The Stable Isotopic Composition of Atmospheric CO2

About half of the CO₂ emitted by fossil fuel combustion currently remains in the atmosphere. The other half is removed by CO₂ uptake on land and in the ocean. Prediction of future CO₂ concentrations relies critically on the understanding of these sources and sinks and their potential variability over time. While such understanding requires quantification of individual carbon fluxes, the means to achieve this in direct measurements are very limited. The isotopic composition of atmospheric CO₂ provides a powerful and indispensable tool in that respect. This chapter reviews the methodological and observational basis underlying the use of the isotopic composition of atmospheric CO₂ in tracing and interpreting the changes in sources and sinks of CO₂ over seasonal, annual, and longer timescales, as well as over different spatial scales. The processes influencing the isotopic 'signatures' associated with specific processes in the contemporary carbon cycle and the usefulness of such signals are discussed. For example: How can annual-scale variations in ¹³C observations be used to partition CO₂ uptake between land and ocean? Why does the decreasing trend in the ¹³C of atmospheric CO₂ over the past 150 years (due to the addition of ¹³C-depleted CO₂ to the atmosphere through fossil fuel combustion) provide an indicator of the carbon turnover rate in the biosphere? Why does the ¹⁸O in atmospheric CO₂ reflect variability in the plant versus soil fluxes within the land biosphere? Finally, the new frontiers in the application of isotopic analysis of atmospheric CO₂, such as the use of laser spectroscopy, our ability to identify variations in the rare, isotopically doubly labeled CO₂ molecules (e.g., containing both ¹³C and ¹⁸O), and the measurement of the ¹⁷O anomaly in CO₂, are explored.