The Arctic is changing fast. As temperatures rise and the tundra's permafrost thaws, it releases carbon dioxide and methane, greenhouse gases, into the atmosphere. With the ice melting earlier in the spring and not freezing until much later in the autumn, the herders are being forced to change centuries-old migration patterns, as the reindeer find it difficult to walk over a snowless tundra. The rising temperatures also affect the tundra's vegetation, the only source of food for the reindeer.