Two numerical experiments were performed by using the Community Atmosphere Model version 3 (CAM3) with different sea ice datasets to assess the contribution of the decline of Arctic sea ice to warming in the Northern Hemisphere. One experiment was driven by observed sea ice cover data; for the other one, the authors used the sea ice data of the 4×CO2 scenario simulated by the fourth-generation European Centre Hamburg atmospheric general circulation Model of Istituto Nazionale di Geofisica e Vulcanologia, Italy (INGV_ECHAM4). The comparison of the two experiments indicates that the de-cline of the Arctic sea ice leads to a dramatic warming over the high latitudes of the Northern Hemisphere, char-acterized by a maximum warming of more than 26°C over the Arctic region. The significant warming is closely re-lated to the enhanced atmospheric heat source. A 40–60 W m?2 increase in the apparent heat source was simulated in winter due to the decline of Arctic sea ice. In contrast, no significant change was found in the atmospheric ap-parent heat source in summer. As a result, the summer temperature change induced by the decline of Arctic sea ice appears to be weak. This study suggests that accurate sea ice cover data is crucial for future climate projection of air temperature in high latitudes.