Date of Award

Spring 1-1-2010

Document Type


Degree Name

Doctor of Philosophy (PhD)


Atmospheric & Oceanic Sciences

First Advisor

David C. Noone

Second Advisor

Darin Toohey

Third Advisor

James White


The 18O composition of atmospheric CO2 is a potentially valuable tracer of global interactions between the hydrologic and carbon cycles. The observed 18O composition of atmospheric CO2 (hereafter ∂Ca, where d =(R/Rstandard-1) ~ 1000 and R is the molar ratio of heavy to light isotopes) does not show a clear long-term trend, though almost all monitoring stations observed an impressive decrease in ∂Ca from 1992 to 1998. The cause(s) of this and other interannual ∂Ca variations are still relatively unknown, and this work aims to better understand the driving mechanisms that caused the observed interannual ∂Ca variations.

Observed interannual ∂Ca anomalies from Mauna Loa were correlated with anomalies of certain meteorological variables that could potentially affect ∂Ca. Negative correlation existed between ∂Ca and both relative humidity and precipitation amount within parts of the tropics. Positive correlations existed between ∂Ca variations and the 18O content of precipitation for the same tropical regions. Rough estimates suggest that about 20% of the decrease in ∂Ca during the 1990s was due to increases in relative humidity and about 80% of the decrease was due to decreases in the d18O value of precipitation (and likely a consequence of increases in the amount of precipitation).

A global model was constructed to simulate atmospheric CO2 and CO18O (and thus ∂Ca). This model employed an isotopic land model (ISOLSM) and the Community Atmosphere Model (CAM). The model is used for a series of sensitivity experiments to better understand how both steady-state and interannual varying ∂Ca respond to changes in relative humidity, d18O values of precipitation and water vapor, temperature, and light levels. ∂Ca responded the most to changes in the d18O values of precipitation and water vapor, with moderate responses to relative humidity changes. Model results suggest that the decrease in ∂Ca during the 1990s was due primarily to decreases in the 18O composition of precipitation with a smaller a contribution from increased relative humidity. Thus, observations of ∂Ca may become a powerful integrative tool in the coming decades for monitoring large scale changes in the hydrological cycle should it accelerate under a warming climate, as predicted.