Carbon dioxide (chemical formula CO2) is a colourless and odourless gas that is naturally found in the atmosphere. The molecule is linear (see illustration). Some people also refer to it as carbonic acid, but in fact what they actually mean is hydrogen carbonate (H2CO3), the solution of carbon dioxide in water.
The Earth’s atmosphere these days contains around 383 ppm of carbon dioxide (Jan, 2007). This concentration is rising year by year, mostly as a result of the large-scale use of fossil fuels, but deforestation also plays a role . Before the Industrial Revolution this concentration was approximately 280 ppm. The rising concentration of carbon dioxide in the atmosphere most probably plays an important role in global warming.
Carbon dioxide is used a lot in canned drinks, being the source of the ‘fizz’. It is also found like this in nature in some mineral waters. Carbon dioxide is used in fire fighting, in particular in situations where water could be dangerous to use as an extinguisher because it could cause chemical reactions, conduct electricity or lead to significant damage, such as in paper archives.
Carbon dioxide was discovered at the beginning of the 17th Century by the Brussels-born aristocrat Jan Baptista van Helmont, who called it "Sylvester gas". He determined that after the combustion of charcoal in a closed bowl, the remaining mass was smaller than the original mass. His conclusion was that difference was transformed into a "wild spirit" (spiritus sylvestre) of gas.
Carbon dioxide is used by plants in the process of photosynthesis. This is the process by which plants take up water and atmospheric carbon dioxide and combine them to produce glucose, while releasing the oxygen (O2) produced back into the atmosphere. Energy is needed for this process and this is supplied by sunlight.
In greenhouses, CO2 gas is used as a sort of nutrient for the plants: in the presence of higher concentrations of CO2 many plants grow more quickly. With an increase of the concentration of carbon dioxide levels on the planet there is the probability that vegetation will also grow faster.
Animals do the reverse of what plants do. They breathe oxygen in and carbon dioxide out, when the oxygen is used to metabolise (burn) fats and carbohydrates to release energy, and carbon dioxide is produced in the process.
The easiest way of making carbon dioxide is by burning carbon (such as charcoal) in the presence of air. It is released in huge quantities from the burning of fossil fuels. This process is one of the most important sources of energy for humans.
Natural terrestrial CO2 exchange arises from sources such as savannah- and forest fires (caused by lightning strikes), eruptions from volcanoes, the fermentation processes in wetlands and mangroves, and by exchange with the seas and oceans. The human/industrial CO2 output amounts to approximately 6% of the total natural terrestrial CO2 exchange according to current scientific theories.
Cells produce carbon dioxide as a waste product of metabolism; extra effort creates extra carbonic acid, which is removed by the blood. Carbonic acid reduces pH, which is carefully kept by the body between 7.35 and 7.45. If the pH threatens to fall too far then the respiratory centre of the brain is stimulated and the breathing rate is speeded up and deepened. By doing this the excess of carbon dioxide is exhaled via the respiratory tract. Although carbon dioxide is not toxic in high concentrations it can be asphyxiating because it displaces oxygen. In the area around Lake Nyos, a volcanic crater lake in Cameroon, on the 26th August 1986 more than 1700 died when a huge pocket of CO2 was released. On 16th August 2008 107 people were taken ill in the German town of Mönchengladbach when a fire extinguisher factory accidentally released 25,000 m3 CO2 and it flowed into a housing estate .
The possibility of escape from large volumes of carbon dioxide is one of the most important areas of concern in the underground storage of CO2.
Two physical properties of carbon dioxide are each in their own way important in daily life. The most prominant property is that of greenhouse gas. Additionally, because of the way that carbon dioxide changes phase it is used for a variety of ends. See Warming of the Earth for the main article on this subject. The concentration of CO2 has increased dramatically since the Industrial Revolution, and explosively since 1960.
Because carbon dioxide absorbs infrared radiation it reduces the emanation of heat that reached the earth from the sun back out into space. This is what is meant by the Greenhouse Effect. It has been found that the concentration of CO2 has increased since the Industrial Revolution . During the last 10,000 years the concentration of carbon dioxide on Earth has generally hovered around 275 ppm. However, from the beginning of the Industrial Revolution the concentration of carbon dioxide has been rising. Since the second half of the 20th Century there has been a significant speeding up of the rate of increase in. Around the year 2000 it was found that the concentration of carbon dioxide had already reached 375 ppm. The increase in CO2 has been primarily caused by the burning of fossil fuels (oil and gas). This can be deduced from the fact that the carbon isotope C-13 in atmospheric CO2 is falling, which would have pointed to a plant source. The isotope C-14 in atmospheric CO2 has also decreased, indicating a source of carbon that has not been in the biosphere for a long time. Finally, highly accurate measurements have determined that the concentration of oxygen has also fallen with a corresponding speed, which points to an oxidation- or combustion process.
According to pretty much all experts the greenhouse effect is increasing as a consequence of increasing carbon dioxide concentration (the so-called amplified greenhouse effect). In November 2007 the Inter-governmental Panel on Climate Change (IPCC) of the United Nations proposed that carbon dioxide from emissions caused by humans was the most important greenhouse gas . While there are other gases with a stronger greenhouse effect, it is the sheer quantities of carbon dioxide emissions that means that it is this gas that at the moment produces the greatest contribution to the greenhouse effect.
The atmosphere of the planet Venus consists almost entirely of carbon dioxide. The greenhouse effect on this planet is very large as a result.
The increase in the carbon dioxide level in the atmosphere can lead to a faster growth of plants on earth, which could counterbalance the effects of the deforestation mentioned above. An other effect of the increase in the carbon dioxide level is that the pH-value of water that is kept in balance with the air, is falling. This means that the water is becoming more acidic which in turn could be having effects on the planet’s water flora and fauna. See also: Heating of the Earth, and: CO2 Capture and Storage.
When cooled to −78.5 °C carbon dioxide turns directly into a solid, when it’s also known as dry ice. Under normal air pressure the dry ice does not melt as it warms up but sublimates directly to its gaseous state. Solid carbon dioxide looks more or less like ice formed from water. At 15 °C carbon dioxide becomes a liquid when the pressure is raised to 50 bar.
There are a number of spectacular and yet relatively non-dangerous experiments that can be done with solid carbon dioxide. If a couple of granules are dropped in to a glass of water the dry ice evaporates thanks to the relative ‘heat’ of the water. The water starts to bubble and a considerable volume of vapour is given off. It looks like the water is boiling, when it is really still at room temperature or even a little colder than it was. Since carbonic acid vapour is heavier than air the vapour dribbles over the edge and runs out of the glass. This effect is often used in the theatre or on TV on a larger scale in fog machines.
Carbonic acid dissolved in (rain) water, hydrogen carbonate, is also of geological importance, because it is in a state to dissolve chalk or limestone. This leads to karst phenomena. In its bound form as calcium carbonate (CaCO3) it occurs as limestone. When calcium carbonate is dissolved in acid or is heated as in the production of clinker and cement, CO2 is released. The cement industry is responsible for around 5% of the worldwide annual emission of the greenhouse gas CO2.
In order to regulate the emission of greenhouse gases, a carbon dioxide emissions trading scheme  has been set up. Participants in the CO2 emissions trade receive at the beginning of the year a certain quantity of rights.
This quantity is established by national governments and ministries on the basis of the national allocation plan (NAP). A business where emission takes place (in law it is referred to as an ‘establishment’) have a share of these rights so long as they have a valid emissions permit. The registration and control of these is carried out in the Netherlands by the Dutch Emissions Authority (NEA), and other participating countries have similar authorities.