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.
Domain One of the Carbon Budget Analysis ToolCBAT is primarily focused on measuring for potential policy-purposes, the path to that UNFCCC Concentration-Objectiveeven if there are unresolved arguments about what the quantitative value of the objective actually is.
In considering this, it is of central importance to realize that UNFCCC-Compliance is a matter of 'Precautionary Policy' [i.e. beyond just 'the science'] and also of being committed, on the basis of the UNFCCC Principles of Precaution and Equity, to the successful realization of that objective.
The UK Climate Act [UKCA] for example is an instance that purports to be a function of of this realization.
