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 bal ance on atmospheric CO2 through time. In this calculation, the residua l flux needed to balance the carbon budget when fossil fuel emissions, ocean uptake, and the documented increase of atmospheric CO2 concentr ations are taken into account, is interpreted as being net terrestrial carbon exchange. In this study, the uncertainties in such a calculati on are investigated and related to the magnitude of the ''missing carb on sink'' as a function of time. The uncertainty in the CO2 growth rat e is found to be of the order of +/-1 GtC/yr (similar to 0.5 ppmv/yr) prior to the start of direct atmospheric measurements in 1959. The dif ficulties in assigning a precise uncertainty estimate for the CO2 grow th rate from the ice core record are illustrated. It is then shown tha t the missing sink is significantly different from zero from the 1950s to the present day, even when all the uncertainties are taken into co nsideration. Finally, it is pointed out that the uncertainties in the cumulative carbon budget imbalance may be larger than previously thoug ht, This has implications for model studies where conclusions are base d on the ability to tune a given model to reproduce the cumulative mis sing sink curve.