Net terrestrial carbon exchange from mass balance calculations: an uncertainty estimate

One classical method of determining the net exchange of carbon between the atmosphere and the terrestrial biosphere is to perform a mass balance on atmospheric CO₂ through time. In this calculation, the residual flux needed to balance the carbon budget when fossil fuel emissions, ocean uptake, and the documented increase of atmospheric CO₂ concentrations are taken into account, is interpreted as being net terrestrial carbon exchange. In this study, the uncertainties in such a calculation are investigated and related to the magnitude of the “missing carbon sink” as a function of time. The uncertainty in the CO₂ growth rate is found to be of the order of ± 1 GtC/ yr (~ 0.5 ppmv/yr) prior to the start of direct atmospheric measurements in 1959. The difficulties in assigning a precise uncertainty estimate for the CO₂ growth rate from the ice core record are illustrated. It is then shown that the missing sink is significantly different from zero from the 1950s to the present day, even when all the uncertainties are taken into consideration. Finally, it is pointed out that the uncertainties in the cumulative carbon budget imbalance may be larger than previously thought. This has implications for model studies where conclusions are based on the ability to tune a given model to reproduce the cumulative missing sink curve.