AP Environmental Science DoDEA Virtual High School

The Carbon Cycle

Name of Student: Date:

Go to the Interactive Lab site: http://www.learner.org/courses/envsci/interactives/carbon/carbon.html

This model is similar to ones presented by the Intergovernmental Panel on Climate Change. It allows you to experiment with how human input to the cycle might change global outcomes to the year 2100 and beyond. One particularly relevant human impact is the increase in atmospheric CO2 levels. Between the years 1850 and 2006, atmospheric concentrations have risen from 290 parts per million (ppm) to over 380 ppm - a level higher than any known on Earth in more than 30 million years. Using the simulator, you will experiment with the human factors that contribute to this rise and explore how different inputs to the carbon cycle might affect the concentrations of the greenhouse gas CO2.

In many scenarios, the atmospheric concentration of CO2 is projected to increase beyond 700 ppm by the end of the century. However, this increase in atmospheric carbon doesn't account for all of the carbon released by burning fossil fuels.

Step 1: Carbon Cycle

To find out where all the carbon really goes, run the simulation, one decade at a time. Record the total amount of carbon in the atmosphere (the number in the sky) and other carbon sinks (terrestrial plants, soil, surface ocean, and deep ocean), as carbon moves through the system. Note that 1 ppm of atmospheric CO2 is equivalent to 2.1 GT (Gigatons) of carbon. As you record your data, keep in mind that this is a simulation of real life.

The simulator is already set to default for this simulation. Simply hit “Run Decade” and record your data with each click.

Step 1 / Total Carbon Emissions / Gaseous Carbon / Ocean Water / Fossil Fuels / Biosphere Gaseous Carbon
To Year / Smokestack / Atmosphere / Ocean Surface / Deep Ocean / Oil and Gas / Coal / Soil / Terrestrial Plants
2000
2010
2020
2030
2040
2050
2060
2070
2080
2090
2100

Answer the following questions:

1. If only one half of the flora in the world existed in 2100 (perhaps due to deforestation), what do you predict the atmospheric carbon level would be?

2. How would you change the simulation to reflect this reduced amount of flora (plants)?

3. What is the relationship between increased carbon in the ocean and increased carbon in the soil? (look at the amount of carbon in the soil and the oceans over the course of your simulation- do they both increase, or is the relationship inverse?)

4. Using the data generated by the simulation, determine the mathematical relationship between the percentage increase in fossil fuel consumption and the increase in atmospheric carbon. Is the relationship linear (line) or exponential (J-curve)?

5. What is the relationship between an increase in fossil fuel consumption and increased carbon in terrestrial plants?

6. How might this change flora populations? What impact could twenty years at this level of consumption have on flora?

7. What is the relationship between an increase in total carbon concentration (the smokestack) and increased carbon in the ocean surface?

8. How might this change marine life populations? What impact could fifty years at this level of emissions have on marine fauna? On marine flora?

9. Which areas are most highly (and quickly) affected by an increase in carbon emissions (and increase in fossil fuel consumption)?

You should now have some understanding of how carbon moves through the system, but you may be wondering about the mechanisms behind this flow. As you read through the following explanations, refer to your Data Table for Lesson 1 Step 2.

Atmosphere: combustion of carbon-based fuel combines carbon, C, and oxygen, O2, adding CO2 to the atmosphere. CO2 is not a by-product of fossil fuel use; it's the direct product of the very reaction that releases the energy.

Biosphere (Terrestrial Plants and Soil): plants (biomass) inhale CO2 and exhale O2. When there's more CO2 available, biomass tends to breathe in more, and therefore grow more. Most scientists now believe that plants have a limited ability to increase their growth rate. However, this is not taken into account in this model.

Surface ocean: The amount of gas dissolved in any liquid is proportional to the partial pressure of that gas in the vapor phase above the liquid (Henry's Law). As a result, if we increase the partial pressure of atmospheric CO2 (i.e. increase the concentration of CO2), then we force more CO2 gas to dissolve into the liquid. (In this case, the liquid is the ocean.) In addition to the CO2 dissolving into the liquid as a gas, CO2 reacts with H2O and forms bicarbonate ions (HCO3-) and carbonate ions (CO3--). This combustion of fossil fuels results in an increase in dissolved surface ocean carbon and a decrease in pH.

Deep ocean: Ocean chemistry involving mineral precipitation, and biological activity, and ocean currents transport the carbon from the surface ocean to the deep ocean over long time-scales.

Step 2: Curbing Emissions

In a best-case (but very unrealistic) scenario, imagine that scientists suddenly discovered an unlimited, clean, and cheap fuel source that emitted no CO2 into the atmosphere, thus bringing fossil fuel use down to zero. What would happen? Would the carbon cycle naturally bring atmospheric CO2 levels back to pre-industrial levels (below 280 ppm)?

Change the Lesson to Curbing Emissions. (click the lesson tab across the top and choose Curbing Emissions) Press the "NONE" button next to fossil fuel use to bring CO2 emissions to zero in the simulation. Then run the simulation for a hundred or more years to see what happens. Record your data and compare it to your previous entries.

Lesson 2:
Step 1 / Gaseous Carbon / Ocean Water / Biosphere Gaseous Carbon
To Year / Atmosphere / Ocean Surface / Deep Ocean / Soil / Terrestrial Plants
2000
2050
2100

Answer the following questions:

1. How have atmospheric carbon levels changed?

2. Without any fossil fuel consumption, which parts of the cycle have improved their carbon levels in comparison to previous data?

3. Which sections of the cycle have improved from the previous levels you have recorded but still are increasing their carbon levels?

As you saw in Step 2, even with no further input from humans, the elevated levels of atmospheric CO2 caused by a century of fossil fuel burning will continue to impact the carbon cycle because the system attempts to reach a state of equilibrium, with the exception of the gradual moving of carbon from the surface to the deep ocean, which happens only over longer time-scales. It could take 2000 years or more for this process to restore atmospheric CO2 to pre-industrial levels.

Reducing carbon emissions to zero is far from realistic. Many scientists agree that a doubling of the pre-industrial CO2 concentration to approximately 550 ppm is a reasonable target to shoot for in order to avoid the most serious impacts on climate and ecosystems.

Step 3: Feedback Effects

So far we have considered only the impact of burning fossil fuels. But there are other human activities that influence the carbon cycle. One major factor is deforestation and land use. Currently, land use (for example, rice paddies) and deforestation outstrip reforestation by roughly 1 GT per year. If deforestation were to increase, perhaps due to increased burning of rainforests, carbon would be transferred first from terrestrial plants to the atmosphere and then through the rest of the carbon cycle.

Change the lesson to Feedback Effects. Change the net deforestation rate and observe how that impacts the carbon cycle. You can do this by clicking and dragging the green block on the Net Deforestation Rate or by typing in a value. Note that deforestation is expressed as GT of carbon released, not as a percentage rate of increase. Realistic deforestation estimates would remain less than 2 GT per year (so use a value less than 2). Record what happens to the system at a steady net deforestation rate of 1.6 GT per year.

Step 3 / Biosphere Gaseous Carbon
To Year / Net Def. Rate / Soil / Terrestrial Plants
2000 / enter your #
2060 / enter the same #
2080 / enter the same #
2100 / enter the same #

Answer the following questions:

1. By 2080, how has the terrestrial flora population changed?

2. What is the carbon level in the soil and how does the carbon level affect the flora populations and species variety?

There are several important natural systems that may be affected by greenhouse warming as atmospheric CO2 rises. Some of these systems may release even more CO2 into the atmosphere, speed up the warming, and cause a positive feedback loop. Which feedback effects will actually take place is hard to predict in such a complex system, but a model for one feedback effect is included in the simulator: melting tundra. If the arctic tundra were to melt as temperatures rise, its stored carbon would enter the system. You will find two possible scenarios. One model assumes that 1/6 of the tundra will melt over 100 years. The other predicts that 1/3 will melt over that same time period.

1. What effect on atmospheric carbon levels would you expect to see as a result of this tundra melt?

Run the simulation by clicking the 1/3 or 1/6 option next to the Melting Tundra option and verify your decision.

As you have seen in this lab, despite the natural tendency of the carbon cycle to regulate the amount of carbon in the atmosphere, the system is currently being overwhelmed by human fossil fuel combustion and deforestation. If this increase in atmospheric CO2 results in temperature rise as scientists predict, there are several possible factors that could cause feedback effects. If human beings are to mitigate these risks, they will have to take strong action soon.

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