The Arctic is a polar region located at the northernmost part of the Earth. The Arctic consists of the Arctic Ocean and parts of Alaska, Canada, Finland, Greenland, Iceland, Norway, Russia, and Sweden. The Arctic region consists of a vast ocean with a seasonally varying ice cover, surrounded by treeless permafrost. The area can be defined as north of the Arctic Circle, the approximate limit of the midnight sun and the polar night. Alternatively, it can be defined as the region where the average temperature for the warmest month is below 10 °C; the northernmost tree line roughly follows the isotherm at the boundary of this region. Socially and politically, the Arctic region includes the northern territories of the eight Arctic states, although by natural science definitions much of this territory is considered subarctic. The Arctic region is a unique area among Earth's ecosystems. The cultures in the region and the Arctic indigenous peoples have adapted to its cold and extreme conditions. In recent years the extent of the sea ice has declined.