It is now clear that fossil fuel combustion and other human activities are perturbing the global carbon cycle [e.g., Heimann, 1997]. Today's patterns of carbon sources and sinks can be downscaled from atmospheric measurements made around the world. Usually, the fluxes are inverted from space and time gradients in carbon dioxide (CO₂) using atmospheric transport models. In the case of a passive tracer such as CO₂, the transport acts as a linear operator between the sources and the modeled atmospheric concentration field. In addition to CO₂ alone, other tracers of the carbon cycle are extremely valuable to improving our estimates of today's carbon sources and sinks.

The ¹³C/¹²C ratios in CO₂ are strongly modified by plant photosynthesis but negligibly altered during air-sea exchange. As terrestrial plants preferentially fix the lighter isotope ¹²C, terrestrial uptake of CO₂ will impart a fingerprint of slightly increased ¹³C/¹²C ratios on the CO₂ remaining in the atmosphere. Though small, this signal can be detected by ultraprecise measurements of the ¹³C isotopic composition of CO₂ using mass spectrometry.

The observed meridional gradient of ¹³C constitutes one of our best clues for understanding the latitudinal distribution of oceanic vs. terrestrial CO₂ fluxes. One of the most striking results that ¹³C data (and now O₂/N₂ ratio data) unveiled is the existence of a very large repository of anthropogenic CO₂ in Northern Hemisphere ecosystems during the early 1990's when the atmospheric CO₂ growth rate had diminished to only one third of its normal value. Still, the long term trend and interannual fluctuations of ¹³C at one given monitoring station is at the limit of detection of mass spectrometers, on the order of 0.01 per mil for ¹³C in CO₂. Thus, even a very slight bias in the isotopic data would translate into different inferred magnitudes of the global land and ocean uptake of anthropogenic CO₂.

Variations in the oxygen:nitrogen ratio (O₂/N₂) with respect to the oxygen:argon ratio in air are due to photosynthesis, respiration, and fossil fuel and biomass burning which are directly linked to the global carbon cycle and its perturbation by man [e.g., Keeling et al., 1996]. These variations can be measured by techniques similar to those used for ¹³C/¹²C ratios. Oxygen has a low solubility in water compared to CO₂ and lacks a buffering system. Oceanic productivity as well as mixing or upwelling of O₂depleted water imprints a distinct signal on the atmosphere, modulated by airsea gas exchange. This leads to relatively large annual O₂/N₂ variations in both hemispheres. The long term trend as well as interannual fluctuations in O₂/N₂ can now be resolved with modern techniques like mass spectrometry. In combination with CO₂ concentration measurements this gives us much information about the global carbon budget, i.e., the socalled "missing sink".

The ¹⁸O/¹⁶O ratio in atmospheric CO₂ has been identified as a unique tracer to constrain separately the gross uptake (photosynthesis) and release (respiration) of carbon by terrestrial biota. CO₂ can exchange an ¹⁸O atom with two isotopically distinct water reservoirs: Evaporating leaf water during photosynthesis and soil moisture during respiration. Thus, the ¹⁸O/¹⁶O isotopic composition of CO₂ is controlled indirectly by the ¹⁸O/¹⁶O ratio of water in the biosphere, and thus is linked to the global water cycle. Atmospheric measurements of ¹⁸O/¹⁶O in CO₂ show a pronounced negative latitudinal gradient, which is interpreted as a fingerprint of respiratory CO₂ emissions. The reason ¹⁸O in CO₂ is useful is that it relates to the gross fluxes of CO₂ exchanged by land ecosystems (photosynthesis, respiration), rather than to the net fluxes which are constrained by CO₂, ¹³C and O₂/N₂.

In pursuing and improving the atmospheric monitoring of CO₂ and its isotopic composition, there is a crucial need to augment the coverage of poorly known oceanic and continental areas, thus better improving the diagnostic of carbon fluxes at the continental to regional scale.