The increase in global atmospheric carbon due to human activity is one of the most prominent contemporary environmental issues.
All living things are built mostly from carbon. But the living world accounts for only a tiny fraction of the total carbon on Earth. More than 99% of Earth's carbon is tied up in rocks, particularly limestone, and cycles very slowly. So the major fluxes of carbon involve less than 1% of the total carbon on Earth, and of that, most involve carbon dioxide (CO2).
Although we think of CO2 as a part of the air we breathe, CO2 from the atmosphere can also dissolve in water. Because oceans are so huge, the biggest pool of CO2 is actually in the deeper ocean waters, and, on the timescale of decades to millennia, oceans are the primary regulator of atmospheric CO2. Ocean pools of carbon are depicted in the diagram on the right. As shown, the ocean comprises two separate carbon pools: one on the surface, in contact with the atmosphere, and a second in deeper waters that, for physical reasons, mixes only slowly with surface waters. Thus CO2 in the deep ocean tends to stay there, while CO2 in the atmosphere and the ocean surface tends to equilibrate.