Simulated sources and sinks of atmospheric CO2, calculated by a threedimensional tracer inversion model of the global carbon cycle, have been subjected to tests to determine their sensitivity to uncertain model specifications and input data. The model, described in a companion article [Piper et al., 2001a], employs regional CO2 source components as boundary conditions to a three-dimensional transport model, TM2, driven by observed winds and a vertical convection scheme based on both observational data and dynamic theory. The model, by an inverse calculation, adjusts the strengths of 7 source components, pertaining to boreal, temperate, and tropical zones, to achieve an optimum fit to measurements of atmospheric CO2 concentration and 13C/12C isotopic ratio from 1981 through 1999 at an array of 9 stations. A standard reference fit was made using initial choices of model parameters and input data for wind, convection, temperature, solar irradiance, remotely sensed plant activity, and related factors, expressed as averages over as many years of recent data as were available for each, but with respect to winds and convection just for the year 1986. This standard case is challenged by means of sensitivity tests in which model specifications and input data have been varied. Although temporal variability in simulated oceanic and terrestrial biospheric sources and sinks deduced in this standard case mainly reflects variability in observations of atmospheric CO2, as much as a fourth is found to be owing to variable wind and convection, and some is likely to be due to temporally varying errors in the specification of CO2 emissions from fossil fuel which lead to nearly compensating departures in inferred biospheric fluxes. Averages of deduced fluxes and the patterns of variability are found to be only slightly sensitive to model specifications, except for the distribution of terrestrial fluxes near the La Jolla sampling site, set a priori, which strongly affects the deduced strength of the northern temperate biospheric sink. As a preferred model solution for further study of relationships to environmental factors and for comparison with other investigations, we adopted the specifications of the standard case in all respects except the distribution of sources and sinks in the vicinity of our observing station at La Jolla, California which is close to an urban area. Both the standard and preferred cases, supported by sensitivity tests, indicate that the terrestrial biosphere has been a sink of atmospheric CO2 in the temperate latitudes of both hemispheres over the past two decades, a source in the tropics and boreal zone. The oceans have been sinks everywhere except in the tropics.