Land-atmosphere carbon exchange makes a significant contribution to the variability of atmospheric CO2 concentration on time scales of seasons to centuries. In this thesis, a terrestrial vegetation and carbon model, VEgetation-Global-Atmosphere-Soil (VEGAS), is used to study the interactions between the terrestrial carbon cycle and climate over a wide-range of temporal and spatial scales.

The VEGAS model was first evaluated by comparison with FLUXNET observations. One primary focus of the thesis was to investigate the interannual variability of terrestrial carbon cycle related to climate variations, in particular to El Niño-Southern Oscillation (ENSO). Our analysis indicates that VEGAS can properly capture the response of terrestrial carbon cycle to ENSO: suppression of vegetative activity coupled with enhancement of soil decomposition, due to predominant warmer and drier climate patterns over tropical land associated with El Niño. The combined affect of these forcings causes substantial carbon flux into the atmosphere. A unique aspect of this work is to quantify the direct and indirect effects of soil wetness vegetation activities and consequently on land-atmosphere carbon fluxes. Besides this canonic dominance of the tropical response to ENSO, our modeling study simulated a large carbon flux from the northern mid-latitudes, triggered by the 1998-2002 drought and warming in the region. Our modeling indicates that this drought could be responsible for the abnormally high increase in atmospheric CO2 growth rate (2 ppm/yr) during 2002-2003.

We then investigated the carbon cycle-climate feedback in the 21st century. A modest feedback was identified, and the result was incorporated into the Coupled Carbon Cycle Climate Model Inter-comparison Project (C4MIP). Using the fully coupled carbon cycle-climate simulations from C4MIP, we examined the carbon uptake in the Northern High Latitudes poleward of 60˚N (NHL) in the 21st century. C4MIP model results project that the NHL will be a carbon sink by 2100, as CO2 fertilization and warming stimulate vegetation growth, canceling the effect of enhancement of soil decomposition by warming. However, such competing mechanisms may lead to a switch of NHL from a net carbon sink to source after 2100. All these effects are enhanced as a result of positive carbon cycle-climate feedbacks.