Carbon Dioxide

Teachers' Information Sheet


This lesson looks at one of the principal greenhouse gases, carbon dioxide (CO2), in more detail. Carbon dioxide is a colourless, odourless gas. Its present day atmospheric concentration is approximately 367 ppmv (parts per million by volume). It is the principal anthropogenic contributor to the greenhouse effect, and there has been a major increase in the amount of the carbon dioxide in the atmosphere over the past 200 years. The suspected main cause for the increase in atmospheric concentrations of this gas has been man-made emissions since the Industrial Revolution from processes such as fossil fuel combustion.

The lesson is structured around the following questions:

  • What are the sources of CO2?
  • What are the sinks of CO2?
  • How much CO2 is in the atmosphere?
  • How long does CO2 remain in the atmosphere?
  • What can be done to reduce the amount of CO2 in the atmosphere?
  • What can we do to reduce the amount of CO2 being produced?

What are the sources of carbon dioxide?

This section looks at the sources of carbon dioxide, with particular emphasis on the anthropogenic or man-made sources, which are of particular relevance to global warming.

Carbon dioxide is produced naturally through the process of respiration, the decay of plant and animal matter, and natural forest fires. However, there are many anthropogenic sources of carbon dioxide. For example fossil fuel combustion, land use changes and cement manufacture are all major anthropogenic sources of CO2.

Fossil fuel combustion: fossil fuels contain large quantities of carbon. When they are burnt, this carbon is released into the atmosphere as carbon dioxide, leading to an increase in its atmospheric concentration. The amount of CO2 released depends on the type of fossil fuel used. Coal, for example, emits more CO2 per unit of useful energy produced than does natural gas.

Land use changes: changes in the use of land, as a result of deforestation, have had a major impact on the amount of carbon dioxide emitted into the atmosphere. Trees and plants absorb carbon dioxide during the process of photosynthesis, and the carbon is stored in the tissue and wood fibre of the tree. The large scale clearing of many of the world's forested areas, such as the Amazonian rainforest and the rainforests of Malaysia, for agricultural and industrial purposes, has reduced the amount of CO2 that is being absorbed by vegetation, allowing more carbon dioxide to be retained in the atmosphere. In addition, the burning or gradual decay of the timber products releases CO2 to the atmosphere.

Cement: the manufacture of cement uses large quantities of limestone, which contains a high percentage of calcium carbonate (CaCO3). During the manufacturing process large quantities of carbon dioxide are released from the limestone into the atmosphere.

What are the sinks of carbon dioxide?

A sink is a method by which a gas is removed from the atmosphere. This section outlines the principal sinks for carbon dioxide, i.e. its absorption by the oceans and terrestrial biota (for example forests).

The oceans contain about 95% of the carbon actively circulating within the biosphere. Carbon dioxide is dissolved within the ocean and it also reacts with the chemical constituents in the water. Scientists believe that a large proportion of the carbon dioxide that has been released into the atmosphere will eventually be held within the oceans. However, the cycling of carbon within the ocean is a slow process that can take between 100-1000 years, so the absorption of CO2 by the oceans is not keeping pace with the amount of carbon dioxide that is being emitted. There is also scientific concern that if the predicted climate changes occur, this will affect the natural exchange of carbon between the ocean and the atmosphere and consequentially the uptake of carbon dioxide by the oceans. (An increase in the temperature of the oceans because of global warming would decrease the solubility of CO2.)

How much carbon dioxide is in the atmosphere?

The atmospheric concentration of carbon dioxide has been steadily increasing since the 18th Century, most likely as a result of human activity. Pupils may graph the rise in the amount of CO2 against time using the data supplied in the pupils notes.

The pre-industrial concentration of carbon dioxide in the atmosphere was 280 ppmv (parts per million by volume). By 1999 this had increased to 367 ppmv, an increase of about 30% on the pre-industrial value.

Since 1957, the amount of carbon dioxide in the atmosphere has been monitored at Mauna Loa in Hawaii in the eastern Pacific Ocean. The station has recorded an increase in the atmospheric concentration of carbon dioxide of 1.2 ppmv or 0.3% per year. This rise can mainly be attributed to anthropogenic emissions of carbon dioxide, since the concentration would not change so quickly under normal circumstances.

How long does carbon dioxide remain in the atmosphere?

This section aims to highlight the fact that CO2 could remain in the atmosphere for up to two centuries before it is removed by various reactions, thereby increasing the importance of CO2 as a greenhouse gas.

The time taken for atmospheric gases to adjust to changes in sources or sinks is known as the atmospheric lifetime of a gas. The atmospheric lifetime of carbon dioxide is in the order of 50-200 years. As a consequence of this, CO2 emitted into the atmosphere today could influence the atmospheric concentrations of carbon dioxide for up to two centuries to come. The Inter-governmental Panel on Climate Change (IPCC) has predicted that if anthropogenic emissions of carbon dioxide continue at present day levels, the atmospheric concentration of carbon dioxide could have increased to 415-480 ppmv by 2050, rising to 460-560 ppmv by 2100.

What can be done to reduce the amount of carbon dioxide in the atmosphere?

Scientific concern over the contribution to global warming of increasing CO2 concentrations has led to an international response. Of particular concern is the growth in energy derived from fossil fuels. This section introduces the international agreements signed to help reduce carbon dioxide levels in future years.

In 1992 the IPCC estimated that in order to stabilise carbon dioxide emissions at present day levels a 60% reduction in global carbon dioxide emissions would be required.

Since the problem of global warming was first highlighted there have been attempts by international governments to reduce the emissions of greenhouse gases and try and solve the potential problem of global warming. The most important such attempt to limit our rapidly increasing carbon dioxide emissions is The United Nations Framework Convention on Climate Change (FCCC)

The Framework Convention on Climate Change was signed at the Rio Earth Summit in 1992, by over 150 Governments. The Convention requires all countries that ratify the Treaty to produce a national programme, outlining measures that will limit the amount of greenhouse gases produced and improve methods for protecting the sinks for carbon dioxide such as the forests and vegetation. The developed countries were committed to return their emissions of greenhouse gases to 1990 levels by the year 2000. Developed countries also provided financial and technological assistance to allow the developing nations to produce their own programmes on emission reductions.

In 1997, the Kyoto Protocol called for developed nations to reduce emissions of greenhouse gases by 5% by 2012. This Protocol will become legally binding when enough countries have ratified it.

The UK Climate Change Programme is the UK Government's action plan for reducing greenhouse gas emissions. The UK Programme goes further than the FCCC and Kyoto Protocol, by calling for a 20% reduction in national carbon dioxide emissions by 2010 (from 1990 levels). This reduction is being achieved through a number of measures, including increased energy awareness, VAT on fuel, energy efficiency and alternative energy sources. Emissions of carbon dioxide in the UK are currently predicted to fall by 15% by 2010 with existing measures.

What can we do to reduce the amount of CO2 being produced?

Although the international agreement will encourage governments to watch their CO2 emissions, it is important to heighten people's awareness of what the individual can do to help reduce global CO2 concentrations. This section is included to provide a few examples of how easy it is to reduce personal and family CO2 levels.

  • Promote energy efficiency in the home by encouraging people to use less electricity and gas.
  • Lag your hot water tank and pipes.
  • Insulate your house.
  • Walk or use public transport instead of taking the car.
  • Plant a tree.
  • Reduce the temperature on your thermostat by 1C.
  • Wash full loads in the washing machine, and dry clothes outside where possible.
  • Only boil as much water in the kettle as you need.
  • Do all the ironing in one go.

Questions and sample solutions

1. Within the notes for pupils is a table of atmospheric concentrations of carbon dioxide. This data can be used to produce a graph similar to the figure shown below.

The pupils should now be able to answer the following questions:

2. What was the concentration in 1880? Answer: 293 ppmv

3. When was the concentration 345 ppmv? Answer: 1982

4. What are the best ways to reduce CO2?
Sample solution: Energy efficiency, reduced use of fossil fuels, more public transport

Points for class discussion

Think about the areas of the world that are being deforested.
How much land is being deforested?
Which areas are most severely affected by deforestation?

Fossil Fuels:
Where do fossil fuels come from?
Why do they release carbon dioxide when they are burnt?
Are they 'renewable'?

Reducing CO2 emissions:
What other sources of energy are there apart from fossil fuels?
How can energy be used more efficiently?
What is 'sustainable' forestry, and why is it important?