Radiocarbon (C-14) is a naturally-occurring radioactive isotope that is produced in the upper atmosphere with a half‐life of 5,730 years. It is oxidized to carbon dioxide and enters the food chain via photosynthesis so that all living land biomass is labeled with the C-14 content of atmospheric carbon dioxide. When an organism ceases carbon uptake upon death, its C-14 content declines due to radioactive decay. The amount of C-14 remaining at a given time indicates the time since the organism’s death (or its age). Radiocarbon dating can be used to calculate the age of organisms that died before 1950 back to approximately 50,000 years (8-10 half-lives).

The year 1950 marks a special time in the carbon cycle and is year zero in radiocarbon dating. In the 1950s and 1960s, thermo‐nuclear weapons testing increased the atmosphere’s C-14 content above natural levels, and therefore, all land biomass that grew after the year 1950 contains more C-14 than biomass that grew before 1950. This extra C-14 is called “bomb-carbon” and samples containing bomb-carbon are described as “modern”. Since the Partial Nuclear Test Ban Treaty was ratified in 1963, the amount of C-14 in the atmosphere (and newly formed land biomass) has been declining. During the first decades, bomb C-14 levels in the atmosphere declined mostly due to the uptake of bomb C-14 into other carbon reservoirs in the global carbon cycle, i.e. the ocean, plants, and soils. The temporal change in the amount of atmospheric C-14 in the past 70 years is a global tracer signal in the carbon cycle that is known as the bomb-spike.

Fossil fuels (e.g. coal, gasoline) formed from plants that lived millions of years ago. All C-14 in this biomass has decayed away, and fossil fuels are devoid of C-14. Their combustion to carbon dioxide has been diluting the natural-occurring amount of C-14 in the atmosphere as well as that of bomb C-14. Today, the combustion of fossil fuel‐derived carbon dioxide has erased the signal of the thermonuclear weapons testing from the atmosphere, and current and future changes in the C-14 content of atmospheric carbon dioxide can be used to monitor fossil fuel consumption by humans.

Many types of samples in environmental science are complex mixtures. For example, carbon dioxide in the air in a city originates from the combustion of fossil fuels (devoid of C-14) as well as from the decomposition of biomass in soils and the respiration of plants and animals (containing a lot of C-14). By measuring the C-14 content in the carbon dioxide and its sources, we can estimate the relative contribution of each source to the mixture.