Deforestation

Mature forests contain large quantities of organic carbon, present in the living and dead biomass of plants, and in organic matter of the forest floor and soil. The quantity of carbon in mature forests is much larger than in younger, successional forests, or in any other type of ecosystem, including human agroecosystems. Therefore, whenever a mature forest is disturbed or cleared for any purpose, it is replaced by an ecosystem containing a much smaller quantity of carbon. The difference in carbon content of the ecosystem is balanced by an emission of carbon dioxide (CO₂) to the atmosphere. This CO₂ emission always occurs, but its rate can vary. The CO₂ emission is relatively rapid, for example, if the biomass is burned, or much slower if resulting timber is used for many years and then disposed into an anaerobic landfill, where biological decomposition is very slow.

Prior to any substantial deforestation caused by human activities, Earth's vegetation stored an estimated 990 billion tons (900 billion metric tons) of carbon, of which 90% occurred in forests. Mostly because of deforestation, only about 616 billion tons (560 billion metric tons) of carbon are presently stored in Earth's vegetation, and that quantity is diminishing further with time. It has been estimated that between 1850 and 1980, CO₂ emissions associated with deforestation were approximately equal to emissions associated with the combustion of fossil fuels. Although CO₂ emissions from the use of fossil fuels has been predominant in recent decades, continuing deforestation is an important source of releases of CO₂ to the atmosphere.

The CO₂ concentration in Earth's atmosphere has increased from about 270 ppm prior to about 1850, to about 360 ppm in 1999, and it continues to increase. Many atmospheric scientists hypothesize that these larger concentrations of atmospheric CO₂ will cause an increasing intensity of an important process, known as the greenhouse effect, that interferes with the rate at which Earth cools itself of absorbed solar radiation. If this theory proves to be correct, then a climatic warming could result, which would have enormous implications for agriculture, natural ecosystems, and human civilization.