Carbon is the 4th most abundant element in the universe. Is it essential to all life as we know it. All living organisms contain carbon. Additionally, carbon is present in rocks, dissolved in rivers, lakes and oceans, and in the atmosphere as carbon dioxide.
The movement of carbon across these reservoirs is the carbon cycle. Life has a significant role in the carbon cycle. Green plants and bacteria use atmospheric carbon dioxide (plus some dissolved carbonates in the case of aquatic organisms) to create the molecules of life - carbohydrates, proteins and fats. Organisms higher up in the food chain eat these plants or other animals.
Plants and animals respire, that is, burn fuel in order to live. The products of respiration include carbon dioxide, which is released into the atmosphere. When organisms die, their remains decompose, also releasing carbon back into the atmosphere and the soil.
In addition to the biological turnover of carbon, the Earth itself has a carbon cycle, with carbon being continually released from carbon sources and removed by carbon sinks. The geological carbon cycle works over millions of years, whereas the biological carbon cycle works over periods from days to a few thousands of years.
The image above shows the carbon cycle. The green numbers next to each label represent the carbon reservoirs, in units of billions of tons (gigatons). For example, the atmosphere contains about 750 gigatons of carbon, mostly in the form of carbon dioxide, but also trace amounts of other gases, such as methane. The soil contains about 1580 gigatons, in the form of organic matter, bacteria, etc. Fossil fuel reservoirs hold about 4000 gigatons.
As can be seen, the bulk of the carbon is in the deep ocean, around 38,100 gigatons. The numbers in red show the carbon fluxes (per year) between different carbon pools. The numbers are also in gigatons.
Geological Carbon Cycle
This occurs over millions of years. Rain washes atmospheric carbon dioxide down to the soil and the sea. Carbon dioxide in the soil exists as carbonic acid, which combines with minerals in the soil to form carbonates - a process known as weathering. Over time, these carbonates are eroded and transported by wind and water back to the sea. Carbonates in the oceans eventually sink to the bottom; therefore the oceans are a net carbon dioxide sink.
Plate tectonics drives the sea floor deep underground at the subduction zones. As the sea floor gets buried deeper, it heats up and eventually releases the carbon dioxide, which makes its way back to the surface through volcanoes, hotsprings, or gradual seeps.
Plate tectonics also affects the land. Deeply buried carbonate rocks can be pushed upwards, exposing them on the surface. This is happening in the Himalayas, which contain sedimentary carbonate rich rocks which were formed at the bottom of some ancient ocean. Once at the surface, the rocks are once again exposed to weathering and erosion.
In the end, the fate of all carbon leaving the atmosphere is to enter the sea, and become incorporated in the sea floor. This sea floor then releases its carbon back to the atmosphere when it is subducted deep enough beneath the crust. But during the course of this cycle, the various reservoirs can hold carbon for hundreds of millions of years.
Biological Carbon Cycle
Biology provides a fast turnover cycle superimposed on the geological carbon cycle. The two biolgical processes - photosynthesis and respiration, together are responsible for carbon turnover at a 1000 times faster rate than the entire geological cycle. These processes happen fast enough that seasonal variations in atmospheric carbon dioxide can easily be detected. For example, in the higher latitudes, sunlight is quite seasonal, with shorter days during the winter. Photosynthesis is therefore greatly reduced during winters, while respiration continues pretty much unhindered. So there is a marked seasonal cycle with carbon dioxide levels being higher in winters than they are in the summers.
In marine environments, there is an additional factor to consider. Many marine organisms (phytoplankton) use carbon to make shells. These shells sink to floor when the organisms die, and the sediment on the ocean floor can be compacted over time to form limestone. Other organic matter can also be buried on the ocean floor, and under certain conditions turn into hydrocarbons such as coal and gas. The oceans can therefore serve as carbon sinks over geological time scales. Eventually, of course, all this carbon will also make its way to the surface due to plate tectonics. But relatively stable reservoirs can last for hundreds of millions of years.
For most of human history, we had no net impact on atmospheric carbon. Like any other form of life, our contribution to the carbon reservoir was our bodies, and our carbon production consisted of whatever carbon dioxide we breathed out. Whatever we burned as fuel through cooking fires and heating our homes was too small to make any significant difference.
Things started changing with the beginning of industrialization around 1850. Two things happened: the population of humans increased dramatically, and the per capita production of carbon increased as well due to the fuel requirements of industrialization. This dramatically increased the production of carbon dioxide. Coal and oil have been carbon sinks for hundreds of millions of years. The carbon in fossil fuels was sequestered (that is, not in the atmosphere and not part of the carbon cycle) during this period. This carbon is now being released into the atmosphere as carbon dioxide.