World of Earth Science | Carbon Dioxide

Carbon dioxide was the first gas to be distinguished from ordinary air, perhaps because it is so intimately connected with the cycles of plant and animal life. Carbon dioxide is released during respiration and combustion. When plants store energy in the form of food, they use up carbon dioxide. Early scientists were able to observe the effects of carbon dioxide long before they knew its function.

About 1630, Flemish scientist Jan van Helmont discovered that certain vapors differed from air that was then thought to be a single substance or element. Van Helmont coined the term gas to describe these vapors and collected the gas given off by burning wood, calling it gas sylvestre. Today, it is known that this gas is carbon dioxide, and van Helmont is credited with its discovery. In 1756, Joseph Black proved that carbon dioxide, which he called fixed air, is present in the atmosphere and that it combines with other chemicals to form new compounds. Black also identified carbon dioxide in exhaled breath, determined that the gas is heavier than air, and characterized its chemical behavior as that of a weak acid. The pioneering work of van Helmont and Black soon led to the discovery of other gases. As a result, scientists began to realize that gases must be weighed and accounted for in the analysis of chemical compounds, just like solids and liquids.

In 1783, French physicist Pierre Laplace (1749–1827) used a guinea pig to demonstrate quantitatively that oxygen from the air is used to burn carbon stored in the body and produce carbon dioxide in exhaled breath. Around the same time, chemists began drawing the connection between carbon dioxide and plant life. Like animals, plants breathe using up oxygen and releasing carbon dioxide. Plants, however, also have the unique ability to store energy in the form of carbohydrates, our primary source of food. This energy-storing process, called photosynthesis, is essentially the reverse of respiration. It uses up carbon dioxide and releases oxygen in a complex series of reactions that also require sunlight and chlorophyll (the green substance that gives plants their color). In the 1770s, Dutch physiologist Jan Ingen Housz established the principles of photosynthesis.

English chemist John Dalton guessed in 1803 that the molecule contains one carbon atom and two oxygen atoms (CO₂); this was later proved correct. The decay of all organic materials produces carbon dioxide slowly, and Earth's atmosphere contains a small amount of the gas (about 0.033%). Spectroscopic analysis has shown that in our solar system, the planets of Venus and Mars have atmospheres rich in carbon dioxide. The gas also exists in ocean water, where it plays a vital role in marine plant photosynthesis.

In modern life, carbon dioxide has many practical applications. For example, fire extinguishers use CO₂ to control electrical and oil fires that cannot be put out with water. Because carbon dioxide is heavier than air, it spreads into a blanket and smothers the flames. Carbon dioxide is also an effective refrigerant. In its solid form, known as dry ice, it is used to chill perishable food during transport. Many industrial processes are also cooled by carbon dioxide, which allows faster production rates. For these commercial purposes, carbon dioxide can be obtained from either natural gas wells, fermentation of organic material, or combustion of fossil fuels.

Recently, carbon dioxide has received negative attention as a greenhouse gas. When it accumulates in the upper atmosphere, it traps the Earth's heat, eventually causing global warming. Since the beginning of the industrial revolution in the mid 1800s, factories and power plants have significantly increased the amount of carbon dioxide in the atmosphere by burning coal and other fossil fuels. This effect was first predicted by Svante August Arrhenius, a Swedish physicist, in the 1880s. Then in 1938, British physicist G. S. Callendar suggested that higher CO₂ levels had caused the warmer temperatures observed in America and Europe since Arrhenius's day. Modern scientists have confirmed these views and identified other causes of increasing carbon dioxide levels, such as the clearing of the world's forests. Because trees extract CO₂ from the air, their depletion has contributed to upsetting the delicate balance of gases in the atmosphere.

In rare circumstances, carbon dioxide can endanger life. In 1986, a huge cloud of the gas exploded from Lake Nyos, a volcanic lake in northwestern Cameroon, and quickly suffocated more than 1,700 people and 8,000 animals. Scientists have attempted to control this phenomenon by slowly pumping the gas up from the bottom of the lake.