Carbon Dioxide
Carbon Dioxide
Carbon dioxide (CO2) is, along with oxygen and nitrogen, one of the major atmospheric gases. Although it only makes up .03 percent of the atmosphere, it is vitally important for all living things. A colorless and odorless gas, carbon dioxide is made up of a central carbon atom joined to two oxygen atoms.
Early scientists were able to observe the effects of carbon dioxide long before they knew exactly what it was. At the start of the seventeenth century, the common air humans and animals breathe was thought to be a single substance or element. However, around 1630, a Flemish physician, Jan Baptista van Helmont (1577–1644), became the first to discover that there were other "vapors" that were different from ordinary air. He coined the word "gas" to describe the vapors that were given off when wood was burned (actually carbon dioxide). He also recognized that this same gas was produced by the process of wine fermentation. In 1756, the Scottish chemist, Joseph Black (1728–1799), proved that carbon dioxide is present in the atmosphere and that it combines with other chemicals to form compounds. Black also learned that the gas he called "fixed air" was present in exhaled breath and that it was heavier than ordinary air. It was also known by then that a candle flame would eventually go out when enclosed in a jar with a limited supply of air and that a small bird would die in the same tight jar. Near the end of the eighteenth century, chemists finally began to realize that gases were important and should be weighed and accounted for whenever chemists analyzed chemical compounds, as was done with solids and liquids. This more scientific method and approach would eventually lead to a real understanding of the nature and role of carbon dioxide.
Following the 1771 discovery by English chemist Joseph Priestley (1733–1804) that plants give off oxygen (which he called "dephlogisticated air"), the Dutch physician Jan Ingenhousz (1730–1799) demonstrated in 1779 that during photosynthesis (the plant process that uses sunlight to make food), green plants take in carbon dioxide (as well as give off oxygen). He therefore established the key fact that plants, in the presence of light, consume the carbon dioxide produced by humans and animals and give off the oxygen which is in turn consumed by both humans and animals.
In the bodies of humans and animals that breathe air, carbon dioxide is generated by the cells as a waste product (similar to the release of carbon dioxide when wood burns). The lungs remove it from the body, which received it from the bloodstream. The amount of carbon dioxide in the blood affects how humans and animals breathe; a rise stimulates the rate of breathing and a drop lowers it. When air consists of 3 percent of carbon dioxide, it is difficult to breathe. If the amount goes above 10 percent, a loss of consciousness occurs. At about 18 percent humans and animals suffocate. Although this rarely happens, in 1986 a huge cloud of carbon dioxide suddenly exploded from Lake Nyos in northwestern Cameroon (Africa) and suffocated more than 1,700 people and 8,000 animals. It was later learned that atmospheric conditions probably caused the colder water at the bottom of the lake to turn over and quickly release a large amount of carbon dioxide at the surface.
Increasingly, we are becoming aware of the critical role carbon dioxide plays in the health of the global environment. The amount of carbon dioxide in the atmosphere affects our climate since it acts to trap the heat escaping from Earth's surface and reflect it back down. Many scientists argue that when there is too much carbon dioxide in the atmosphere, a "greenhouse effect" will occur and cause the planet to overheat. There is no doubt that atmospheric levels of carbon dioxide have increased over the years as a result of massive amounts of forests that would consume carbon dioxide being destroyed. Also, the amount of carbon dioxide in the atmosphere has also increased because of the steady burning of fossil fuels (which releases carbon dioxide as a by-product). Fermentation and the breakdown of organic matter also play a role toward increasing the amount of carbon dioxide in the atmosphere. Many argue that Earth's temperature could be raised by these influences to a level at which all life is threatened.
Despite these dire predictions, recent scientific calculations of the amount of carbon dioxide in the atmosphere suggest that there is significantly less than should be expected. So far, scientists have not been able to account for the "missing" carbon dioxide. Many think that we may be underestimating the amount or rate (or both) of photosynthesis that takes place on a global scale. If that is the case, Earth is in better shape than first thought.
[See alsoCarbon Cycle; Carbon Family; Greenhouse Effect; Photosynthesis; Pollution ]