Carbon, Life, and Health HASPI Medical Biology Lab 13 Background/Introduction

Carbon, Life, and Health HASPI Medical Biology Lab 13 Background/Introduction You will die, but the carbon will not; its career does not end with you. It will return to the soil, and there a plant may take it up again in time, sending it once more on a cycle of plant and animal life. Quote by Jacob Bronowski The Carbon Cycle Carbon, represented by the letter C in chemical formulas and molecules, is arguably the most important atom on Earth. It is the backbone for most of the molecules that form living matter and is necessary for survival of proteins, carbohydrates, nucleic acids, and lipids. Yet, the Earth s crust only contains 0.032% carbon as compared to oxygen, which makes up 45%, and silicon, which is 29%. Additionally, carbon dioxide (CO2) makes up less than 0.03% of the Earth s atmosphere. As a result, it is vitally important that carbon has the ability to be cycled. Carbon is stored and cycled between five major reservoirs: carbon that is stored as compounds, such as carbohydrates (C6H12O6), in living and dead organisms. carbon that is stored in the atmospheric gases carbon dioxide (CO2) and methane (CH4). carbon that is stored in the oceans as calcium carbonate (CaCO3) and hydrocarbons dissolved in water. Geosphere carbon that is stored as organic matter in soil, fossil fuels (like coal, which is 85% C), and sedimentary rocks. The carbon cycle involves Carbon Source Carbon Source Geosphere Carbon Source http://www.physicalgeography.net/fundamentals/images/carboncycle.jpg Carbon Source http://d32ogoqmya1dw8.cloudfront.net/images/esla bs/carbon/soil_hands_361.jpg the movement of stored carbon between the biosphere, atmosphere, hydrosphere, and geosphere. An area where carbon is stored is called the source, and the area where carbon is transferred is called the sink. An example of how carbon is transferred is the burning of fossil fuels from the geosphere, which releases carbon dioxide and methane into the atmosphere. Fossil fuels are the source, and the atmosphere is the sink. 361

The Role of Photosynthesis and Cellular Respiration Photosynthesis and cellular respiration are important components of the carbon cycle. These two processes transfer carbon as carbon dioxide from the atmosphere during photosynthesis, to the biosphere as starches and sugars. Cellular respiration returns the carbon that was stored in starches and sugars to the atmosphere as carbon dioxide. Why is it necessary for this cycle to occur if it is simply transferring carbon back and forth? Photosynthesis not only transfers carbon, but also captures and stores solar energy that is required by living organisms to function. This energy is released during cellular respiration. A healthy balance between photosynthesis and cellular respiration results in a relatively stable amount of carbon dioxide in the atmosphere, which is capable of trapping the heat radiated from the Earth s surface, and therefore maintaining a relatively balanced environment. The carbon dioxide concentration in the atmosphere rises and falls slightly throughout the year due to changes in the rate of photosynthesis during different seasons. Global Warming and Human Health As a result of changes in land use, such as deforestation and burning of fossil fuels, the amount of carbon dioxide in the atmosphere has been steadily rising since the late 1950s. Scientists and researchers predict that this increase will trap more heat, raising temperatures, and impact the environment and organisms that live there. This increase in temperature has been called global warming, and is intensely debated. Currently, researchers are using computer and mathematical modeling of the carbon cycle under different scenarios to build relationships and predict future outcomes of global warming on our environment, and on our health. Climate and weather have a significant role in human health. While the impacts of climate change are dependent on many factors, the following weather-related risks to health are predicted to increase with the onset of global warming: Heat-related illness and death due to longer and more frequent heat waves Trauma-related injury and death due to increase in severity and frequency of extreme weather events (storms, high winds, floods, etc.) Illness and death due to an increase in the concentration of air and water pollutants Increase of food-borne, water-borne, and animal-borne diseases due to changes in precipitation patterns and temperature. Review Questions answer questions on a separate sheet of paper USGCRP (2009). Projection of the number of 100 days per year. 1. Why is carbon important to life on Earth? 2. Where is carbon found in the biosphere? The atmosphere? The hydrosphere? The geosphere? 3. What is the difference between a carbon source and sink? Provide an example. 4. How do photosynthesis and cellular respiration impact the carbon cycle? 5. Why does the amount of carbon dioxide in the atmosphere fluctuate? 6. What causes global warming? Give two examples of how global warming could impact human health. 7. Using the USGCRP map above, determine how many 100 days could occur in the lower emission scenario in 2080-2099 where you live? How many 100 days in the higher emission scenario? 362

HASPI Medical Biology Lab 13 Background Information Models (e.g., physical, mathematical, computer models) can be used to simulate systems and interactions including energy, matter, and information flows within and between systems at different scales. A large variety of these models are available online for education, simulations, and real-time data. In this activity, you will use Internet-based models to learn more about the carbon cycle and its impact on our lives. Materials Computer/Internet Directions Graph paper/graphing software You will be given tasks, or directions, to perform on the left. Record your questions, observations, or required response to each task on the right. Part A. Learning About the Carbon Cycle Task Response 1 2 3 4 5 6 Go to the following website: http://sepuplhs.org/high/sgi/teachers/carbon_sim.html This website, created by the Science Education for Public Understanding Program (SEPUP), will provide an integrated animation and simulation of the carbon cycle. Click the of the page. Click on the by the button at the bottom button, followed on the left-hand side of the page. Click the button. The image provides an interactive visual of the carbon cycle through the Pre-Industrial Era. The star symbols represent carbon reservoirs, while the flashing arrows indicate how carbon is moved between carbon reservoirs. Click on each carbon reservoir to learn more about it, and use the information provided to complete the questions in Table 1. Click on the information box to close it. Click on each of the arrows to learn more about how carbon moves between reservoirs, and use the information provided to complete the questions in Table 2. Click on the information box to close it. 363

Task Image 7 8 9 10 Click the button at the bottom of the page. This button will not become available until all of the stars and arrows on the Pre-Industrial Era image have been opened. The image provides an interactive visual of the carbon cycle through the Post-Industrial Era. The star symbols represent carbon reservoirs, while the flashing arrows indicate how carbon is moved between carbon reservoirs. Click on each carbon reservoir to learn more about it, and use the information provided to complete the questions in Table 3. Click on the information box to close it. Click on each of the arrows to learn more about how carbon moves between reservoirs, and use the information provided to complete the questions in Table 4. Click on the information box to close it. 11 Create a line or bar graph comparing the Gigatons (Gt) of carbon stored in each reservoir between the Pre-Industrial and Post-Industrial Eras. Use the graph to answer the analysis questions below. 12 Click on the button at the bottom of the page to move on to the simulation. You must go through all of the stars and arrows before the button will become active. Part A Analysis Questions answer questions on a separate sheet of paper 1. How has the carbon cycle changed between the Pre-Industrial and Post-Industrial eras? 2. In which reservoir was the most carbon stored in the Pre-Industrial era? 3. In which reservoir was the least carbon stored in the Pre-Industrial era? 4. In which reservoir is the most carbon stored in the Post-Industrial era? 5. In which reservoir is the least carbon stored in the Post-Industrial era? 6. What is the difference between the source and sink in this activity? 7. Between which source and sink was the most carbon exchanged in the Pre-Industrial era? 8. Between which source and sink is the most carbon exchanged in the Post-Industrial era? 364

Table 1. Pre-Industrial Era Carbon Reservoirs Carbon Reservoir Average Amount of Carbon in Gigatons (Gt) Description Rocks Soil & Detritus Land Plants Fossil Fuels Ocean Biomass Ocean Waters Table 2. Pre-Industrial Era Carbon Cycling Carbon Cycle Average Amount of Carbon in Gigatons (Gt) What is the Source and Sink? Description Soil & Detritus Plants Fossil Fuels Fires/Combustion Ocean Biomass Ocean Waters 365

Table 3. Post-Industrial Era Carbon Reservoirs Carbon Reservoir Average Amount of Carbon in Gigatons (Gt) Description Rocks Soil & Detritus N/A Land Plants Fossil Fuels N/A Ocean Biomass N/A Ocean Waters N/A Table 4. Post-Industrial Era Carbon Cycling Carbon Cycle Average Amount of Carbon in Gigatons (Gt) What is the Source and Sink? Description Soil & Detritus Plants Fossil Fuels Fires/Combustion Ocean Biomass Ocean Waters 366

Part B. Simulating the Carbon Cycle 1 Remain on the SEPUP website. AFTER learning about carbon reservoirs and cycles in the Pre- and Post-Industrial Eras, the button will take you to the simulation. Pre-Industrial Era Simulation 2 The simulation will start in the Pre-Industrial Era. Click the button. 3 On average, 600 Gt of carbon was stored in the atmosphere, 610 Gt of carbon was stored in the biosphere, and 38,000 Gt of carbon was stored in the hydrosphere (oceans) per year during the Pre-Industrial Era. 4 The Simulation Controls represent the carbon added to the atmosphere through respiration, and removed from the atmosphere through photosynthesis, in Gigatons (Gt) per year. When you click the button, the simulator will adjust the amount of carbon in 5 the atmosphere, biosphere, and hydrosphere each year according to how much is added and removed through respiration and photosynthesis over a 100-year period. Simulation A Normal Respiration and Photosynthesis Carbon Levels 6 Leave Plant Respiration at 60 Gt and Photosynthesis at 120 Gt. 7 Hypothesize how the amount of carbon in the atmosphere, biosphere, and hydrosphere will change throughout the simulation in Table 5. Click the button. The simulation will begin. Pause the simulator using the pause button on the bottom right at Year 25. Record the amount of carbon in the 8 atmosphere, biosphere, and hydrosphere in Table 6. NOTE: A blank space for in the simulation indicates that the amount of photosynthesis and respiration is balanced. Record this as 0 in the Table. 9 Start the simulation again with the play button. Pause and record the amount of carbon at Year 50 and Year 75 in Table 6. Allow the simulation to run to Year 100. Record the amount of carbon in the atmosphere, 10 biosphere, and hydrosphere in Table 6. Document how the carbon levels actually changed in Table 5, and determine whether your hypothesis was correct. 11 Once the data has been recorded, click the button. Simulation B Increased Respiration For Simulation B, we are going to increase the amount of carbon added to the 12 atmosphere through respiration. Set Plant Respiration to 60.5 and leave Photosynthesis at 120. Hypothesize how this will impact carbon levels in Table 5. Click the button. The simulation will begin. Pause the simulator using the 13 pause button on the bottom right at Year 25. Record the amount of carbon in the atmosphere, biosphere, and hydrosphere in Table 6. Start the simulation again with the play button. Pause and record the amount of carbon 14 at Year 50 and Year 75 in Table 6. Allow the simulation to run to Year 100. Record the amount of carbon in the atmosphere, 15 biosphere, and hydrosphere in Table 6. Document how the carbon levels actually changed in Table 5, and determine whether your hypothesis was correct. 16 Once the data has been recorded, click the button. 367

17 18 19 20 Simulation C Decreased Photosynthesis For Simulation C, we are going to decrease the amount of carbon removed from the atmosphere through photosynthesis. Leave Plant Respiration at 60, and set Photosynthesis to 120.5. Hypothesize how this will impact carbon levels in Table 5. Click the button. The simulation will begin. Pause the simulator using the pause button on the bottom right at Year 25. Record the amount of carbon in the atmosphere, biosphere, and hydrosphere in Table 6. Start the simulation again with the play button. Pause and record the amount of carbon at Year 50 and Year 75 in Table 6. Allow the simulation to run to Year 100. Record the amount of carbon in the atmosphere, biosphere, and hydrosphere in Table 6. Document how the carbon levels actually changed in Table 5, and determine whether your hypothesis was correct. 21 Once the data has been recorded, click the button. A graph will appear with the carbon levels from the simulation. 22 Click the button at the bottom of the graph. Post-Industrial Era Simulation 1 The simulation now focuses on the Post-Industrial Era. On average, 720 Gt of carbon is stored in the atmosphere, 60 Gt of carbon is stored in the 2 biosphere, and 38,000 Gt of carbon is stored in the hydrosphere (oceans) per year in the Post- Industrial Era. The Simulation Controls represent the carbon added to the atmosphere through respiration and removed from the atmosphere through 3 photosynthesis in Gigatons (Gt) per year. The Post-Industrial simulation also accounts for carbon added to the atmosphere through fossil fuel burning and changes in land use. Simulation D Current Carbon Levels Released Into the 4 Leave Plant Respiration at 60 Gt, Fossil Fuel Burning at 6 Gt, Land Use Change at 2 Gt, and Photosynthesis at 120 Gt. 5 Hypothesize how the amount of carbon in the atmosphere, biosphere, and hydrosphere will change throughout the simulation in Table 5. Click the button. The simulation will begin. Pause the simulator using the 6 pause button on the bottom right at Year 25. Record the amount of carbon in the atmosphere, biosphere, and hydrosphere in Table 7. 7 Start the simulation again with the play button. Pause and record the amount of carbon at Year 50 and Year 75 in Table 7. Allow the simulation to run to Year 100. Record the amount of carbon in the atmosphere, 8 biosphere, and hydrosphere in Table 7. Document how the carbon levels actually changed in Table 5, and determine whether your hypothesis was correct. 9 Once the data has been recorded, click the button. 368

Simulation E Increased Carbon Released Into the What if we increased the amount of carbon released into the atmosphere by fossil fuel burning and land use changes slightly? Increase Fossil Fuel Burning to 7 Gt and Land 10 Use Change to 3 Gt. Leave Plant Respiration at 60 Gt, and Photosynthesis at 120 Gt. Hypothesize how the amount of carbon in the atmosphere, biosphere, and hydrosphere 11 will change throughout the simulation in Table 5. Click the button. The simulation will begin. Pause the simulator using the 12 pause button on the bottom right at Year 25. Record the amount of carbon in the atmosphere, biosphere, and hydrosphere in Table 7. Start the simulation again with the play button. Pause and record the amount of carbon 13 at Year 50 and Year 75 in Table 7. Allow the simulation to run to Year 100. Record the amount of carbon in the atmosphere, 14 biosphere, and hydrosphere in Table 7. Document how the carbon levels actually changed in Table 5, and determine whether your hypothesis was correct. 15 Once the data has been recorded, click the button. Simulation F Decreased Carbon Released Into the What if we decreased the amount of carbon released into the atmosphere by fossil fuel 16 burning and land use change by half? Decrease Fossil Fuel Burning to 3 Gt and Land Use Change to 1 Gt. Leave Plant Respiration at 60 Gt and Photosynthesis at 120 Gt. Hypothesize how the amount of carbon in the atmosphere, biosphere, and hydrosphere 17 will change throughout the simulation in Table 5. Click the button. The simulation will begin. Pause the simulator using the 18 pause button on the bottom right at Year 25. Record the amount of carbon in the atmosphere, biosphere, and hydrosphere in Table 7. Start the simulation again with the play button. Pause and record the amount of carbon 19 at Year 50 and Year 75 in Table 7. Allow the simulation to run to Year 100. Record the amount of carbon in the atmosphere, 20 biosphere, and hydrosphere in Table 7. Document how the carbon levels actually changed in Table 5, and determine whether your hypothesis was correct. Create a line or bar graph comparing and displaying the carbon levels in the 21 atmosphere, biosphere, and hydrosphere for simulations A-F using the data collected in Tables 6 and 7. Part B Analysis Questions answer questions on a separate sheet of paper Pre-Industrial Era Simulation 1. How did the amount of carbon exchanged through photosynthesis and cellular respiration impact the amount of carbon stored in the atmosphere over a 100-year period? The biosphere? The hydrosphere? 2. How did small changes in the amount of carbon put into the atmosphere by cellular respiration, and removed from the atmosphere by photosynthesis, impact the amount of carbon in the atmosphere? The biosphere? The hydrosphere? Post-Industrial Era Simulation 3. How did the amount of carbon exchanged through photosynthesis and cellular respiration impact the amount of carbon stored in the atmosphere over a 100-year period? The biosphere? The hydrosphere? 4. How did small changes in the amount of carbon put into the atmosphere by cellular respiration, and removed from the atmosphere by photosynthesis, impact the amount of carbon in the atmosphere? The biosphere? The hydrosphere? 369

Table 5. Hypothesizing the Impact of Respiration and Photosynthesis Simulation A B C D E F Hypothesize how the amount of carbon in the atmosphere, biosphere, and hydrosphere will change over 100 years. How did the amounts of carbon in the atmosphere, biosphere, and hydrosphere actually change over 100 years? Table 6. Pre-Industrial Era Carbon Reservoir Simulation Year 25 Year 50 Year 75 Year 100 Simulation Respiration & Photosynthesis A Respiration 60 Photosynthesis 120 B Respiration 60.5 Photosynthesis 120 C Respiration 60 Photosynthesis 120.5 Table 7. Post-Industrial Era Carbon Reservoir Simulation Year 25 Year 50 Year 75 Year 100 Simulation Respiration & Photosynthesis D E F 370 Respiration 60 Fossil Fuel Burning 6 Land Use Change 2 Photosynthesis 120 Respiration 60 Fossil Fuel Burning 7 Land Use Change 3 Photosynthesis 120 Respiration 60 Fossil Fuel Burning 3 Land Use Change 1 Photosynthesis 120

Name(s): Period: Part C. Investigating the Carbon Cycle Image Task 1 Date: Go to the following website: http://www.esrl.noaa.gov/gmd/dv/iadv/ The Earth System Research Laboratory Global Monitoring Division collects atmospheric carbon dioxide, carbon 2 monoxide, methane, and a variety of other gaseous samples throughout the world, and ranging from 1969 present in select locations. The map contains sampling locations. Red circles are sites actively collecting data, 3 yellow circles are sites that are inactive, and blue boxes are main collecting sites. Choose the site from the drop-down menu at the top of the map, or click the site dot on the actual 4 map. For now, choose the site as Mauna Loa, Hawaii. 5 6 Once the site has been chosen, it will appear in blue in the column to the left of the map. Click on the menu and select drop-down as the plot type. A new page will appear that will create custom graphs from the data collected at the selected location. From the Options box, choose: Carbon Dioxide (CO2) as the Parameter Flask Samples as the Data Type Discrete as the Data Frequency 8 Some - a subset of the available data with the Start Year as 1980 and the End Year as 1981 as the Time Span. Click on the button at the bottom of the Options box. A plot graph will appear with atmospheric carbon 9 dioxide levels from each flask collected from JanuaryDecember in 1980 in Mauna Loa, Hawaii. 7 371

The blue dots ( ) indicate data that has been confirmed, orange dots ( ) high are data that is preliminary, and the green + symbols 10 indicated questionable data. For this activity, ignore the green + symbols, and only use the blue or orange dots as data points. Find and record in Table 8 the highest and lowest carbon dioxide levels for 1980. In this example for 11 Mauna Loa, Hawaii, the highest level is about 344 low and the lowest level is about 335. Change the Time Span to Start Year as 1985 and the End Year as 1986, and hit the submit 12 button to get a new plot graph with data for the selected years. 13 Find and record in Table 8 the highest and lowest carbon dioxide levels for 1985. Continue changing the Time Span, finding and recording in Table 8 the highest and lowest carbon dioxide levels for 1990, 1995, 2000, 2005, 2010, and the current year. If you 14 come across a location that does not have data for that year, record N/A for not available. Change the Parameter to Methane (CH4), and return the Time Span Start Year to 1980 15 and End Year to 1981. Record the highest and lowest methane levels in Mauna Loa, Hawaii for 1980 in Table 9. Continue changing the Time Span, finding and recording in Table 9 the highest and lowest carbon dioxide levels for 1985, 1990, 1995, 2000, 2005, 2010, and the current year. If 16 you come across a location that does not have data for that year, record N/A for not available. Return to the map page, and change the site to Barrow, Alaska. Repeat steps 5 16 for 17 Barrow, Alaska. Return to the map page, and change the site to Tutuila, American Samoa. Repeat steps 18 5 16 for Tutuila, American Samoa. Return to the map page, and change the site to South Pole, Antarctica. Repeat steps 5 19 16 for South Pole, Antarctica. Return to the map page, and change the site to Summit, Greenland. Repeat steps 5 16 20 for Summit, Greenland. Return to the map page, and change the site to Trinidad Head, California. Repeat steps 21 5 16 for Trinidad Head, California. Return to the map page, and CHOOSE any other site from the map. Record the name of 22 the site on the last row of Tables 8 and 9. Repeat steps 5 16 for the chosen site. Create a plot, line, or bar graph summarizing the data in Table 8, and a separate graph 23 summarizing the data in Table 9. Answer the analysis questions based on your graphs. 372

Table 8. Carbon Dioxide (CO2) Levels (μmol mol -1 ) Location 1980 1985 1990 1995 2000 2005 2010 Current Mauna Loa, Hawaii 344 335 Barrow, Alaska Tutuila, American Samoa South Pole, Antarctica Summit, Greenland Trinidad Head, California N/A N/A N/A N/A N/A N/A N/A N/A N/A Table 9. Methane (CH4) Levels (nmol mol -1 ) Location 1980 1985 1990 1995 2000 2005 2010 Current Mauna Loa, Hawaii Barrow, Alaska Tutuila, American Samoa South Pole, Antarctica Summit, Greenland Trinidad Head, California N/A N/A N/A N/A N/A N/A N/A N/A N/A Part C Analysis Questions answer questions on a separate sheet of paper 1. Hypothesize why it is important to collect samples of atmospheric gases worldwide, instead of from only a few locations. 2. Why is it important to compare collected data from year-to-year? 3. Do you think the Global Monitoring Division should be continued? Why or why not? 4. According to your graphs, have atmospheric carbon dioxide levels decreased, increased, or remained the same over the last 30 years? Explain your answer and hypothesize what may be affecting atmospheric carbon dioxide levels. 5. According to your graphs, have atmospheric methane levels decreased, increased, or remained the same over the last 30 years? Explain your answer and hypothesize what may be affecting atmospheric methane levels. 373

Part D. The Carbon Cycle and Health Question Response Use an Internet search engine to research and answer the questions in the space provided. Explain how increased or decreased carbon 1 dioxide in the environment can impact human health. Explain how increased or decreased carbon 2 dioxide in the environment can impact plant health. Why is it important for there to be a 3 balance between plant health and human health? What is black carbon, and how 4 can it impact human health? 5 6 7 How can increased methane in the environment impact human health? What is carbon monoxide poisoning? What are the causes of carbon monoxide poisoning? Will climate change and global warming impact human health? Explain your answer. 374

Connections & Applications Your instructor may assign or allow you to choose any of the following activities. As per NGSS/CCSS, these extensions allow students to explore outside activities recommended by the standards. 1. MODELING THE CARBON CYCLE: Develop a model to illustrate the role of photosynthesis and cellular respiration in the cycling of carbon among the biosphere, atmosphere, hydrosphere, and geosphere. The model can be computer-generated, illustrated, or a physical model (clay, craft items, food items, etc.) Design a plan, and check with your instructor on resources before constructing your model. 2. BURIAL OR CREMATION? Death is not only a part of the life cycle, but integral to the continuation of all life on Earth. In the natural world, once living organisms die, decomposers break down the body and allow matter to be recycled. In terms of carbon, decomposition allows carbon that would be trapped to be released and reenter the carbon cycle. Some human burial methods prevent decomposition, and therefore prevent carbon from returning to the carbon cycle. An average of 1.8 million burials per year occur in the United States alone. a. Research human burial and cremation methods. b. Determine how human burial and cremation impact the environment, specifically the carbon cycle. c. Construct an oral or written argument that aims to convince your audience of whether burial or cremation should be the preferred method of disposing of human remains. Cite your references. i. Oral argument should be 4-5 minutes in length ii. Written argument should be a minimum of 5 paragraphs (4 sentence min) 3. RESEARCH THE HEALTH IMPACT: Carbon dioxide (CO2) and carbon monoxide (CO) differ by a single oxygen molecule, and yet an environment with a 0.0035% concentration level of CO can produce symptoms of carbon monoxide poisoning. Research and answer the following questions. a. A description of carbon monoxide poisoning (how specifically does carbon monoxide poison the body?) b. Symptoms of carbon monoxide poisoning c. Sources of carbon monoxide poisoning in the home d. Sources of carbon monoxide poisoning in the environment e. Prevalence: How many individuals are poisoned yearly by carbon monoxide f. Cite at least 2 sources. Following each source, assess the accuracy and credibility of the source by determining who endorses the site and who monitors information placed on or within the source 375

Resources & References EPA. 2013. Climate Impacts on Human Health. Unites States Environmental Protection Agency, http://www.epa.gov/climatechange/impacts-adaptation/health.html. NOAA. 2014. Earth System Research Laboratory, Global Monitoring Division. U.S. Department of Commerce, National Oceanic & Atmospheric Administration, http://www.esrl.noaa.gov/gmd/dv/iadv/. SEPUP. 2012. The Carbon Cycle. Science Education for Public Understanding Program, Lawrence Hall of Science, University of California, Berkeley, http://sepuplhs.org/high/sgi/teachers/carbon_sim.html. Thompson, J.N. 2013. The Carbon Cycle., Encyclopedia Britannica, www.britannica.com. 376