Carbon dioxide 1
Carbon dioxide molecule

A carbon dioxide molecule consists of one carbon atom linked to two oxygen atoms.

Carbon dioxide is introduced into the atmosphere through the burning of fossil fuels and other solid materials such as trees, and wood products. It is also produced by a number of chemical processes such as the manufacture of cement.

Carbon dioxide is also generated by natural biological processes such as respiration and biological decay. It is also generated by natural biological processes that are being utilised to generate electricity (anaerobic digestion) and to recycle organic wastes (composting). However, these two technologies can be operated in a way that can enable the gas to be captured and used to make commercial products. We will examine these technologies in detail later.

Carbon dioxide is the primary greenhouse gas produced through human activities. For example, in the USA in 2016, the carbon dioxide produced represented  81.6% of all greenhouse gas emissions produced by human activity in the USA that year.

Carbon dioxide is removed from the atmosphere through the process of photosynthesis by plants, microalgae and macroalgae (seaweeds). One other possible mechanism for limiting the quantity of carbon dioxide in the atmosphere is by its reaction with silicate-containing rocks to form limestone.

It has been estimated that about 10% of atmospheric carbon dioxide is taken up each year by photosynthesis,  almost all of which is returned to the atmosphere by the process of respiration (oxidation of complex organic substances).

Oceans have a great capacity to absorb CO2 through its reaction with carbonate. There is about 60 times as much inorganic carbon in the oceans as in the atmosphere. However, efficient transfer only takes place in the top 100m of the oceans. This region contains only about one atmosphere equivalent.

Carbon dioxide is normally present in the atmosphere as part of the Earth’s natural carbon cycle – the circulation of carbon among the atmosphere, oceans, soil, microorganisms, plants and animals.

However, human activity is affecting the carbon cycle by introducing more carbon dioxide into the atmosphere by, for example, burning fossil fuels, while at the same time reducing the ability of forests to remove carbon dioxide from the atmosphere by deforestation. 

Many types of industries produce carbon dioxide for a range of purposes. A recent analysis (2016) of carbon dioxide production in the USA showed the following breakdown:

Electricity generation

In 2016 the production of electricity accounted for 34% of the industrial carbon dioxide generated, and 24% of all greenhouse gases produced in the USA that year.


The use of gasoline and diesel in transportation accounted for 34% of the carbon dioxide generated, and 27% of all greenhouse gases emitted in the USA.


Many industries produce carbon dioxide by fuel combustion. Others produce carbon dioxide through different chemical reactions during the production of cement, iron steel, and other chemicals. These industrial sources accounted for 15% of all carbon dioxide produced. 

Residential and commercial activity

These activities accounted for 10% of carbon dioxide production.

Other sources not involving fossil fuel combustion

This source accounted for the remaining 6% of carbon dioxide production. 

Back to greenhouse gases to look at the others.