5: The Carbon Cycle

Key questions for this investigation

  • How does carbon exist and flow within the earth’s climate system?
  • How does the cycling of carbon regulate climate?
  • How might human interaction with this cycle shape future climate?

Tool used in this investigation

Carbon Cycle Diagram – Figure 1 of this activity.  An on-line, interactive version of this diagram is forthcoming, along with additional activities.


A major player in changing climate is carbon in its various forms. This is in large part because of how increasing levels of carbon dioxide in the atmosphere are impacting the energy budget of the earth’s climate. Key to understanding this impact is understanding how carbon moves through the climate system via a process known as the carbon cycle.

The carbon cycle is the continuous exchange of carbon between living organisms, the oceans, atmosphere, and the earth’s crust. During this exchange various carbon compounds are continuously created, destroyed, and stored. Chief among these compounds are carbon dioxide (CO2), methane (CH4), calcium carbonate (CaCO3), and hydrocarbons (HxCy). The chief mechanisms by which this happens include…

  • Respiration of animals, plants, and microbial life
  • Production of methane via digestion and decomposition
  • Photosynthesis by plants and microbial life
  • Atmosphere-ocean exchanges
  • Combustion of fossil fuels and vegetation
  • Volcanic eruption
  • Weathering of rock at the earth’s surface

Figure 1 is a diagram of this cycle that you will use for activity A. The diagram shows principle carbon flows and storages. One of your tasks in this activity will be to identify these exchanges and major carbon reservoirs. Your task in Activity B will be to investigate how we influence this cycle and the consequences of that influence.




Figure 1 is a diagram of the global carbon cycle showing principal flows and storage of carbon in its various forms. The diagram is divided into three “spheres”, the atmosphere, ocean, and geosphere (earth’s crust). The yellow arrows show the flow of carbon compounds between these three spheres, initials identify the principle mechanisms by which these exchanges take place, and chemical formulas identify principle forms of carbon. Additional information is available in the legend below.


Table 1

Legend for Diagram 1

L1 – Principal forms of carbon shown in the diagram

  • Carbon dioxide (CO2)
  • Methane (CH4) – Principal component of natural gas, also a byproduct of digestion and decomposition.
  • Hydrocarbons (HxCy)- Principal components of living organisms, detritus (organic waste and dead organisms) and many fossil fuels (principally oil and coal).
  • Calcium carbonate (CaCO3) – Principal component of limestone and some seafloor sediment

L2 – Principal natural carbon flows

  • Ocean/atmosphere exchanges (oe) of carbon-based gasses between ocean water and the atmosphere. This also includes gas exchange between shallow and deep ocean as seawater mixes.
  • Respiration (rp) of plants, animals, and microbial life
  • Runoff carrying weathered rock and sediment (ro) in the form of dissolved calcium that combines with carbonate ions in seawater to form CaCO3.
  • Volcanic eruptions (ve) expelling large amounts of CO2
  • Weathering (wt) of surface rock and sediment

L3 – Principal human related carbon flows

  • Burning of fossil and bio fuels. The latter includes wood, ethanol, biogas, syngas, and biodiesel.
  • Construction – Principally from the use of concrete since CaCO3 is a key component in the cement used in concrete. CaCO3 releases CO2 into that atmosphere as it solidifies.
  • Agriculture – Principally CH4 from raising animals and crops such as rice.
  • Deforestation – Reducing the amount of CO2 removed from the atmosphere by logging forests for wood products, land for agriculture and development, and getting access to fossil fuels and mineral resources.

L4 – Principal carbon reservoirs

Capacity of these reservoirs in GT or gigatons where 1 GT = 10 billion tonnes values from UNEP/GRID-Arendal Carbon Cycle Diagram

< https://www.grida.no/resources/5050 >


      • Surface waters – 1020 GT
      • Deep ocean – 38,000 GT

Atmosphere – 750 GT

Geosphere (the earth’s crust)

      • Seafloor sediment and sedimentary rock – 66,000 to 100,000,000 GT
      • Oil and gas – 300 GT
      • Coal – 3000 GT
      • Soil and organic matter (principally detritus) – 1580GT

Biosphere (the total living community of the planet) – 540 to 610 GT


L5 – Principal carbon flows

These flows are in GT/yr or gigatonnes per year.  These values are derived from UNEP/GRID-Arendal Carbon Cycle Diagram < https://www.grida.no/resources/5050 >

  • Between the ocean and the atmosphere (oe) – 92 GT/yr to the ocean / 90 GT/yr to the atmosphere
  • Between the shallow and deep ocean – 92 GT/yr to deep ocean / 100 GT/yr to shallow ocean
  • Between the ocean and seafloor sediment – 150 GT / yr in both directions
  • Between the soil and the atmosphere resulting from changes in soil use associated with deforestation and agriculture – 0.5 GT/yr to the soil / 1.5 GT/yr to the atmosphere
  • Between industries, mining, agriculture, and cities in the form of fossil fuel burning – 6 to 8 GT/yr to the atmosphere
  • Due to plant growth and decomposition – 121 GT/yr to the biosphere / 60 GT/yr to the atmosphere


Residence time – In the case of the carbon cycle, this is the average length of time that carbon compounds remain in the various reservoirs listed in L4. Understanding how major greenhouse gasses influence the climatic energy balance of our planet is largely a matter of looking at the warming potential of these gasses and their abundance in the atmosphere. In turn, atmospheric abundance of these gasses is dependent on their atmospheric residence time.

The table below (Table 2) lists five key greenhouse gasses. In addition to showing their approximate abundance in the atmosphere and their contribution to the greenhouse effect, it also shows their Global Warming Potential (GWP) and their atmospheric residence time. The latter is a measure of the warming potential of each gas relative to an equal mass of CO2. For instance, a kilogram of CH4 has 28 times the warming potential of a kilogram of CO2. However, CO2 contributes more to the greenhouse effect because of its higher abundance. Something that is strongly influenced by its longer residence time.

Table 2

Key characteristics of principal Greenhouse Gasses. From Kiehl and Trenberth (1997) and Blasing T.J (2016)   < https://cdiac.ess-dive.lbl.gov/pns/current_ghg.html >


Abundance in the atmosphere (%)

Atmospheric  residence time


Greenhouse Effect Contribution (%)

Water vapor (H2O)

1 to 3

Hours – days


36 – 72

Carbon dioxide (CO2)

~ 0.038

100 – 300 years


9 – 26

Methane (NH4)

~ 0.00018

12 years


4 – 9

Nitrous oxide (N2O)

~ 0.0006

121 years



Ozone (O3)

~ 0.0006

Hours – days


3 -7


Activity – Examining the Carbon Cycle

Using figure 1 and the information from its legend, address the following questions…

  1. What is the total carbon storage in gigatons for each of the four major spheres of the earth? These spheres are the atmosphere, geosphere, oceans, and biosphere.
  2. How do these spheres compare in term of their carbon storage? To answer this question rank them in terms of their total storage and calculate what percentage of the total global storage each represents.
  3. Which of the carbon exchanges listed in box L5 of Table 1 remove more carbon from the atmosphere than they put into it? Which put more carbon into the air than they remove?
  4. Based on the carbon emission rates listed in box L5, how long would it take industrial society to double the amount of carbon in the atmosphere if emission stayed fixed at these rates?
  5. Based on the flow and storage estimates in boxes L4 and L5, how many years would it take to exhaust the fossil fuels reserves shown in the model?
  6. The carbon flow rates listed in box L5 are for short term carbon exchanges (transfer that take less than a 1000 years). Based on this information what might be some longer terms exchanges that show up on figure 1 that do not appear in box L5?

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