. The adjoint model to a global threedimensional atmospheric transport model can be used to efficiently perform a sensitivity analysis, i.e. the computation of the partial derivatives of a particular model output feature with respect to many control variables. We demonstrate this approach by investigating the dependence of the magnitude of the modeled seasonal cycle of CO 2 at remote monitoring stations with respect to the magnitudes of the seasonal cycle of the net CO 2 surface fluxes prescribed from a simple diagnostic terrestrial biosphere model. The technique results in global maps of those source regions that predominately influence the magnitude of the seasonal cycle at the different monitoring stations. 1 Introduction Observations of the atmospheric CO 2 concentration from the global networks of remote background air monitoring stations provide an indispensable tool to constrain spatial and temporal variations of net surface fluxes of carbon between the atmosphere, the oceans ...