In the late 1970s, Keeling’s group undertook a long-planned expansion of the CO2 program that looked more closely at sources of the CO2 that are present in the atmosphere.
Carbon can be distinguished by its isotopes—carbon atoms with varying numbers of neutrons. Carbon atoms containing six protons and six neutrons (known as carbon-12 or 12C) are the most common isotope and comprise about 99% of all carbon on Earth. Those that contain an additional neutron are carbon-13, 13C, which comprise about 1% of all carbon. Carbon-14, 14C, with two extra neutrons, makes up only a trace amount. The three isotopes behe very similarly in chemical reactions, so they move through the environment in nearly identical ways.
The isotopes 12C and 13C are stable, meaning they are conserved over time, while 14C is unique among the three for being unstable. This isotope, also called radiocarbon, is created in nature when cosmic rays act upon nitrogen atoms in the atmosphere (atomic tests he also contributed some 14C). It has a constant, well-known half-life, meaning that 14C disappears at a predictable rate by turning back into nitrogen.
Because of that regular disintegration, radiocarbon can be used to determine the age of carbon-based objects like fossilized plants and animals. The less 14C an object has, the older it must be. In contrast, the amount of 12C and 13C remains unchanged after many thousands of years. Therefore, modern sources of CO2 such as living organisms he about the same amount of 14C as the atmosphere, whereas ancient sources like fossil fuels he none left.
Scientists use analytical tools to determine the signature ratios of 12C, 13C and 14C found in different reservoirs (the oceans, atmosphere, biosphere, fossil fuels and others). These ratios are sometimes called carbon fingerprints. Using these fingerprints, scientists can compare the ratios of carbon isotopes over time to track carbon as it moves from one reservoir to the next. For example, as fossil fuels are burned, they add 12C and 13C to the atmosphere, but no 14C. The result is that the proportion of 14C in the atmosphere goes down.
Keeling had made isotopic measurements previously while conducting experiments in the woodlands of California, and he had planned to continue isotopic measurements of his air samples as part of the International Geophysical Year program. Difficulties in analysis at that time led him to sideline this research. In 1978, Keeling returned to the isotopic studies, working with a group at the University of Groningen in the Netherlands to study the isotopic ratios of carbon present in samples dating back to 1955.
The first paper on isotopic measurements from this period was published in 1979. It showed a shift in 13C/12C ratio that matched predictions associated with fossil fuel combustion. Further analysis of 14C was used as evidence that the current accumulation of CO2 in the atmosphere was linked to the liberation of long-sequestered banks of carbon—the burning of fossil fuels by mankind.
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