Novel applications of carbon isotopes in atmospheric CO2: What can atmospheric measurements teach us about processes in the biosphere?

Conventionally, measurements of carbon isotopes in atmospheric CO ₂ (δ ¹³CO ₂) have been used to partition fluxes between terrestrial and ocean carbon pools. However, novel analytical approaches combined with an increase in the spatial extent and frequency of δ ¹³CO ₂ measurements allow us to conduct a global analysis of δ ¹³CO ₂ variability to infer the isotopic composition of source CO ₂ to the atmosphere (δ _s). This global analysis yields coherent seasonal patterns of isotopic enrichment. Our results indicate that seasonal values of δ _s are more highly correlated with vapor pressure deficit (r = 0.404) than relative humidity (r = 0.149). We then evaluate two widely used stomatal conductance models and determine that the Leuning Model, which is primarily driven by vapor pressure deficit is more effective globally at predicting δ _s (RMSE = 1.6‰) than the Ball-Woodrow-Berry model, which is driven by relative humidity (RMSE = 2.7‰). Thus stomatal conductance on a global scale may be more sensitive to changes in vapor pressure deficit than relative humidity. This approach highlights a new application of using δ ¹³CO ₂ measurements to validate global models.