EarthLabs > Climate and the Carbon Cycle: Unit Overview > Lab 2: The Carbon Cycle > 2A: A Forest Carbon Cycle

The Carbon Cycle: What Goes Around, Comes Around

Part A: A Forest Carbon Cycle

Consider the image of the forest carbon cycle on the right and think about the analogy between the Earth System and New York City. Can you identify the "neighborhoods" in this image? Which of the four Earth's spheres do these neighborhoods represent? The orange arrows in this image tell you that there is movement of carbon compounds between these neighborhoods. What makes the carbon compounds move from one neighborhood to another? How long do the carbon compounds stay in these neighborhoods? You will find out the answers to these questions by taking a carbon journey through a forest ecosystem carbon cycle. Before you begin your journey, let's check in to make sure that you understand which of the Earth's spheres are represented in this forest carbon cycle.


Checking In

  1. Which of the four Earth System's spheres are represented in the forest carbon cycle image? Check all that apply and then click the Check Answers button at the end of the list.
    [CORRECT]
    [CORRECT]
    [CORRECT]
    [CORRECT]
  

The Forest Carbon Cycle Game

You can't see the carbon flowing through the image of the forest carbon cycle above, but taking on the role of a carbon atom moving through a forest carbon cycle game will help you understand how carbon moves into and out of the Earth's spheres. Before beginning the forest carbon cycle game, there are two carbon cycle terms you need to know before starting the game:

Reservoir: A carbon reservoir is a place in the Earth System where carbon is stored. Carbon reservoirs can be large or small and somewhere in-between. We might say that houses and buildings in New York neighborhoods are analogous to these reservoirs.

Process: A process causes carbon move from one reservoir to another. Processes in the forest carbon cycle game include photosynthesis, respiration, decomposition, ingestion, excretion/defecation, combustion and diffusion.

Instructions:
Your teacher will download and set up the Forest Carbon Cycle game in your classroom. As you play the Forest Carbon Cycle game developed by the University Corporation for Atmospheric Research, you will take on the role of carbon atoms traveling through the various carbon reservoirs of a forest. The stations that you see posted around the classroom represent reservoirs of carbon typically found in a forest ecosystem. For example, if you are at the "ATMOSPHERE" station, then you are a carbon compound currently stored in the atmosphere reservoir. Carbon can stay in these reservoirs sometimes for short periods of time and in some cases, for much longer periods of time.

Step 1: You will be given a Forest Carbon Cycle Passport Chart (Microsoft Word 2007 (.docx) 118kB Mar4 14) and assigned a carbon reservoir station to start at. This chart will be the record of your journey through the carbon reservoirs. Fill-in the name of reservoir and the Earth's sphere(s) in your Forest Carbon Cycle Passport Chart.

Step 2: Draw a ticket. The ticket will tell you where you will be going next and how you will get there. The ticket may also have special instructions such as "count to 100 and then choose a different ticket."

Step 3: For each round, record the required information Forest Carbon Cycle Passport Chart.

Step 4: When your class has finished, find a peer(s) and compare your carbon cycle journeys. How were they the same? How were they different?

Discussion

  • Was everyone's carbon journey through the forest the same? Why or why not?
  • In a real ecosystem, does carbon cycling ever stop? Why or why not?
  • What is the role of food chains in the forest carbon cycle?
  • Carbon does not spend the same amount of time in each of the carbon reservoirs. Why do you think that is?

Next watch a short animation Carbon Dioxide and The Carbon Cycle. Click on the carbon cycle button and spend some time following the carbon dioxide molecules as they cycle through the various carbon reservoirs. Note that the combustion of fossil fuels and wildfires also adds carbon to the forest carbon cycle. Finally, answer the Checking In questions below.

Checking In

  1. In a forest carbon cycle, which process(s) moves carbon out of the atmosphere to another reservoir? Check all that apply and then click the Check Answers button at the end of the list.
    [INCORRECT]
    [CORRECT]
    [INCORRECT]
    [CORRECT]
  2. Which process(s) moves carbon into the atmosphere? Check all that apply and then click on the Check Answers button at the end of the list.
    [CORRECT]
    [CORRECT]
    [CORRECT]
    [CORRECT]
  3.   

How much carbon moves? Gigatons, Carbon Sinks and Carbon Sources

As you were moving from carbon reservoir to carbon reservoir in the forest, you represented only a few carbon atoms. In reality, enormous amounts of carbon continually move between reservoirs in a carbon cycle. How enormous is enormous? So enormous that the amount of carbon moving between reservoirs per year is expressed in Gigatons(Gt). One gigaton is equivalent to 1,000,000,000 metric tons. That's one billion metric tons in just one Gigaton(Gt)! It is hard to visualize just how much carbon is in one gigaton so lets use an elephant to help us. Although elephants vary in size and weight, the average weight of an Asian elephant is about four metric tons. If the average weight of an elephant is 4 metric tons, simply divide 1,000,000,000 metric tons by four metric tons. Thus, one Gigaton of carbon is equivalent to 250,000,000 elephants!


Next, consider this simple representation of the global carbon cycle on the left. The arrows represent the movement of carbon between four reservoirs: the ocean, vegetation and land, fossil fuels, and the atmosphere. The numbers within these arrows represent gigatons of carbon moving per year. Note that more carbon is moving into the vegetation and land reservoir than is moving out to the atmosphere. This tells you that the vegetation and land reservoir is currently behaving as a carbon sink, taking in more carbon from the atmosphere than it is releasing to the atmosphere. A carbon source is just the opposite. A reservoir that behaves as a carbon source releases more carbon to the atmosphere than it takes in. Examine the gigatons of carbon moving into and out of the atmosphere and then answer the Checking In questions below.


Checking In

  1. Is the ocean behaving as a carbon sink or a carbon source?
    [INCORRECT]
    [CORRECT]
  2. Are Fossil fuels and land use(burning forests for agriculture) currently behaving as a carbon sink or a carbon source?
    [CORRECT]
    [INCORRECT]
  3. Which biosphere process(s) are responsible for moving 450 GT of carbon from the atmosphere into the vegetation and land?
    [INCORRECT]
    [INCORRECT]
    [INCORRECT]
    [CORRECT]
  4. Which biosphere process is responsible for moving 439 GT of carbon from the vegetation and land into the atmosphere?
    [INCORRECT]
    [INCORRECT]
    [CORRECT]
    [INCORRECT]
  5. Which chemical process is responsible for moving gigatons of carbon from the fossil fuels into the atmosphere?
    [CORRECT]
    [INCORRECT]
    [INCORRECT]
    [INCORRECT]


A carbon cycle out of balance?

Scientists consider the carbon cycle to be in balance when the total amount of carbon moving into vegetation and oceans is equal to or greater than the amount of carbon going into the atmosphere. Examine the "Gigatons and The Carbon Cycle" diagram again and then answer the Stop and Think questions below.

Stop and Think:


1. Does the carbon cycle in this diagram appear to be in balance or out of balance? Use specific evidence from the diagram to support your answer.

2. Imagine that we could go back to pre-industrial times before fossil fuels were being burned for energy. Would a pre-industrial carbon cycle be in balance or out of balance? What makes you think so?

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