There are 3 main reservoirs through which carbon moves: the atmosphere, terrestrial organisms, referred to as the biota, and the ocean. The atmosphere plays a central role in the carbon cycle. Here, carbon is found as carbon dioxide. Atmospheric carbon dioxide enters terrestrial food chains through plants when they perform photosynthesis. Some of the carbon picked up by plants returns to the atmosphere as carbon dioxide when plants respire. The rest of the carbon is used to build plant tissues. The carbon then either moves through the food chain, beginning with herbivores when they eat plants, or to decomposers, when plants die. Animals and decomposers return the carbon to the atmosphere as carbon dioxide when they respire. Oceans play a major role in determining carbon dioxide levels in the atmosphere. Carbon dioxide dissolves in ocean water and returns to the atmosphere when it spontaneously comes out of solution. Carbon leaves the water when it enters aquatic food chains via photosynthesis. The carbon is returned to the water when aquatic organisms respire. The burning of fossil fuels releases carbon dioxide from carbon stores long-buried in the earth. This adds to the levels of carbon dioxide in the atmosphere, increasing the greenhouse effect, and contributing to global warming.