Inverse modeling of annual atmospheric CO2 sources and sinks 2. Sensitivity study

Atmospheric transport models can be used to infer surface fluxes of atmosph eric CO2 from observed concentrations using inverse methods. One of the mai n problem of these methods is the question of their sensivity to all the pa rameters involved in the calculation. In this paper we study precisely the influence of the main parameters on the net CO2 fluxes inferred by an annua l Bayesian three-dimensional (3-D) inversion of atmospheric CO2 monthly con centrations. This inversion is described as the control inversion (S-0) of Bousquet et nl. [this issue]. Successively, at regional to global spatial s cales we analyze the numerical stability of the solution to initial fluxes and errors, the influence of a priori flux scenario, the sensitivity to the atmospheric transport model used, the influence of delta(13)C measurements , and the influence of the atmospheric network. We find that the atmospheri c transport model introduces a large uncertainty to the inferred budget, wh ich overcomes our control run uncertainties. The effects of vertical transp ort on CO2 concentrations appear to be a critical point that has to be inve stigated further. Spatial patterns of fluxes also have significant influenc e on a regional basis. We notice that accounting for the Baltic Sea station (BAL) deeply modifies the Europe versus Asia partition of the land uptake at mid and high latitudes of the Northern Hemisphere. We also analyze the w eak influence of using delta(13)C measurements as additional constraints. I n the tropics we find that the low level of constraints imposed by the atmo spheric network limits the analysis of fluxes to zonal means. Finally, we c alculate overall estimates of CO2 net sources and sinks at continental scal e, accounting for all sensivity tests. Concerning the controversial partiti on of CO2 sink at mid and high latitudes of the Northern Hemisphere, we fin d (on average, for the 1985-1995 period) an overall partition of the sink o f 0.7+/-0.7 Gt C yr(-1) for North America, 0.2+/-0.3 Gt C yr(-1) for the No rth Pacific Ocean, 0.5+/-0.8 Gt C yr(-1) for Europe, 0.7+/-0.3 Gt C yr(-1) for the North Atlantic Ocean, and 1.2+/-0.8 Gt C yr(-1) for north Asia. Thi s overall partition tends to place an important land uptake over north Asia . However, uncertainties remain large when we account for all the sensitivi ty tests.