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The Carbon Cycle for High School
Photosynthesis, respiration, combustion, ocean exchange
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Carbon moves continuously among the atmosphere, oceans, land, living organisms, rocks, and fossil fuels. This movement is called the carbon cycle, and it helps regulate Earth’s climate, ocean chemistry, and the growth of ecosystems. High school environmental science uses the carbon cycle to connect biology, chemistry, geology, and human activity. Understanding it helps explain why rising carbon dioxide levels are linked to climate change.
Carbon changes reservoirs through processes such as photosynthesis, cellular respiration, decomposition, combustion, ocean exchange, sedimentation, and rock weathering. Some carbon fluxes happen quickly, such as plants taking in CO2 during photosynthesis, while others take thousands to millions of years, such as carbon becoming limestone or fossil fuels. Human activities add carbon to the atmosphere mainly by burning fossil fuels and changing land use. The key idea is conservation of matter: carbon atoms are not created or destroyed, but they can move into forms and places that strongly affect Earth systems.
Key Facts
- Photosynthesis removes carbon dioxide from the atmosphere: 6CO2 + 6H2O + light energy = C6H12O6 + 6O2.
- Cellular respiration returns carbon dioxide to the atmosphere: C6H12O6 + 6O2 = 6CO2 + 6H2O + energy.
- Combustion of fossil fuels releases stored carbon: hydrocarbon + O2 = CO2 + H2O + energy.
- A carbon reservoir is a place where carbon is stored, such as the atmosphere, ocean, biomass, soil, rocks, or fossil fuels.
- A carbon flux is the rate of carbon movement between reservoirs, often measured in gigatons of carbon per year, GtC/yr.
- If carbon inputs to the atmosphere are greater than carbon outputs, atmospheric CO2 increases: change in atmospheric carbon = inputs minus outputs.
Vocabulary
- Carbon reservoir
- A carbon reservoir is any part of Earth that stores carbon for a period of time, such as forests, oceans, soils, rocks, or the atmosphere.
- Carbon flux
- A carbon flux is the movement of carbon from one reservoir to another over a specific amount of time.
- Photosynthesis
- Photosynthesis is the process in which plants, algae, and some bacteria use light energy to convert carbon dioxide and water into sugars and oxygen.
- Respiration
- Respiration is the process in which organisms break down sugars to release energy and produce carbon dioxide and water.
- Combustion
- Combustion is the burning of carbon-containing fuels that releases energy, carbon dioxide, and water vapor.
Common Mistakes to Avoid
- Thinking carbon disappears when organisms die is wrong because decomposition transfers carbon to the soil, atmosphere, water, or sediments instead of destroying it.
- Confusing reservoirs with fluxes is wrong because reservoirs store carbon while fluxes describe carbon moving between reservoirs.
- Treating all carbon cycle processes as equally fast is wrong because biological exchanges can happen in days to years, while rock formation and fossil fuel creation can take millions of years.
- Assuming oceans only absorb carbon dioxide is wrong because oceans both absorb and release CO2 depending on temperature, circulation, biology, and the difference in CO2 concentration between air and water.
Practice Questions
- 1 A forest absorbs 1200 metric tons of carbon per year through photosynthesis and releases 850 metric tons of carbon per year through respiration and decomposition. What is the forest’s net carbon change per year?
- 2 A power plant burns fuel and releases 2.5 million metric tons of CO2 in one year. Carbon makes up about 12/44 of the mass of CO2. How many metric tons of carbon are released?
- 3 Explain why burning fossil fuels increases atmospheric carbon even though carbon is conserved in the carbon cycle. Include the idea of reservoirs and fluxes in your answer.