Atmospheric concentration of carbon dioxide (CO2), oxygen (O2), and 13C/12C stable isotope ratio in CO2 recorded over the last decades at representative stations. Top panel: CO2 (green lines) from Mauna Loa Northern Hemisphere (MLO) and South Pole Southern Hemisphere (SPO) atmospheric stations, and O2 (blue lines) from Alert Northern Hemisphere (ALT) and Cape Grim Southern Hemisphere (CGO). Lower panel: δ13C in CO2 from MLO and SPO. The ratio of the 13C to 12C isotopes, relative to a standard, is measured by δ13C (delta C 13), which is defined as δ13C = [ (13C/12C)sample / (13C/12C)standard − 1 ] × 1000 and has units of permil. Samples with a larger value of δ13C are said to be enriched, while samples with a lower δ13C are said to be depleted.
Ciais et al., 2013. Fig. 6-3 modified to include only the top two panels.