4.3 Carbon cycling

4.3 Carbon cycling


Lesson 1: Carbon cycle and measuring changes in the carbon cycle.

Big questions: Why is it that energy cannot be recycled in a food chain, while nutrients can?

Objectives: Know the basic components of the carbon cycle. Distinguish between a source, a sink, and a reservoir. Carry out basic calculations using fluxes. Realise that methane is another gas that contains carbon, and is significant in the context of global warming.


  • Source – a process that releases a substance eg. combustion of fossil fuels releases carbon dioxide into the atmosphere.
  • Sink – a process that removes a substance eg. photosynthesis removes carbon dioxide from the atmosphere
  • Reservoir- a store of a substance eg. the atmosphere stores carbon dioxide.
  • Flux – any movement of a substance eg. carbon. Usually used in a quantitative context eg. combustion of fossil fuels releases 6 gigatonnes of carbon a year.



Spend some time playing the carbon cycle game

(click here https://www.windows2universe.org/earth/climate/carbon_cycle.html. Be prepared to share something that you learned from the exercise.

Activity 1: The carbon cycle.

Study the diagram below. You may wish to make a copy.

Screen Shot 2016-09-01 at 6.57.57 AM

Instructions: After studying the carbon cycle above, in your own notes

  • a) Make a list of sources and sinks of atmospheric carbon dioxide
  • b) Make a list of reservoirs (pools of Carbon), and fluxes (movement of carbon between pools). This may take the form of a table.
  • Check your list with others.
  • Check the summary at the bottom of the page

Activity 2. Fluxes of methane 

First watch this bizarre video. This is the kind of environment where methane is produced naturally – a peat bog. That is, permanently flooded soil.


Environments that produce methane (methanogenesis) are characteristed by:

  • flooded conditions
  • acidic soils
  • low oxygen concentrations
  • relatively slow rates of decomposition

Peat is a kind of soil characterised by layers of slowly decomposing vegetation, because of acidic conditions and / or a high soil water content. A peat bog is an area of wetlands that contain peat.

Atmospheric methane levels are raised by:

  • Methanogenesis – by archaen bacteria (other bacteria contribute by making the substances which arcane bacteria use to make methane, namely hydrogen, carbon dioxide and acetate)

Atmospheric methane levels are lowered by:

  • Oxidation – converted to carbon dioxide and water in the atmosphere, in a process involving free radicals (free radicals are highly reactive molecules which have unpaired electron pairs).

Methane is also produced in the intestines of many animals, including humans – this is carried out by the same bacteria found in peat bogs.

Activity 3 – Methane vs. carbon dioxide


You will debate the relative importance of methane:

  • read the following document on methane:

Debate with the people at your table. Should we be more focussed on carbon dioxide, or methane in our strategy to combat global warming.

Summarise in one single statement your conclusion regarding the relative importance of carbon dioxide and methane in climate change.


Recommended question of the day: Interpreting data on atmospheric CO2 concentrations. Data-based questions Page 221 1-4

before answering the question it is recommended you watch a clip from the movie An Inconvenient Truth.


Fluxes of carbon – summary (for help with activity 1).

Atmospheric carbon dioxide levels are raised by:

  • Combustion – producing Carbon, Carbon monoxide and Carbon dioxide gases (and H2O)
  • Respiration – releasing Carbon dioxide (and H20)
  • Acid erosion of limestone – releasing Carbon dioxide (and H2O and producing a salt)

Carbon dioxide levels are lowered by:

  • Dissolving in the ocean – forming hydrogen carbonates, and carbonic acid
  • Reef building – which produces calcium carbonate
  • Photosynthesis – which produces organic compounds like glucose


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