Carbon dioxide enters the atmosphere mainly from the decay of vegetation, but it also comes from volcanic eruptions, the exhalations of animal life, from the burning of fossil fuels (such as coal, oil, and natural gas), and from deforestation. The removal of CO2 from the atmosphere takes place during photosynthesis, as plants consume CO2 to produce green matter. The CO2 is then stored in roots, branches, and leaves. The ocean acts as a huge reservoir for CO2, as phytoplankton (tiny drifting plants) in surface water, fix CO2 into organic tissues. Carbon dioxide that dissolves directly into surface water mixes downward and circulates through greater depths. The increase in atmospheric CO2 appears to be due mainly to the burning of fossil fuels, such as coal and oil; however, deforestation also plays a role as cut timber, burned or left to rot, releases CO2 directly into the air, perhaps accounting for about 20 percent of the observed increase.