One of the most popular methods of estimating global distribution of CO2 sources and sinks is referred to as the "top-down" approach. In this inverse calculation method, observed background atmospheric CO2 concentration values are used in an atmospheric transport model to estimate CO2 source distribution functions. However, there are many problems associated with this approach. One of these problems is associated with the covariance between the atmospheric transport and the biospheric flux. This is commonly referred to as the rectifier effect in the carbon cycle community. Unless the rectifier effect is correctly represented in the transport model, inverse calculation will give wrong estimates of carbon source/sink distribution functions. The presentation will identify some of the key aspects of the processes that need to be incorporated into the transport model in order to obtain a better estimate of the distribution and magnitude of CO2 sources and sinks.