By 1950, Willard Libby and his group at the University of Chicago had worked out ways to measure this proportion precisely.

An example of the ingenious technical work and hard-fought debates underlying the main story is the use of radioactive carbon-14 to assign dates to the distant past.

For other examples, see the essays on Temperatures from Fossil Shells and Arakawa's Computation Device.

The radioactive isotope carbon-14 is created in the upper atmosphere when cosmic-ray particles from outer space strike nitrogen atoms and transform them into radioactive carbon.

Some of the carbon-14 might find its way into living creatures.

Climate science required the invention and mastery of many difficult techniques.

These had pitfalls, which could lead to controversy.

Any contamination of a sample by outside carbon (even from the researcher's fingerprints) had to be fanatically excluded, of course, but that was only the beginning.

Delicate operations were needed to extract a microscopic sample and process it.

The best way to transfer the exacting techniques was in the heads of the scientists themselves, as they moved to a new job.

Tricks also spread through visits between laboratories and at meetings, and sometimes even through publications.

Comparing the old wood with modern samples, he showed that the fossil carbon could be detected in the modern atmosphere.(5) Through the 1950s and beyond, carbon-14 workers published detailed tables of dates painstakingly derived from samples of a wondrous variety of materials, including charcoal, peat, clamshells, antlers, pine cones, and the stomach contents of an extinct Moa found buried in New Zealand.(6) The measurements were correlated with materials of known dates, such as a well-documented mummy or a log from the roof of an old building (where tree rings gave an accurate count of years).