According to the technical monograph "Origin and Destiny of the Earth's Magnetic Field," the magnetic field is decaying as a first order exponential with half life of 1400 years, a number much less than the 5700 year half life of C14. The consequence of the decay is that there is corresponding exponential increase of the generation rate of C14. Using present conditions as a reference will result in an increase in the apparent age of older samples."
The
current rate of change of the magnetic field does not necessarily have anything to do with its rate of change in the past. In fact, paleomagnetic data show that the magnetic field is cyclic, has reversed many times in the past, and therefore its rate of change is constantly changing; sometimes it's increasing and sometimes it's decreasing. See
Claim CD701 and
Is the Earth's Magnetic Field Young?.
However, none of this has any relevance to radiocarbon dating! That's because radiocarbon dating does not use present conditions as a reference!
Radiocarbon dates are reported in two kinds of years. One is "radiocarbon years", which
are based on assuming constant radiocarbon production and other assumprions, but everyone involved understands that
radiocarbon years are not calendar years; a sample with a "radiocarbon year" age of 10,000 radiocarbon years is
not 10,000 calendar years old.
The other kind of years are calendar years, based on calibrating the radiocarbon method against other independent methods. The most commonly used calibration curve is based on tree rings (dendochronology). For example, if we have a sample of wood that dates to 10,000 radiocarbon years old and we know from tree ring measurements that the sample is 11,438 calendar years old, we have established one calibration point. Do this many times and we can draw a calibration curve through the calibration points, and use that curve to translate radiocarbon years to calendar years for any sample (within the range of the calibration curve).
The nice thing about calibration curves is that it
just doesn't matter how radiocarbon production varied in theh past; that's automatically compensated for.
See
Radiocarbon calibration. A widely used calibration curve is shown at
INTCAL 1998: Tree-Ring Section and a more recent one, including data from other sources and covering a wider range, is at
CALPAL 2004 January: there's even an online radiocarbon-to-calendar-year converter (that uses the latter curve) at
CalPal Online.
[This message has been edited by JonF, 04-26-2004]