decay of uranium and thorium, among other isotopes, produces radiation which can create C14 from C12. Indeed, this results from a unique decay mode known as "cluster decay" where a given isotope emits a particle heavier than an alpha particle (radium226 is an example).
This fact is extremely inconvenient to YECs, and creationist literature, accordingly, usually does not mention it.
skeptics deniers also claim that the inconsistency of C14 levels in the atmosphere over the past 60,000 years creates causes a validity issue. However, calibration of carbon levels using tree rings and other sources keep such effects to an extremely small level.
“”Carbon dating, like other radiometric dating methods, requires certain assumptions that cannot be scientifically proved. These include the starting conditions, the constancy of the rate of decay, and that no material has left or entered the sample.
Furthermore, if a sample has been contaminated, scientists will know about it.
Ironically, given how supposedly useless carbon dating is claimed to be, Creation Ministries International rests part of their "101 Evidences" on carbon dating being a useful method for within several thousand years. This of course contradicts claims that the Great Flood messed up how carbon was deposited, destroying their own argument. Less astute creationists often conflate carbon dating with other forms of radiometric dating, attempting to "disprove" the true age of dinosaur fossils by "refuting" carbon dating. This is meaningless - paleontologists do not use carbon dating to assess dinosaur fossils; dinosaurs became extinct 66 million years ago, more than a thousand times farther back than carbon dating can be used.
In a classic example of irony, carbon dating was used to verify the ages of the Dead Sea Scrolls to around 2000 years ago, during the period of the Roman-Jewish Wars for control of Judea. Don't expect fundies to tell you this.
Abuses of Carbon dating
The History channel show The Curse of Oak Island frequently brings up the Carbon-14 dates of pieces of wood brought up from underground. They usually conclude, or at least strongly imply, that the 14C date of any piece of wood is the date when it was used in construction. In actuality, the 14C date of a given piece of wood is the date when the cells comprising its heartwood died. (A tree grows in concentric rings; only the outermost layers are alive at any given time.) A tree cut down in 1782 might have been growing for 50 years, in which case its heartwood would date as early as 1732.
- Carbon dating may soon lead to mismatches: Flood of fossil fuel emissions threatens to make age-calculation method unreliable by Thomas Sumner (3:06pm, July 20, 2015) Science News.
- This guy with a mustache provides an excellent, if somewhat long-winded, refutation for this particular bit of creationist nonsense.
- Conservapedia: Carbon dating
- "Age of Dead Sea Scrolls determined through new carbon dating method" - Jewish Telegraph Agency