There has been a large observed reduction in the Arctic Sea Ice cover in the late 20th century. It has been generally attributed to be one of consequences of global warming. However, many scientific questions remain to be solved. For example, what is the most important mechanism controlling the Arctic climate response and the Arctic Sea Ice cover to changes in climate and climate forcing, e.g. (1) GHG forcing directly through change in radiative fluxes, (2) atmospheric or oceanic heat transports, (3) local feedback processes, (4) Aerosols influencing clouds, or radiative forcing in the atmosphere and at the surface. In this project, we will utilize the fully coupled CESM1 Earth System model, and the slab-ocean model (SOM) version of the Community Atmospheric Model (CAM) version 5 to explore this question. We will focus particularly on how oceanic heat transport, affects the Arctic Sea Ice cover in the 20th century. Our first goal is to reproduce simulation of the fully coupled CESM1 with CAM5-SOM. Then, various forcings including CO2, Aerosol, and ocean heat fluxes will be varied to determine contribution of each on change of Arctic Sea Ice in the 20th century.