In CBAT atmospheric concentrations of CO2 are shown as 'Gigatonnes' of Carbon [Gt C] on the upper left vertical axis & as the equivalent in Parts Per Million by Volume [PPMV] on the upper right axis [as in the image below].
Converting PPMV to a weight of carbon is straightforward. The conversion factor from PPMV to a weight of carbon in tonnes is 1 PPMV CO2 weighs 2.13 Billion Tonnes Carbon or 'Gigatonnes' of Carbon [Gt C].
As atmospheric concentrations of CO2 are most often measured as 'Parts Per Million by Volume' [PPMV], it is difficult to relate these directly to the 'carbon-weight' [in Gt C] of the 'carbon-emissions' affecting that rise in concentrations in a manner related to UNFCCC-Compliance.
So doing this conversion from PPMV to Gt C makes it easier to generally measure and compare Green House Gas [GHG] emissions with atmospheric concentrations and also 'Contraction & Concentration' with other models that are related to UNFCCC-compliance.