These exchanges are both large and ubiquitous, stymieing any attempt at direct measurement on a global scale. We therefore resort to either predictive modelling of the exchanges or the use of inverse methods. In the latter, we combine knowledge of atmospheric concentrations and atmospheric advection and diffusion to estimate, statistically, compatible patterns of surface exchanges. This has been the focus of my research over the last decade.

By the late 1990s there were records of CO₂ concentration from several observing sites stretching back at least a decade. There were also two other vital clues to how CO₂ concentration had changed. With remarkable prescience, scientists had been collecting and storing atmospheric samples from Cape Grim in Tasmania since the late 1970s, against the hope that atmospheric species they could not measure at the time might later turn out to be interesting. These samples provided a long and precise record of atmospheric oxygen concentration and the isotopic composition of atmospheric CO₂. An inverse calculation using this oxygen, isotope, and CO₂ data formed the basis of a paper we published in Tellus in 1999 (PJ Rayner, IG Enting, RJ Francey, R Langenfelds, "Reconstructing the recent carbon cycle from atmospheric CO2, delta C-13, and O-2/N-2 observations," Tellus B-Chem. Phys. Meteorol. 51[2]: 213-32, April 1999). The combination of data gave us our clearest window yet into the problem that had occupied the carbon-cycle community through the 1980s and 1990s: How much of the natural removal of CO₂ was due to ocean uptake and how much was due to land? Oxygen concentration and the isotopic composition of CO₂ are affected differently by the processes causing CO₂ exchange in either land or ocean, so we could use variations in these species to partition the CO₂ exchange. The data suggested that the net removal was predominantly oceanic, although the terrestrial uptake was about as large once we considered the large source believed to come from deforestation. The result has since been refined to account for some processes we neglected, but it roughly stands, i.e., that similar amounts of CO₂ are taken up by the ocean and the land.

The simultaneous use of the three sets of measurements also gave us a hint of the year-to-year variability and spatial structure of the uptake. We noted, for example, that the land was responsible for most of the year-to-year variability and that most of this occurred in the tropics. More recent results have increased the role of tropical land in year-to-year variations in carbon flux.

Although we could make our conclusions on global carbon budgets with some confidence, the extreme lack of data, especially over the tropical continents, made our comments on the spatial distribution highly uncertain. Also, the representation of atmospheric transport in numerical models is subject to uncertainty and, with our one model, we could not easily comment on this. Quite a bit of our work in the following years has focused on possible improvements in the spatial coverage of data, for example by the use of remote sensing of atmospheric composition, or checking the robustness of atmospheric inversions using many different numerical models of atmospheric transport. There are now satellite missions under development which may provide the dense coverage of measurements needed to reduce the spatial uncertainties in our inverse calculations and allowing us to make clear divisions, for example, between the continents in the northern hemisphere.