In other words, those hoping that uncertainties in radiocarbon dating, say in the assumption of constancy of atmospheric carbon-14 levels, will mean that specimens are really much younger than the measured dates, are in for a big disappointment -- it is now clear that specimens are actually somewhat older than the raw, uncalibrated reckonings.
As mentioned above, young-earth creationist writers have cited various anomalies and potential difficulties with radiocarbon dating, and have used these examples to justify their conclusion that the entire scheme is flawed and unreliable.
Perhaps you have heard of Ice Man, a man living in the Alps who died and was entombed in glacial ice until recently when the ice moved and melted.
The man's body was recovered and pieces of tissue were studied for their C content by accelerator mass spectroscopy.
In 2009, several leading researchers in the field established a detailed calibration of radiocarbon dating, based on a careful analysis of pristine corals, ranging back to approximately 50,000 years before the present epoch [Reimer2009].
Here is a graph showing radiocarbon dates on the vertical axis and the calibrated age on the horizontal axis (shown here with permission from Johannes van der Plicht, one of the authors of the 2009 study).
For instance, even in the 1950s, when Willard Libby first developed the process, it was recognized that the scheme assumes that the level of carbon-14 in the atmosphere is constant.
But researchers have known at least since 1969 that the carbon-14 level has not been constant, so that the radiocarbon clock needs to be "calibrated." As a result, various schemes are used to correct and calibrate radiocarbon dates, including: In each case, radiocarbon dates, determined by well-established procedures and calculations, are compared directly with dates determined by the above methods, thus permitting the radiocarbon dates to be accurately calibrated with distinct and independent dating techniques.
Because of this relatively short half-life, radiocarbon is useful for dating items of a relatively recent vintage, as far back as roughly 50,000 years before the present epoch.
Radiocarbon dating cannot be used for older specimens, because so little carbon-14 remains in samples that it cannot be reliably measured.
In any event, it must be emphasized once again that radiocarbon dating has no relevance one way or the other for the overall question of whether the earth is many millions of years old, since the scheme can only be used to reliably date specimens less than approximately 50,000 years old.
Additional background is available in a well-written Wikipedia article on the topic [Radiocarbon2011], and in Richard Wiens' article [Wiens2002].
The relative width of the red calibration curve indicates the range of uncertainty: In October 2012, a team led by Christopher Ramsey of Oxford University published a new study, based on analyses of varves (alternating light/dark bands in sediments) from Lake Suigetsu, which is located about 350 kilometers west of Tokyo, near the coast of the Sea of Japan.