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 we begin our journey, lets check in to make sure that you understand which of the Earth's spheres are represented in this forest carbon cycle.

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 carbon passport record and assigned a carbon reservoir to start at. The passport will be the record of your journey - the carbon reservoirs you traveled through and the processes by which you traveled (examples - photosynthesis, respiration, combustion, ingestion, decomposition etc.)

Step 2: Once at a station, you will draw a ticket. The ticket will tell you where you will be going next and how you will get there [in brackets]. Record the carbon reservoir you are going to next and the responsible process [in parentheses] on your passport record and then follow the ticket's instructions on what to do next.

Step 3: When you finish recording the information on your carbon passport record, you can move on to your next carbon reservoir unless the ticket gives you different directions. For example, the ticket may tell you to count to 100 and then choose a different ticket.

Step 4: When finished, find a partner and compare your carbon cycles. How were they the same? How were they different?

Download a Forest Carbon Passport Chart (Microsoft Word 2007 (.docx) 93kB Dec9 12) and begin your journey. When you have finished your journey, complete the two Checking In questions below and watch the short animation.
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 also adds carbon dioxide to the forest carbon cycle. Does the animation reinforce your understanding of the carbon journey you have just taken?

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 is expressed in Gigatons(Gt) or 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.

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.