The observed atmospheric CO2 growth rate consists of a large interannual variability superimposed on a gradual increase due to human fossil fuel carbon release. This talk will review the characteristics of such variabilities on interannual to interdecadal time scales and how the corresponding carbon sources and sinks might be related to known climate variabilities such as ENSO. We will also discuss the implication of understanding the natural carbon variability to the projection of future carbon cycle and climate change.