Department of Earth System Science

This study investigates the response of the global mean and spatial variations of the δ ¹⁸O value of atmospheric CO₂ (δC_a ) to changes in soil CO₂ hydration rates, relative humidity, the δ ¹⁸O value of precipitation and water vapor, visible radiation, temperature, and ecosystem flux partitioning. A three-dimensional global transport model was coupled to a mechanistic land surface model and was used to calculate isotopic fluxes of CO₂ and H₂O and the resulting δC_a . The model reproduced the observed global mean and north-south gradient in δC_a . The simulated seasonal amplitude and phases of CO₂ and δC_a  agreed well at some but not all locations. Sensitivity tests with relative humidity increased by 3.2% from its original value decreased δC_a  by 0.21‰. Similarly, a global 3.3‰ decrease in the isotopic composition of both precipitation and water vapor (δW_P  and δW _AV, respectively) caused a 2.6‰ decrease in δC_a . A 1 K increase in atmospheric temperatures also affected δC_a , but there was a very small δC_a  response to realistic changes in light levels. Experiments where leaf and soil CO₂ fluxes were repartitioned revealed a nontrivial change to δC_a . The predicted north-south δC_a  gradient increased in response to an increase in soil CO₂ hydration rates. However, the δC_a  gradient also had a large response to global changes in δW_P  and δW _AV. This result is particularly important since most models fail to deplete δW_P  enough at middle and high latitudes, where the influence of δW_P  andδW _AV on the δC_a  gradient is strongest.