The response of the 18 O/ 16 O composition of atmospheric CO 2 to changes in environmental conditions

This study investigates the response of the global mean and spatial variations of the δ 18 O value of atmospheric CO 2 ( δC a ) to changes in soil CO 2 hydration rates, relative humidity, the δ 18 O 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 CO 2 and H 2 O 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 CO 2 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 CO 2 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 CO 2 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 and δW AV on the δC a gradient is strongest.