The carbon cycle

The cyclic circulation of essential chemical components through the regions of the biosphere — the atmosphere, the hydrosphere and the lithosphere — is called the biogeochemical cycles.

Water, as well as carbon, nitrogen, oxygen and phosphorus, circulate cyclically in this way. Because of these cycles, natural ecological balance can be maintained. Some of them are the carbon cycle, the nitrogen cycle and the phosphorus cycle; information on the carbon and nitrogen cycles is given here.

The main way carbon is fixed in an ecosystem is photosynthesis. Animals get their food by feeding on green plants, and through that food they obtain carbon. Some decomposers obtain carbon by breaking down dead organisms. All living things release carbon to the air as carbon dioxide in respiration. Where decomposers are absent, the carbon in the bodies of dead plants and animals becomes fossil fuels, a process taking millions of years; burning releases the carbon in fossil fuels. Micro-organisms also play an important part in the carbon cycle: they quickly release the carbon in dead bodies to the atmosphere.

Why the cycle is out of balance

For most of history, the carbon taken out of the air by photosynthesis was roughly matched by the carbon returned by respiration and decomposition, so the amount in the air stayed steady. Fossil fuels are carbon that was taken out of the cycle millions of years ago and locked underground. Burning coal, oil and gas — the hydrocarbons of chapter 14 — puts that old carbon back in a few centuries, far faster than plants can take it up, while clearing forests removes the plants that would. The extra carbon dioxide is the main cause of the global warming studied later in this chapter.