The carbon cycle is the means by which carbon is moved between the world's soils, oceans, atmosphere, and living organisms. Northern tundra ecosystems play a key role in the carbon cycle because the cold, moist, and frozen soils trap organic material and slow their decomposition. This very slowly decaying organic material has caused carbon to build up in the Arctic during the past thousands of years. Historically, the tundra has stored large amounts of carbon. Now warming in the Arctic is slowly causing the permafrost to thaw and the tundra to become warmer and dryer.