At the same time, atmospheric nuclear testing, which throws huge amounts of carbon into our atmosphere, was begun in the 1940's.
Radiocarbon dates after 1950 are virtually useless unless and until we can figure out a way to calibrate for the excessive amount of carbon still being deposited in our atmosphere.
(Ham et al., page 68.) C ratio in the past, or that this is "the technique's Achilles' heel" is incorrect.
The whole validity of radiocarbon dating for the past 10,000 years---the time span of interest to biblical chronology---hangs only on the tree-ring chronologies which are used to calibrate it. .) This process does not involve any assumption about historic radiocarbon to stable carbon ratios because the radiocarbon concentration in the tree-ring samples would be affected in exactly the same way as the radiocarbon concentration in the specimen to be dated. To quote again from The Answers Book: Some recent, though controversial, research has raised the interesting suggestion that c (the speed of light) has decreased in historical times. If it is correct, then radioactive decay rates would automatically be affected, and would show artifically high ages.
We need a fixed point in time as a starting point so that all the BP dates are equivalent no matter when they are published.
Since the BP designation was originally associated with radiocarbon dating, archaeologists chose the year 1950 as a reference point for 'the present.' That date was chosen because radiocarbon dating was invented in the late 1940s.
Scholars use the science of dendrochronology, which matches those annular rings to known carbon fluctuations.
So, when you see 2000 cal BP, think "2000 years before the calendar year 1950" or what calculates to the calendar year 50 BCE.
No matter when that date is published, it will always mean that.
Radiocarbon dating was invented in the late 1940's, and within a few decades, it was discovered that while the dates retrieved from the method have a sound, repeatable progression, they are not a one-to-one match with calendar years.
Most importantly, researchers discovered that radiocarbon dates are affected by the amount of carbon in the atmosphere, which has fluctuated greatly in the past for both natural and human-caused reasons (such as the invention of iron smelting, the Industrial Revolution, and the invention of the combustion engine).