Ten simulations are performed over the last millennium with a global three-dimensional atmosphere-sea-ice-ocean model driven by both natural (solar and volcanic) and anthropogenic (increase in greenhouse gas concentrations and tropospheric aerosols) forcings. The model consists of ECBILT, a spectral T21, 3-level quasi-geostrophic atmospheric model, coupled to CLIO, a coarse resolution sea-ice ocean general circulation model. The results are compared to available reconstructions in order to evaluate the relative contribution of internal and forced variability during the last millennium. At hemispheric scale, the impact of the forcing is clear in all the simulations and knowing the forced response provides already a large amount of information about the behavior of the climate system. Besides, at the scale of the Arctic, the forcing has a weaker contribution to the simulated variability compared to internal variability. This result could be used to refine our conception of “Medieval Warm Period” and “Little Ice Age” (MWP and LIA) in the Arctic. The temperature averaged over the Arctic is generally higher (lower) during those periods because of a stronger (weaker) external forcing at that time. Nevertheless, at decadal time scales, the sign of the internal temperature variations determines to what extent the forced response will be actually visible or masked by internal noise.