The carbon cycle describes how carbon is exchanged between the atmosphere, biosphere, geosphere, and hydrosphere via natural processes. Carbon cycles as it is absorbed by plants through photosynthesis and released by animals through respiration, and it cycles between the atmosphere and lithosphere as carbon dioxide is dissolved in oceans and released from weathering rocks. However, the large amounts of carbon released from burning fossil fuels has unbalanced the natural carbon cycle and increased atmospheric carbon dioxide levels, contributing to global warming.