he stable isotope ratios of atmospheric CO2 (18O/16O and 13C/12C) have been monitored since 1977 to improve our understanding of the global carbon cycle, because biosphere–atmosphere exchange fluxes affect the different atomic masses in a measurable way. Interpreting the 18O/16O variability has proved difficult, however, because oxygen isotopes in CO2 are influenced by both the carbon cycle and the water cycle.

This year marks the 50th anniversary of the start of the Mauna Loa CO2 record, the longest continuous record of CO2 in the atmosphere. Initiated by Charles D. Keeling of the Scripps Institution of Oceanography, the record provided the first compelling evidence that the concentration of CO2 in the atmosphere was rising.