Produced by cosmic radiation in the upper atmosphere, ¹⁴C is cycled through the major earth carbon reservoirs (atmosphere, biosphere, the oceans, etc.). Time is important as ¹⁴C disappears from the scene with a half life of 5,730 years. Comparing the initial ¹⁴C activity (which is assumed to be identical to a 19^th century level) in plant material with remaining activity yields the radiocarbon age. The assumption of constant initial activity leads to great torment as it produces radiocarbon ages that are only first-order approximations of calendar-year ages.

Samples of known age (such as tree-ring dated wood up to 12,000 years old) are needed for the calibration (conversion to calendar-year ages) of radiocarbon dates. Large numbers of samples have to be measured with high precision. Counting times are several days, and the University of Washington’s Quaternary Isotope Laboratory data set took 30 years to develop. The combined measurements of several laboratories are used for an internationally accepted calibration curve which, by graphical or computerized means, converts a radiocarbon age to a cal (calibrated) age.

Beyond the determination of radiocarbon ages, there is an astonishing multitude of research topics that can be investigated with the ¹⁴C isotope. This isotope greatly facilitates the quest for scientific diversity. The interdisciplinary nature of ¹⁴C is evident from its use. For instance, one application is the determination of deep water residence times in the global ocean. Another is the calculation of atmospheric carbon dioxide residence times from releases of 20^th century nuclear bomb ¹⁴C and fossil fuel carbon dioxide (fossil fuels lack ¹⁴C). The ¹⁴C-derived history of cosmic ray flux modulation leads to an evaluation of solar-induced climatic change, and the isotope signals associated with the carbon transfer between atmosphere, biosphere, and oceans yield global deforestation estimates. Techniques used for radiocarbon dating also have evolved considerably during the search for higher precision, greater age, and smaller sample size. Another example is the use of the global carbon cycle in interpreting latitude-dependent radiocarbon age offsets.

An isotope with similar applications in the Earth Sciences is ¹⁸O (Oxygen-18). The ¹⁸O/¹⁶O ratio of water, which depends on condensation temperature, mirrors climate (temperature) change in ice cores over long time intervals. The records are very detailed with transition times lasting only a few decades between major climate changes. For GISP-2 (Greenland Ice Sheet Project) and Taylor Dome (Antarctica) tenths of thousands isotope ratios were measured in cores up to 3 km long. Ice is not the only medium for this type of research, calcium carbonate in lakes (marl) and oceans (deep sea sediment and corals) as well as tree-ring cellulose can be used.

Work can be tedious, despite the diversity of topics. Forty years of ¹⁴C counting with utmost precision, day and night, year around, takes its toll. The thousands of ¹⁸O/¹⁶O ratio measurements are easier to take because present-day mass spectrometers are highly automated. A proper antidote is the lively interaction with colleagues and students. And nothing beats a data set that generates additional "earthly" knowledge. Unfortunately, there has been a major shift over the last 40 years away from observations towards "modelling" of existing results. By now the pendulum has swung quite far. Observations, after all, generate new science. For instance, the very rapid transitions between climate modes observed in the ice core ¹⁸O record were not predicted by modern science.

Many parameters determine the use of published work. Quality and discipline are important. After all, there are many more biologists than glaciologists. Crossing the boundaries between disciplines seems to increase citation frequency. The top-cited papers ("Extended C-14 data-base and revised calib 3.0 C-14 age calibration program," Radiocarbon, 35[1]: 215-30, 1993 and "Comparison of oxygen-isotope records from the GISP2 and GRIP Greenland ice cores," Nature, 366[6455]: 552-4, 9 December 1993) are used worldwide by practitioners of Archaeology, Geology, Oceanography, Botany, Atmospheric Sciences, and Glaciology.