Option 1: For the carbon cycle simulation in Doing the Activity, provide colored beads at each of the stations (for example, green=tree, clear=atmosphere, brown=log, red=animal, black=firewood, orange=wood product). Students collect the beads as they move through the stations, putting them on craft cord or a lanyard to make a keychain for their backpack to help them recall their journey.
You might also have students leave a poker chip at each station they visit. After the simulation, students can graph the poker chips from each station to see how many visits the class made to each.
Option 2: As an alternative or in addition to the carbon cycle diagrams in Doing the Activity, have students write about their life as a carbon atom, using a first-person narrative to tell their story.
Option 3: Have students investigate different carbon emissions reduction strategies and carbon sequestration strategies. Then ask students to develop a climate change mitigation plan for their state or region and present it to the class. Should the carbon generated within each state be sequestered by the trees and other plants in that state? If not, who should be responsible for addressing the remaining emissions (other states, federal government, industry, other countries, etc.)?
Option 4: Have students use Google Earth to visit a green space in your neighborhood or other specific location you choose and use the History tab to explore its history. They should identify four dates approximately evenly spaced between the earliest date available to present day, and use the information for those dates to complete the chart and answer the questions on the Carbon Cycle in Action student page.
See Additional Resources for more ideas to enrich this activity.
Option 1: As a formative assessment, read the following statements and have the students put their thumbs up if they think a statement is true and thumbs down if they think it is false. If they are not sure, they can hold their thumb out to the side.
- Carbon cycles through the environment. (True)
- Carbon can be found only in the atmosphere. (False)
- Carbon does not stay in any one place for a long time. (False)
- Photosynthesis is one of the processes that drives the carbon cycle. (True)
- The carbon stored in trees comes mainly from the atmosphere. (True)
- Carbon can be dissolved in ocean water. (True)
To grant students access, simply provide them with this abbreviated, case-sensitive link: http://bit.ly/TheCarbonCycle. For troubleshooting tips, visit the Carbon & Climate Introduction or Technical Support page.
Option 4: If your students are English language learners, have students retell the story of their carbon cycle journey, describing the sequence of events and the main ideas they drew from it. You may use the Carbon Cycle Evaluation Rubric teacher page to assess their retelling.
- To introduce the topic of the carbon cycle, ask students to brainstorm what they already know about carbon. If needed, remind students that carbon is an atom on the periodic table, that it is the fourth most abundant element, and that it can be found in different forms everywhere we look. Use these questions for class discussion or to assess students’ prior knowledge:
- Is there a fixed amount of carbon on the planet? (Like water, there is a relatively constant amount of carbon on the planet. Also like water, carbon constantly moves through different states and places.)
- Do you have any carbon in you? (Yes, in the carbohydrates, fats, proteins, enzymes, and tissues of your body. These are called organic molecules).
- What is photosynthesis? How is it related to the carbon cycle? (During photosynthesis, plants convert carbon dioxide and water into sugar in the presence of sunlight. This process moves carbon from the atmosphere to sugar in plant cells. Then, the sugars are either converted to energy through the process of respiration or used as building blocks to form starch, cellulose, lignin, wood, leaves, or roots. Energy is used to construct these complex molecules. This process, where carbon is removed from the atmosphere and stored in biomass, is known as sequestration.)
- Explain to students that they will model a carbon atom moving through the carbon cycle. Make sure they understand the terms carbon pool, carbon flux, carbon cycle, and carbon sequestration (see Background). If you choose, you may use the Global Carbon Cycle teacher page in your explanation.
- Give each student a copy of the Carbon Pathway student page and provide instructions for how to use this worksheet to record the journey through the carbon cycle. Ask students to count off by six and move to their first station according to their number.
- Students should write their first station on the first row in the table, roll the die, read the outcome from the card at that station, write down the next destination, and move to that station. Even if they end up staying at the same location, they should write down the station name and describe what happened at it.
- Have students complete 10 or more rounds and record their journeys through the carbon cycle. If you have a small class, you may wish to have students complete additional rounds to increase the chance that they will experience diverse outcomes and visit all the stations; this will lead to richer discussion after the activity.
- When everyone is finished, discuss the following questions as a class:
- At which station (carbon pool) did you spend the most time? At which did you spend the least time?
- Did anyone get stuck in one station or between two stations? Why do you think that happened?
- While each of your journeys was different, was there anything similar about them?
- At which pools can carbon be stored? At which is carbon released into the atmosphere?
- Point out that there are many different ways to enter and leave most carbon pools. The time carbon atoms spend in each pool varies. For example, some atoms might cycle very quickly between the atmosphere and forests through photosynthesis and respiration, while others may get sequestered in a tree for hundreds of years.
- Remind students that the activity focused on the biological portion of the carbon cycle. Ask students if there are other places that carbon is stored and other ways that carbon moves. (Carbon moves through organisms in the ocean. It also moves through a geological cycle, in which it is stored in rocks, fossil fuels, and sediments on the ocean floor and includes fluxes like weathering, compaction, and volcanic eruptions.)
- Explain to students that they will work in groups to draw a carbon cycle diagram on a large sheet of paper that represents the places each member in their group visited.
- When the groups finish, have the students place their diagram on the wall. Direct students to walk around the room and use sticky notes to write their observations on the various diagrams. They should be looking for similarities and differences in the other diagrams compared with their group’s diagram.
- After the students have observed each group’s carbon cycle diagram, have them retrieve their own diagram and return to their seats. Let them make any changes they feel they would now like to incorporate into their own diagrams. Discuss:
- What were some of the ways your diagram was like other diagrams and different from them? (Answers will vary.)
- According to your diagram, how does carbon enter the atmosphere? (Both biologic and human-caused processes.)
- How do trees and other plants help remove carbon from the atmosphere? (Trees take in carbon [sequester it] through photosynthesis and store it in their tissues. In this way, forests can act as carbon sinks.)
- Ask your students how they might calculate how much carbon a tree can store. (The amount of carbon an individual tree can store depends on the species of tree, how large the tree is, and how old it is. In general, the larger the tree, the more carbon it can store. The denser the wood, the more carbon it stores. Also, the faster growing the tree, the more quickly it will add carbon to its stores.)
- Optional: To explore the concept of density, give each group a sample of wood (see Getting Ready), and have them use the balance and the graduated cylinder to determine the density. Density is found by taking the mass (weighing) and dividing it by the volume (using the graduated cylinder to get the volume of an irregular shaped object). Ask groups to share their findings with the class. A simpler variation on this idea would be to select one wood sample and model the experiment for the whole class. Lead a discussion, asking such questions as:
- Why might different samples have a different density? (Some of the answers might include measurement error and or that different types of wood have different densities. Students may want to say that size is an issue, but this would be a misconception. Remind them that density is a ratio of mass divided by volume. If a piece of wood is denser it has more matter per volume than a less dense piece of wood, but it is not necessarily larger.)
- What elements do you think are present in a piece of wood? (It is mostly carbon, but also includes hydrogen, oxygen, and other elements.)
- What does density tell us about how much carbon is in the wood? (Wood with higher density has more carbon.)
- Explain to students that they will select a specific tree and will determine the approximate amount of carbon stored in it. Point out that foresters use a formula for calculating carbon in a tree that takes into account four measurements: its diameter at breast height, its height, its wood density, and the amount of water in the tree. Since wood density and the amount of water in a tree are difficult to measure in a living tree, students will use just two of these measurements to approximate the amount of carbon.
- Divide the class into small groups, and have each group select a tree to measure. Using the directions on the Tree Data student page, have students measure the diameter and height of the tree using either metric or English units.
- Have students use either the How Much Carbon Is in a Tree? (Metric Units) or How Much Carbon Is in a Tree? (English Units) student page to estimate the amount of carbon in the tree they measured.
- Lead a class discussion about the carbon storage results that students calculated.
- What was the greatest amount of carbon that was stored by a single tree? Was it clearly the biggest tree? What was the least amount of carbon stored by a tree?
- What was the average amount of carbon stored by the sample of trees we measured?
- For how long will the carbon in each of those trees be stored? (As long as the tree is alive or the wood exists in a product.) What is likely to kill trees in this area? (Possible answers include development, road widening and construction, lightning, old age, insects, disease, etc.)
- Where does the carbon go after it leaves the tree? (Some will be stored in soil; some will move to the atmosphere; some will be eaten and become part of animal biomass.)
- How does carbon storage relate to growth? (The bigger the tree grows, the more carbon it stores.)
- What does carbon content tell us about the age of the tree? (More carbon means an older tree.)
Carbon flows through biological, physical, and geological systems with different processes and at different timescales. The carbon cycle refers to the movement of carbon, in its various forms, from one pool to another (see Figure 1). A carbon pool is any place where carbon can be found, such as plants, the atmosphere, or soil. Carbon pools can also be called stocks or reservoirs. Carbon flux is the process by which carbon moves from one pool to another. For example, carbon moves from the atmosphere to plants through photosynthesis and can move back from plants to the atmosphere through plant or soil respiration during the decomposition process, or during a wildfire when trees and plants are burned.
When a carbon pool absorbs more than it releases, it becomes a carbon sink. For example, the ocean and soil are carbon sinks. Forests can also become carbon sinks. Alternatively, when a carbon pool releases more than it absorbs, it is called a carbon source. Because fossil fuels cannot be created as fast as we are extracting and combusting them, they are considered a carbon source.
The Biological Carbon Cycle
In the biological system, carbon cycles through living organisms, land, water, and the atmosphere by processes including photosynthesis, respiration, and decomposition. Different quantities of carbon reside in each pool, and the carbon moves at different rates between the pools. Some of these processes occur very quickly, while others can take hundreds to thousands of years. For example, some carbon atoms may cycle daily through plant photosynthesis and respiration, while other atoms may be trapped in a tree trunk for hundreds of years. Soil can also trap carbon for long time periods, up to thousands of years.
Carbon sequestration is the process of transferring atmospheric carbon dioxide into other carbon pools, such as into plant biomass via photosynthesis or into soils from decaying plant and animal matter. Carbon storage in plant biomass is relatively easy to visualize, as plants and composed largely of carbon that they have obtained from atmospheric carbon dioxide through photosynthesis. During photosynthesis, plants convert carbon dioxide and water into sugar in the presence of sunlight. Some of the sugars are used as building blocks to form plant biomass, such as cellulose, lignin, bark, and leaves. Through this process, carbon moves from the atmosphere and is stored in plant cells. Building and maintaining these complex structures requires energy, which the plants create by converting some of the sugars made during photosynthesis into energy through the process of respiration. The process of respiration releases some carbon dioxide back into the atmosphere.
Many people do not realize how important soils are in carbon sequestration. Carbon is a major ingredient of soil, and as Figure 1 indicates, soils contain well over twice the amount of carbon found in all terrestrial plants and animals. As plants grow, die, and decay, some of their carbon is stored in soils. This carbon can build continuously over long periods of time and will likely play a significant role in carbon sequestration efforts. The deep ocean and soils represent very large carbon pools, where carbon can stay for hundreds to thousands of years. On the other hand, the exchange of carbon between the atmosphere and plants, along with the exchange between the atmosphere and the surface ocean, occurs continuously. The amount of carbon flowing through the biological system without human impact (through plants and animals to soil and atmosphere) is fairly stable and is considered to be in balance. In Figure 1, the biological fluxes in terrestrial and marine systems account for all their carbon.
For educational materials to support The Carbon Cycle, see the Additional Resources section for this activity.
How Much Carbon Is In a Tree?
The amount of carbon that an individual tree contains depends on tree species and tree size. To calculate carbon storage, foresters and scientists first measure a tree’s height and diameter. Often measurements are taken of trees within a sample plot that is representative of the larger forest, and then the average of these data is used to estimate the biomass in the larger forested area. Tree diameter can be calculated by measuring the tree’s circumference with a measuring tape and dividing this number by pi (π, or 3.14). Because diameter can vary with tree height, measurements are taken at a standard height from the ground—1.5 meters (4.5 feet). This is called diameter at breast height, or DBH. To measure the height of a tree, foresters use instruments called clinometers to calculate tree height. The height of a tree can also be estimated using a ruler. Together, tree height and diameter are used to estimate the tree’s weight. The weight of a living tree is called the green weight and includes both the biomass of the tree and all the water in the tree. The dry weight is calculated by subtracting the weight of the water, which is about 50 percent of a living tree. In general, the amount of carbon stored in an individual tree is about half of the tree’s dry weight.
Carbon Storage vs. Carbon Sequestration
Stored carbon is the amount of carbon that exists in a tree’s leaves, wood, stem, roots, and bark at a particular point in time. Because older trees are larger than younger trees, they are able to store more carbon (see “C storage” line in Figure 3). Knowing the amount of carbon stored in a tree and the tree’s age allows us to calculate the rate of carbon sequestration. This represents the net intake of carbon over a period of time (for example, in one year or for the average per year over the tree’s lifespan). Because young, growing trees add biomass at a faster rate than older trees, they are sequestering carbon at a faster rate than older trees (see “C sequestration” line Figure 3). Although the age at which maximum carbon sequestration occurs varies with tree species, the general shape of this relationship is similar for all species.
The Geological Carbon Cycle
The geological carbon cycle occurs over millions of years and includes processes such as rock formation, large-scale land shifts (plate tectonics), weathering, dissolution, volcanic eruptions, and fossilization. Fossil fuels— petroleum, natural gas, and coal—are created over hundreds of millions of years as layers of rock, soil, and sediment covers the remains of dead plants and animals. As this organic matter becomes compacted, pressure and heat turn the carbon into long chains of hydrocarbons. Without human intervention, the carbon would mostly remain trapped far beneath Earth’s surface—making fossil fuels a long-term carbon sink. Limestone is another long-term carbon sink. Limestone forms when ocean sediments, containing the shells of marine animals, are compressed and buried under the ocean floor.
It is important to note that these flows—of carbon—occur at different time scales. While the generation of new fossil fuels is an ongoing process, it is a process that takes place on a time scale of hundreds of years. In contrast, people have only been burning fossil fuels and emitting large quantities of carbon since the 1800s.
(Source: Background adapted from Activity 7: Carbon on the Move, and Activity 8: Measuring Carbon Sequestration in Project Learning Tree’s Southeastern Forests and Climate Change Secondary Education Module.)
- What is the biological carbon cycle, and how does it differ from the geological carbon cycle?
- What role do trees and forests play in the carbon cycle?
- What are potential effects of releasing carbon that has been stored in geological carbon pools?
In this activity, students model the movement of carbon atoms in the carbon cycle and explore the relationship between atmospheric carbon and plants.
The purpose of the activity is to teach students that carbon may be found in a variety of places and that it moves from location to location. While carbon cycles geologically over millions of years, this activity’s primary focus is the biological component of the carbon cycle that happens on a much shorter timeframe.
- Students will participate in an active simulation of the carbon cycle.
- Students will create a model of the carbon cycle.
- Students will calculate the amount of carbon a tree can sequester.
- Global Carbon Cycle teacher page (optional)
- Copies of Carbon Pathway, Tree Data, and How Much Carbon Is in a Tree? (Metric Units) or How Much Carbon Is in a Tree? (English Units) student pages
- Cards cut apart from the Carbon Cycle Station Cards teacher pages
- Three pairs of dice (optional)
- Six blank sheets of paper for station labels
- Large pieces of paper for group diagrams
- Marking pens
- Tape or thumb tacks
- For each group: Small sample of wood (see Getting Ready below), scale or triple beam balance, large graduated cylinder, calculator (all optional)
- Tree or trees to measure
- Measuring tapes
- Getting Ready: 60 minutes
- Doing the Activity: two to three 50-minute periods
- Evaluate: Varies, depending on option selected
- Print and cut apart the Carbon Cycle Station Cards teacher pages, and make copies of the student pages.
- Use the blank paper to create labels for each of the six stations: Air (Atmosphere), Tree, Firewood, Wood Product, Fallen Log, and Animal.
- Set up six stations, each with one die and a label. Post the label (with tape or thumb tacks) so that it is visible from any place in the room. Place the die and corresponding Carbon Cycle Station Card on a table or desk. Students need enough space to gather at each station and move between stations.
- (Optional) Collect samples of wood for the Elaborate section of Doing the Activity (see Step 13). Pieces should fit easily into a graduated cylinder. Groups might have the same or different types of wood. Have enough wood samples to provide new ones throughout the day. (If a sample absorbs too much water, it will skew the results.)
- (Optional) Consider inviting a forester or interpreter from a local state park or nature area to talk with students about the important role that trees play in the carbon cycle and to help them with their tree carbon measurements and calculations.
- See Additional Resources to find other supports for teaching this activity.
PLT Conceptual Framework
- 2.1. Organisms are interdependent and depend on nonliving components of the Earth.
- 3.1. In biological systems, energy flows and materials continually cycle in predictable and measurable patterns.
- 5.2. Healthy ecosystems are in a state of dynamic equilibrium, with steady inflows and outflows.
See Standards Connections in the Appendices for a list of standards addressed in this activity.