Human influences

The main human influence on the carbon cycle has been the release of carbon by the burning of fossil fuels, for energy. These fossil fuels have formed over geological time, for example the huge coal deposits that developed from buried plant remains in the Carboniferous period. The burning of fossil fuel has had a major effect on the atmosphere since about 1750 with the onset of the Industrial Revolution. Fossil fuel burning is increasing the store of carbon in the atmosphere by about 6 gigatons each year and since the 1700s the atmospheric levels of CO₂ have increased from about 265 parts per million to 350 parts per million. This is the major source of the increase in CO₂ levels in the atmosphere, but is not the only one.

Another major source for the increased release of carbon to the atmosphere is the changes in land use, in particular the extension of intensive farming throughout the world and increasing deforestation, such as occurs in the Amazon Basin. Both of these processes have the effect of reducing the amount of organic matter, hence carbon, in the topsoil. It has been estimated that the soils under natural ecosystems can store many times more CO₂ than can intensive agricultural systems, because during cultivation, soil organic matter is broken down and CO₂ released to the atmosphere.The soil is now recognised as a major source of carbon that needs to be managed. By changing agricultural practices, it is possible to increase the amount of organic carbon in the soil and to create a situation where a larger proportion is transferred into medium and long-term storage in the soil. However, these changes need to be balanced against the need to produce more food for an ever growing population. Decisions like these will need to be taken if we are to make a significant impact on global warming.

Relative Contributions of the Pools of Carbon

Given the significant attention that is being given to the carbon cycle, there have been many attempts at estimating the world's carbon reservoirs. The following are values commonly used:

Reserves	Estimates of Size (Gigatons C)
Atmosphere	750
Forests and Vegetation	550-610
Soils	1,580-2,100
Fossil Fuels	4,000-5,000
Surface Ocean	1,000-1,050
Deep Ocean	38,000-40,000
Carbonate rocks and sediments	75,000-100,000

Note, a 'gigaton' is a billion metric tons