carbonic acid (H2CO3) A weak acid formed in small amounts when carbon dioxide dissolves in water:
CO2 + H2O ^ H2CO3 ^ H+ + HCO3-Carbonic acid occurs in rainwater. Although it is a relatively weak acid, in runoff water it is capable of dissolving calcium carbonate in rocks such as limestone to form caves.
Carboniferous The second most recent period of the Paleozoic era, some 355-280 million years ago. It is named for the extensive coal deposits that formed from the remains of vast swamp forests which thrived in the warm, humid climate. In the United States the Carboniferous is often divided into the Mississippian or Lower Carboniferous and the Pennsylvanian or Upper Carboniferous. In the United States coal formation was restricted to the Penn-sylvanian period. On land, amphibians and a few primitive early reptiles evolved, and in addition to treelike forms of clubmosses and giant horsetails, the first true trees appeared - primitive gymnosperms. Aquatic life included sharks and coelacanths. See geological time scale.
carbon monoxide (CO) A colorless flammable highly toxic gas produced by the incomplete combustion of carbon, especially during the burning of fossil fuels in vehicle engines. Motor vehicles may account for up to 98% of CO emissions in urban areas. Emissions can be reduced by fitting car exhausts with suitable catalytic converters. Vehicles running on liquefied petroleum gas also have lower CO emissions than gasoline-fueled ones. Carbon monoxide forms a complex with the red blood pigment hemoglobin, preventing the red blood cells from carrying oxygen, hence its toxicity.
carbon sequestration The net removal of carbon dioxide from the atmosphere into long-lived pools of carbon in terrestrial, marine, or freshwater ecosystems. Such pools (called carbon sinks) may be living biomass in soils and vegetation or inorganic forms of carbon in soils and rocks and in the calcareous shells of marine plankton and invertebrates.
carbon sink See carbon sequestration.
carbon tetrachloride (CCl4) (tetra-chloromethane) A colorless toxic liquid used in some dry-cleaning processes and in certain fire extinguishers. Under the montreal protocol, its use is being phased out, because its vapor is a greenhouse gas. See global warming.
carbon to nitrogen ratio (C:N ratio) The ratio of carbon to nitrogen in organic matter. When organic material is decomposed in the soil, the C:N ratio has an effect on the rate of decomposition and hence on the amount of humus formed and the rate at which soil nitrogen is released or immobilized. The rate of decomposition increases as the C:N ratio becomes smaller, as microorganisms in general require a ratio of about 30-35:1 for efficient digestion of compost or other organic material, materials with higher ratios having insufficient nitrogen for rapid decomposition. Decomposition of material with high C:N ratios results in release of nitrogen from the decomposing organic material into the soil. Most fresh plant material contains about 40% carbon, but species differ in their nitrogen content. During decomposition, up to 35% of the carbon present will be converted into humus if there is sufficient nitrogen present, the remainder being respired as carbon dioxide. The C:N ratio of humus is about 10:1. See also carbon cycle; nitrogen cycle.
carcinogen Any substance or agent that causes living tissues to become cancerous. Chemical carcinogens include many organic compounds and certain inorganic compounds such as asbestos. Physical agents that can cause cancer include radioactive materials and x-rays. Many carcinogens achieve their effect by causing mutations: they are mutagens.
Carnivora An order of mammals (see Mammalia) that contains flesh-eating mammals such as dogs (Canis), foxes and
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