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Carbon cycle
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Carbon cycle

Carbon cycle - sustaining life on earth

The element carbon can exist in both a gaseous form like carbon dioxide or in liquid or solid form by combining with other elements to form compounds. Such compounds may be in the form of petrol or diesel, living tissue or as inert compounds like carbonates present in many rocks constituting the earth's crust. The process by which carbon is cycled between various forms is called the carbon cycle and this cycle has been implicated in many of the changes in the earth's climate over geological time scales.

carbon cycle

Tiny structures inside green leaves called chloroplasts use the energy of sunlight to break apart the molecules of carbon dioxide (CO2). Plants use the carbon to form organic compounds which in turn are used to build living tissue comprising leaves, flowers, bark and wood while the release of oxygen helps humans, animals and insects to breathe. Carbon dioxide is also absorbed by the oceans where it is used by phytoplankton, the organism at the start of the oceanic food chain, to form their skeletons.

The combustion of fossil fuels transforms the element carbon from a solid or liquid form to carbon dioxide which being lighter than air, then congregates in the upper atmosphere. There it becomes a greenhouse gas as it has the ability to trap some of sun’s rays reflected back into space at a longer wavelength.

If more carbon dioxide is absorbed than given off, then the carbon dioxide level in the atmosphere will fall and this has been associated with the onset of previous ice ages in which global temperatures have fallen. The current ice caps of Greenland and Antarctica and Alpine glaciers are remnants of the last ice age. Conversely, if more carbon dioxide is given off than absorbed, then the carbon dioxide concentration in the atmosphere will rise and there is clear evidence over geological time that this has resulted in periods of global warming.

Reference: Life on Earth by T Flannery (Allen Lane, London) 2011

Increasing use of fossil fuel

Since the start of the industrial era (ca 1750), increasing amounts of energy have been required to undertake work which might have previously been undertaken by humans beings themselves. A product of the combustion process involving fossil fuels is the production of carbon dioxide which, being lighter than air is congregating in the upper atmosphere. As our rate of consuming fossil fuels is still increasing, ever increasing amounts of carbon dioxide are being recorded in the upper atmosphere (Figure 1).

fossil fuel
Figure 1: Atmospheric concentration of carbon dioxide since 1750 [ACE, MMU]

The more recent concentrations of carbon dioxide are being monitored continuously at the Mauna Loa Observatory, Hawaii and these measurements are illustrated in Figure 2. The concentration of CO2 is steadily increasing even though significant efforts have been made since the 1997 Kyoto agreement to limit the emissions of greenhouse gases.

Figure 2: CO2 Concentration at Mauna Loa Observatory, Hawaii [Scripps CO2 program]

The consequence is that ever larger amounts of solar radiation are being absorbed and not reflected back into space so resulting in global warming.

If the average increase in world temperature is not to exceed 1.5 ºC then the CO2 concentration in the upper atmosphere should not exceed 430 parts per million. To stay within this limit will require actions NOW to limit our use of energy particularly that of fossil fuels.

rise temperature
Rise in average global temperature since 1850

These emissions and temperature rise differs from previous eras when our energy use was much less and more sustainable. Then carbon was taken up by biomass, animals and humans to build their tissues and when these died, the organic matter was often secreted beneath sediment which gradually heated up and formed the fossil fuels we are exploiting today

Energy usage

Our usage of energy is still increasing both individually and collectively for reasons which include –

  • increased mobility such as living in one town and working in another
  • living and working in buildings which may be heated in winter and cooled in summer
  • possessing more energy using products such as washing machines, TVs and mobile phones
  • globalisation in which products are manufactured in one country and sold in other countries
  • increase in tourism
KITH # Activity Age
1.1 Energy use over past 100 years
science, citizenship, geography
9 - 14
1.5 Managing without energy
Science, citizenship
9 - 14


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