CDIAC Reporting Units

CDIAC expresses CO₂ emissions in terms of the mass of carbon, the unit commonly used in carbon cycle budgets. This is done because the carbon atom changes atomic partners many times through the course of the carbon cycle, going from CO₂ in the atmosphere to C₆H₁₂O₆ after photosynthesis, or carbonic acid (H₂CO₃) in the ocean and eventually forming calcium carbonate (CaCO₃), carbonate rocks, and so forth. We track the carbon through the carbon cycle, so we keep track of the carbon atom only.

Other data sources give CO₂ emissions in mass of the carbon dioxide molecule. This makes sense for regulatory agencies and organizations concerned with atmospheric CO₂ and climate change. To convert CDIAC data for carbon emissions to comparable CO₂ emissions, multuiple CDIAC values by the ratio of the molecular mass of carbon dioxide to the atomic mass of carbon (44/12 or 3.667).

Carbon has an atomic weight of 12; oxygen has an atomic weight of 16; therefore:

CO₂/C = 12 + 16 + 16 = 44/12 (or 3.667) times the mass of carbon.