Radiocarbon dating was invented in the 1950s by the American chemist Willard F.
Libby and a few of his students at the University of Chicago: in 1960, he won a Nobel Prize in Chemistry for the invention.
Ocean corals, dated by another radiometric method - Uranium-Thorium dating - have also helped to extend the calibration curve beyond the age of the most ancient treering chronologies.
A., Kromer, B., Mc Cormac, G., van der Plicht, J., Spurk, M. INTCAL98 radiocarbon age calibration, 24,000-0 cal BP.
In fact, the natural production of radiocarbon has varied as well.
Before the industrial revolution, from 1800 - 1400 AD, the natural production of radiocarbon was high, so dates are "too young." From 1400 AD to 300 BC they are "too old," and prior to 300 BC , they are too young.
The rate of bombardment is greatest near the poles, where the Earth's magnetic field is dipping into the Earth and therefore does not deflect incoming cosmic rays. 3.5 decays/gram/minute of carbon would be produced by a sample 11,460 years old. However, atmospheric testing of nuclear weapons in the late 1950's and early 1960's greatly increased the amount of radiocarbon in the atmosphere, so the decay rate of 14 decays per minute more than doubled. It was the first absolute scientific method ever invented: that is to say, the technique was the first to allow a researcher to determine how long ago an organic object died, whether it is in context or not.Shy of a date stamp on an object, it is still the best and most accurate of dating techniques devised.