Recent studies suggest that Arctic ecosystems may be more vulnerable to future climate change than most other parts of the globe. However changes in its carbon-cycle and their feedbacks on climate are not yet fully understood. Using the MIT/MBL GLS (Global Land System) model coupled to a two-layer ocean model and a box- atmosphere model, we have studied carbon dioxide (CO2) fluxes between the major Arctic ecosystem components (atmosphere-ocean and atmosphere-land) and identified the dominant mechanisms that determine these fluxes for the Arctic region (52 N° and above). In most scenarios, we find that the Arctic represents a regional net sink of atmospheric CO2. The size of this Arctic sink depends on assumptions about lateral inputs and biological export of dissolved inorganic carbon in the Arctic Ocean, as well as on assumptions associated with the CO2 fertilization effect and warming temperatures in the terrestrial Arctic. Horizontal transport of atmospheric CO2 from extra-Arctic regions is a major control on local atmospheric CO2 concentrations in the Arctic. This suggests that anthropogenic processes in mid-latitudes may partly influence Arctic carbon dynamics. In addition, model runs with two hypothetical land cover change scenarios (conversion of Arctic land area to all tundra or all boreal) demonstrate that changes in land cover associated with potential future climate change may modify the land carbon fluxes by altering soil wetness and available soil nitrogen.