HS-ESS2-6NGSS*ScienceEarth and Space ScienceGrades 9-12
HS-ESS2-6: A Quantitative Model of the Carbon Cycle
In plain English: HS-ESS2-6 is the NGSS high school performance expectation that asks students to develop a quantitative model of how carbon cycles among the hydrosphere, atmosphere, geosphere and biosphere. Students track reservoirs in gigatonnes of carbon and fluxes per year, and use the balance of inflows and outflows to explain changes such as rising atmospheric CO2. It is usually taught in Earth and Space Science.
Develop a quantitative model to describe the cycling of carbon among the hydrosphere, atmosphere, geosphere, and biosphere.
Clarification Statement: Emphasis is on modeling biogeochemical cycles that include the cycling of carbon through the ocean, atmosphere, soil, and biosphere (including humans), providing the foundation for living organisms.
Next Generation Science Standards · Disciplinary Core Idea: Earth's Systems · Official standard
Students build a box model of the carbon cycle with numbers: each reservoir (the atmosphere, the ocean, plants, soils, fossil fuels and rocks) holds a stock of carbon measured in gigatonnes of carbon (GtC), and each process moves carbon at a rate in GtC per year. The rule that runs the model is a budget: the change in a reservoir each year equals what flows in minus what flows out. Students use that rule to explain why the atmosphere is gaining carbon, why only part of human emissions stays in the air, and how fast carbon moves through each sphere.
The NGSS Clarification Statement says: "Emphasis is on modeling biogeochemical cycles that include the cycling of carbon through the ocean, atmosphere, soil, and biosphere (including humans), providing the foundation for living organisms." There is no official Assessment Boundary for this performance expectation. All reservoir sizes and fluxes on this page are rounded estimates close to published global values, simplified so that the budget balances; real estimates carry uncertainties of several percent or more. Lab and scenario data are invented and labeled that way. One part per million (ppm) of CO2 in the atmosphere is taken as 2.12 GtC, and 1 GtC is 44/12 ≈ 3.67 Gt of CO2. Students do the practice of developing a model: they choose the reservoirs and fluxes, put numbers on them, test the model against data and revise it.
Learning Objectives
By the end of this lesson, students will be able to:
Identify carbon reservoirs in the hydrosphere, atmosphere, geosphere and biosphere, including soils and humans, and the processes that move carbon between them
Build a box model with reservoir sizes in GtC and fluxes in GtC per year, and use inflow minus outflow to find how each reservoir changes
Convert among ppm of CO2, gigatonnes of carbon and gigatonnes of CO2, and compute residence times and the airborne fraction
Run and revise the model for scenarios such as land clearing or reduced fossil fuel use, and explain which assumptions limit its predictions
Use lab evidence of carbon moving between rocks, water, air and living things to test parts of the model
Prior Knowledge Required
Students should already be comfortable with:
Cycling of matter and flow of energy among living and nonliving parts of an ecosystem MS-LS2-3
The cycling of Earth's materials and the energy that drives it MS-ESS2-1
Photosynthesis and cellular respiration as chemical reactions that take in or release CO2
Rates and unit conversions, including scientific notation 8.EE.A.4
Draw a bathtub on the board with a tap running and the drain open.
Warm-Up Prompt
"The tap adds 205 liters per minute and the drain removes 200 liters per minute. What happens to the water level? What would you need to change to keep the level steady? Now replace 'water' with 'carbon' and 'bathtub' with 'atmosphere'."
Students should see that the level rises by 5 liters each minute, and that a steady level needs inflow equal to outflow, not zero inflow. Record the words stock (how much is in the tub) and flux (how fast it moves): the model uses both.
Direct Instruction15-20 minutes
Reservoirs, fluxes and the budget rule. Show Diagram 1. Each box is a reservoir in one of the four spheres: the atmosphere; the hydrosphere (the ocean, which holds dissolved CO2, bicarbonate and marine life); the geosphere (fossil fuels, limestone and other rocks, and sediments); and the biosphere (plants, soils, animals and humans). Arrows are fluxes. The model follows one rule for each box: change per year = sum of inflows - sum of outflows. Carbon is conserved, so what one box loses another gains.
Converting between ppm, GtC and CO2
The atmosphere holds about 410 ppm of CO2. How much carbon is that, and how much CO2?
Result: 410 × 2.12 ≈ 869 GtC, so the model uses 870 GtC. As CO2: 870 × 44/12 ≈ 3,190 Gt. Carbon is 12/44 ≈ 27% of the mass of CO2.
Budget for the atmosphere in one year
Into the air: plant respiration 60, land clearing 1, decomposition 57, release from the ocean 77, fossil fuel burning 10. Out of the air: photosynthesis 120, dissolving into the ocean 80 (all GtC/yr).
Result: In = 205, out = 200, so the atmosphere gains 5 GtC per year, which is 5 ÷ 2.12 ≈ 2.4 ppm per year.
Where human emissions go
Humans add 11 GtC/yr (fossil fuels 10, land clearing 1). Natural land uptake is 120 - 60 - 57 = 3 GtC/yr and net ocean uptake is 80 - 77 = 3 GtC/yr.
Result: Atmosphere keeps 11 - 3 - 3 = 5 GtC/yr, about 45% of emissions (the airborne fraction). The ocean takes about 27% and the land about 27%.
Residence time in a reservoir
Residence time = reservoir size ÷ total outflow. The atmosphere holds 870 GtC with 200 GtC/yr leaving; soils hold 1,700 GtC with 57 GtC/yr leaving by decomposition.
Result: Atmosphere: 870 ÷ 200 ≈ 4.4 years for a typical carbon atom. Soils: 1,700 ÷ 57 ≈ 30 years. Rocks: tens of millions of GtC with fluxes of tenths of a GtC per year, so hundreds of millions of years.
Humans speed up a geosphere flux
Volcanoes release about 0.1 GtC/yr from the geosphere. Burning fossil fuels, which formed from buried plants and plankton, releases about 10 GtC/yr.
Result: 10 ÷ 0.1 = 100: humans move carbon from the geosphere to the atmosphere about 100 times faster than volcanoes do, while the slow return path (weathering and burial) stays near 0.2-0.3 GtC/yr.
After the examples, ask: "Which fluxes are large but nearly balanced, and which are small but one-way?" Photosynthesis and respiration move the most carbon but nearly cancel. Fossil fuel burning is smaller but has no matching return flux of similar size, so it controls the trend. This is why a quantitative model matters: a diagram without numbers cannot show it.
Guided Practice15 minutes
Pairs use the fluxes in Diagram 1 to finish the budget for the other reservoirs. Land clearing (1 GtC/yr) comes out of the plants box.
Guided practice: annual budget by reservoir (GtC per year)
Reservoir
Inflows
Outflows
Change per year
Atmosphere
205
200
+5 (done in the example)
Plants
photosynthesis 120
respiration 60, dead plants 58, clearing 1
?
Soils
dead plants 58
decomposition 57
?
Ocean
dissolving 80
release 77
?
Fossil fuels
none that matter on this timescale
burning 10
?
Fill in the last column for plants, soils, ocean and fossil fuels.
Add all five changes. Explain why the total must be zero, and check that it is.
Which sphere is losing carbon, and which three are gaining it? Name the human activity behind the loss.
Answers: plants +1, soils +1, ocean +3, fossil fuels -10; total 5 + 1 + 1 + 3 - 10 = 0. The geosphere loses carbon; the atmosphere, biosphere and hydrosphere gain it.
Independent Practice10-15 minutes
Each student revises the model for an invented scenario: land clearing doubles to 2 GtC/yr, and because fewer trees remain, photosynthesis drops to 118 GtC/yr. All other fluxes stay the same. Students recompute the atmosphere's inflows and outflows, the change per year in GtC and in ppm, and the new airborne fraction.
Answers: in = 60 + 2 + 57 + 77 + 10 = 206, out = 118 + 80 = 198, so the atmosphere gains 8 GtC/yr (about 3.8 ppm per year). Emissions are now 12 GtC/yr, so the airborne fraction rises to 8 ÷ 12 ≈ 67%. Ask students which assumption in the scenario is weakest (respiration and decomposition would also change when forest is lost).
Closure5 minutes
Exit ticket: "Name one reservoir in each of the four spheres, and write one flux that moves carbon out of each. Circle the flux that humans have changed the most." Look for all four spheres and for fossil fuel burning or land clearing circled.
Differentiation Strategies
For Struggling Students
Give the budget table with inflows and outflows already sorted, so students only add, subtract and compare
Use a physical model first: cups of dried beans as reservoirs, and move beans between cups each "year" by the flux amounts
Provide a conversion card: 1 ppm = 2.12 GtC, and 1 GtC = 3.67 Gt of CO2, with one solved example of each
For Advanced Students
Make ocean and land uptake in the Activity 2 spreadsheet depend on how far the atmosphere is above its pre-industrial 590 GtC, and find the level where it stops rising for a given emission rate
Split the ocean into surface and deep boxes, choose an exchange flux, and explain how it changes how fast the ocean takes up extra carbon
Compare the residence time of one carbon atom in the air (a few years) with how long an added pulse of carbon stays above its starting level, and explain why the two times differ
Assessment Guidance
What to Look For
Check that students apply inflow minus outflow to each reservoir and that their budgets conserve carbon (the changes add to zero). Look for correct units (GtC versus GtC per year, carbon versus CO2). Listen for the idea that large fluxes do not matter by size alone: the imbalance matters. Watch for students who leave out the geosphere or treat humans as outside the biosphere.
02
Classroom Activities
3 Activities
1
Limestone and Vinegar: Carbon Locked in the Geosphere
25 minGroups of 3-4
Groups measure how much carbon a sample of limestone (calcium carbonate, CaCO3) stores by dissolving it in vinegar and weighing the CO2 that escapes. The result connects a rock in the geosphere with carbon in the atmosphere and hydrosphere, and gives a real number to compare with the formula.
Procedure
Put on goggles. Measure 100 mL of household white vinegar into a plastic cup. Weigh 2.00 g of powdered limestone or crushed marble chips on a folded paper.
Place the cup of vinegar and the paper with the powder on the balance together and record the total mass.
Add the powder to the vinegar a little at a time so it does not foam over, and return the paper to the balance. Record the total mass every 2 minutes until it stops changing (about 15-20 minutes). Swirl gently.
The mass lost is the CO2 that escaped. Compare it with the prediction: CaCO3 is 100 g/mol and CO2 is 44 g/mol, so 2.00 g of pure CaCO3 can release 0.88 g of CO2.
Safety: wear goggles the whole time. Vinegar is a weak acid: rinse skin or eyes with plenty of water if it splashes, and do not taste anything. Never seal the cup, and do not mix the vinegar with any other chemical, especially bleach. No heating is needed. The finished mixture can be poured down the drain with running water, and spills wiped up at once.
Sample Data (invented)
Total mass at the start 104.36 g; after 20 minutes 103.54 g. Mass of CO2 lost 0.82 g, which is 93% of the 0.88 g predicted. Carbon in the escaped gas: 0.82 × 12/44 ≈ 0.22 g, from a sample that is 12% carbon by mass.
Discussion Questions
Give two reasons the measured CO2 could be less than predicted (some CO2 stays dissolved in the liquid; some powder may not react; the sample may not be pure CaCO3).
In the real carbon cycle, what plays the role of the vinegar? (Rainwater made slightly acidic by dissolved CO2.) Which spheres does the carbon move between?
Limestone forms mostly from the shells of ocean organisms. Trace one carbon atom from the air, through the ocean and a shell, into rock and back out, naming a sphere at each step.
Modification for Distance Learning
Students work from the sample data and a short teacher video of the reaction, compute the carbon released, and post a four-sphere path for one carbon atom.
2
Spreadsheet Box Model of the Carbon Cycle
20 minPairs
Pairs turn Diagram 1 into a spreadsheet that runs year by year. In the first version the ocean and land take up a fixed 3 GtC/yr each. In the revised version, uptake depends on how much extra carbon is in the air, which is closer to how the real sinks behave.
Model Rules
Start the atmosphere at 870 GtC. Emissions from fossil fuels and land clearing: 11 GtC/yr.
Version 1: next year = this year + 11 - 6.
Version 2: total uptake by ocean and land = (6/280) × (atmosphere - 590), where 590 GtC is the pre-industrial atmosphere. At 870 GtC this gives 6 GtC/yr, matching Version 1 in year 1.
Run 20 years and convert each year to ppm by dividing by 2.12.
Sample Output (Version 2, from these rules)
Atmosphere 870.0, 875.0, 879.9, 884.7, 889.4 GtC in years 0-4, and 952.0 GtC (about 449 ppm) in year 20. If emissions are set to 6 GtC/yr, the atmosphere stays at 870 GtC, because inflow then equals outflow.
Discussion Questions
In Version 2, the yearly gain shrinks from 5.0 to about 3.3 GtC over 20 years. Which part of the rule causes this?
What emission rate would hold the atmosphere steady at 870 GtC in Version 2? Why is that not zero?
Version 2 keeps growing toward a level where uptake equals 11 GtC/yr. Find that level and say why the real ocean might not follow this rule so far above today.
Challenge Variation
Split uptake into ocean and land, and make the land part fall by 10% for every 100 GtC above 870 to stand for warming soils releasing more carbon (an assumption for exploring, not a measured value). Compare year 20 with Version 2.
3
Bromothymol Blue Lab: Plants, Water and Air
25 minGroups of 3-4
Bromothymol blue (BTB) is an indicator that turns from blue to yellow as dissolved CO2 makes water more acidic, and back toward blue as CO2 is removed. Groups add CO2 to BTB water with their breath, then use an aquatic plant, Elodea, to test whether living things move carbon between the hydrosphere and the biosphere in light and in dark.
Procedure
Put on goggles. Fill a beaker with 100 mL of dilute BTB solution. One student gently blows out through a clean straw into the solution until it just turns yellow-green. Blow out only, never suck in, and use your own straw; stop if you feel dizzy.
Fill four test tubes with the yellow-green solution. Tubes 1 and 2 get a 6 cm sprig of Elodea; tubes 3 and 4 get none. Stopper all four.
Wrap tubes 2 and 4 in aluminum foil. Place all four about 30 cm from a lamp for 40 minutes, then record the color (and pH if you have pH paper or a probe).
Safety: wear goggles and wash hands after handling the solution and plants. Do not drink the solution or put it near your mouth except through your own straw. Keep the lamp away from water and do not touch the hot bulb. Return plants to the class aquarium, not to a local pond or stream.
Sample Data (invented)
Starting pH 6.2 in all tubes. After 40 minutes: tube 1 (plant, light) blue, pH 7.3; tube 2 (plant, dark) yellow, pH 6.0; tube 3 (no plant, light) yellow-green, pH 6.2; tube 4 (no plant, dark) yellow-green, pH 6.2.
Discussion Questions
Which tube shows carbon moving from the hydrosphere into the biosphere? Which process did it?
Tube 2 became more acidic than at the start. Which process added CO2 to the water in the dark, and which sphere did the carbon come from?
Why are tubes 3 and 4 needed? What would you conclude if tube 3 had also turned blue?
How would you turn this color test into a number that could go into the box model?
Modification for Distance Learning
Students analyze the sample data and a photo of the four tubes, then add the two plant fluxes to their own sketch of Diagram 1 with arrows pointing the right way.
03
Diagrams & Visual Aids
2 diagrams
Diagram 1: Box Model of the Carbon Cycle Across Four Spheres
Reservoirs in gigatonnes of carbon (GtC) and fast fluxes in GtC per year, rounded so that the budget balances. The ocean total includes about 900 GtC near the surface and about 37,000 GtC in deep water. Fossil fuels are the part of the geosphere that humans are moving into the air.
Diagram 2: Two Model Runs for the Atmosphere
Version 1 of the box model, drawn to scale, with ocean and land uptake held at 6 GtC per year in total. Run A keeps emissions at 11 GtC/yr and gains 5 GtC per year; run B cuts fossil fuel emissions to 7 GtC/yr and gains 2 GtC per year. Holding the sinks fixed is an assumption that students test in Activity 2.
04
Homework Assignment
~30 min
HS-ESS2-6 Homework: Building and Using a Carbon Cycle Model
Directions: Use 1 ppm of CO2 = 2.12 GtC and 1 GtC = 44/12 Gt of CO2. All data are invented or rounded. For every problem, show the calculation with units and name the spheres the carbon moves between.
Part 1: Building the Budget (Problems 1-2)
An invented year has these fluxes in GtC/yr: fossil fuel burning 9, land clearing 2, photosynthesis 121, plant respiration 61, decomposition 58, air to ocean 79, ocean to air 76. Treat the land (plants and soils together) as one box. (a) Find the change per year for the atmosphere, the land, the ocean and fossil fuels. (b) Show that the changes add to zero. (c) Which reservoir gains the most?
Burning one gallon of gasoline releases about 8.9 kg of CO2. (a) How many kilograms of carbon is that? (b) A car burns 500 gallons a year. How much carbon does it add to the atmosphere each year? (c) About how many such cars would it take to add 1 GtC (1012 kg) per year?
Part 2: The Four Spheres (Problems 3-4)
The surface ocean holds about 900 GtC and returns 77 GtC/yr to the air. The soils hold about 1,700 GtC and lose 57 GtC/yr by decomposition. (a) Find the residence time of carbon in the surface ocean with respect to release to the air. (b) Compare it with the residence time in soils. (c) Which of the two reservoirs could respond faster to a change in the atmosphere, and why?
Invented monthly readings at a Northern Hemisphere station show CO2 at 419.0 ppm in May and 412.6 ppm in September of the same year. (a) How many GtC left the atmosphere between May and September? (b) Which sphere took it up, and by which process? (c) Explain why the readings rise again from October to May.
Part 3: Using and Revising the Model (Problems 5-6)
An invented scenario clears 1 million km² of tropical forest that stores 150 tonnes of carbon per hectare (1 km² = 100 hectares). (a) How many GtC does the forest store? (b) If half is released to the atmosphere by burning within a year, how many ppm does that add? (c) Where does the other half go, and over what timescale might it reach the atmosphere?
A student's model has only two boxes, the atmosphere and fossil fuels, with emissions of 11 GtC/yr. (a) Predict the rise in atmospheric CO2 over 10 years in ppm with this model. (b) Predict it with the full model from class, where the atmosphere keeps 5 GtC/yr. (c) Which spheres did the student leave out, and which fluxes did that remove? (d) Write one sentence of feedback that would help the student revise the model.
Rubric
Criterion
Full Credit (2 pts)
Partial Credit (1 pt)
Budget Rule
Applies inflow minus outflow to every reservoir and shows that carbon is conserved
Correct method for most reservoirs, or conservation not checked
Units and Conversions
Correct conversions among ppm, GtC and CO2, with units on every quantity
One conversion or unit error
Four Spheres
Names the sphere and the process for every flux used
Spheres named, processes missing or vague
Model Revision
Identifies a missing part or weak assumption and explains how fixing it changes the prediction
Names a weakness without saying how it affects the result
05
Quiz: 20 Questions
Interactive, with answers
Instructions
Use 1 ppm of CO2 = 2.12 GtC and 1 GtC = 44/12 Gt of CO2. All data are invented or rounded. Your score updates as you answer, and Reset quiz clears everything so you or your students can try again.
Multiple choice: pick an option to check it. Short answer: write your answer, then reveal the model answer.
0 of 20 answered · 0 correct
Question 1 of 20 · Multiple Choice
Which reservoir in Diagram 1 stores the most carbon?
Answer: D
Carbonate rocks such as limestone and carbon in sediments hold tens of millions of GtC, far more than any other reservoir. Choice C, soils, is the largest land reservoir (1,700 GtC), but it is tiny next to the geosphere. Choice A holds only 870 GtC.
Question 2 of 20 · Multiple Choice
One year the atmosphere gains 4.0 GtC. About how much does its CO2 concentration rise?
Answer: B
4.0 ÷ 2.12 ≈ 1.9 ppm. Choice A multiplies by 2.12 instead of dividing. Choice C treats one GtC as one ppm, and choice D divides 2.12 by 4.0.
Question 3 of 20 · Multiple Choice
Photosynthesis on land moves carbon between which two spheres?
Answer: A
Plants take CO2 from the air and build it into sugars and tissue, so carbon moves from the atmosphere to the biosphere. Choice B describes respiration and decomposition, the reverse flux.
Question 4 of 20 · Multiple Choice
A reservoir holds 600 GtC. It receives 50 GtC/yr and loses 48 GtC/yr. If the fluxes stay the same, how much will it hold after 10 years?
Answer: A
The net change is 50 - 48 = +2 GtC/yr, so after 10 years it holds 600 + 20 = 620 GtC. Choice C adds only one year of change. Choice D subtracts the net change instead of adding it, and choice B adds the two fluxes instead of taking their difference (600 + 10 × 98).
Question 5 of 20 · Multiple Choice
A reef coral builds its skeleton from calcium carbonate, using carbon dissolved in seawater. Which move of carbon does this describe?
Answer: B
The carbon starts as dissolved bicarbonate in the ocean (hydrosphere) and becomes part of a living coral (biosphere). Choice A describes a later step, when dead reef material is buried and becomes limestone. Choice D describes CO2 dissolving into the ocean, which happens before the coral uses it.
Question 6 of 20 · Multiple Choice
A group of countries reports combined emissions of 22 Gt of CO2 in a year. How much carbon is that?
Answer: D
Carbon is 12/44 of the mass of CO2: 22 × 12/44 = 6.0 GtC. Choice B multiplies by 44/12 instead. Choice A ignores the difference between CO2 and carbon. Choice C divides 22 by 12.
Question 7 of 20 · Multiple Choice
Warmer water can hold less dissolved CO2. If the surface ocean warms while everything else stays the same, what does the model predict?
Answer: B
Less CO2 dissolves and more is released, so the ocean's net uptake falls and more of each year's emissions stay in the air. Choice A reverses the effect. Choice D confuses the size of a reservoir with the balance of its fluxes.
Question 8 of 20 · Multiple Choice
In one year, emissions are 12 GtC and the atmosphere gains 4.8 GtC. What is the airborne fraction?
Answer: A
Airborne fraction = 4.8 ÷ 12 = 0.40, or 40%. Choice B is the fraction taken up by the ocean and land (7.2 ÷ 12). Choices C and D copy a number from the question without dividing.
Question 9 of 20 · Multiple Choice
Which process removes carbon from the atmosphere and stores it in the geosphere for millions of years?
Answer: D
Rain made acidic by CO2 weathers rock; rivers carry the dissolved carbon to the sea, where organisms build carbonate shells that are buried as sediment and become rock. Choices A, B and C all return carbon to the air within years.
Question 10 of 20 · Multiple Choice
Soils hold 1,500 GtC and lose 60 GtC/yr by decomposition. What is the residence time of carbon in the soils?
Answer: B
Residence time = size ÷ outflow = 1,500 ÷ 60 = 25 years. Choice A divides the wrong way (60 ÷ 1,500). Choice C multiplies instead of dividing, and choice D subtracts.
Question 11 of 20 · Multiple Choice
In a closed four-sphere model for one year, the atmosphere gains 6 GtC, the ocean gains 2.5 GtC and fossil fuels lose 9.5 GtC. What must happen to the land biosphere?
Answer: C
Carbon is conserved, so the changes add to zero: 6 + 2.5 - 9.5 + x = 0, which gives x = +1 GtC. Choice A has the right size but the wrong sign. Choice B adds all the numbers without signs.
Question 12 of 20 · Multiple Choice
The Clarification Statement includes humans in the biosphere. Which human activity moves carbon from the biosphere to the atmosphere?
Answer: A
Trees are part of the biosphere, and burning them releases their carbon as CO2. Choices B and C move carbon that is stored in the geosphere, not the biosphere. Choice D does not directly move carbon between these two spheres.
Question 13 of 20 · Multiple Choice
A student's model treats the whole ocean as one well-mixed box of 38,000 GtC that can take up extra carbon from the air right away. Which is the strongest criticism of this model?
Answer: B
Only about 900 GtC sits in surface water that trades carbon with the air within years; the deep ocean exchanges with the surface slowly, so a one-box ocean takes up extra carbon too fast. Choice A is only a choice of units, which can be converted. Choices C and D ignore that the ocean trades about 80 GtC/yr with the air and holds far more carbon than the atmosphere.
Question 14 of 20 · Multiple Choice
In an invented lab, water with dissolved CO2 and a pond plant under a lamp rises from pH 6.4 to pH 7.5 in an hour. A tube with no plant stays at 6.4. What does this evidence support?
Answer: C
A higher pH means less dissolved CO2, and only the tube with the plant changed, so the plant took carbon from the hydrosphere into the biosphere. Choice A is ruled out by the control tube under the same lamp. Choice B would lower the pH, not raise it.
Question 15 of 20 · Short Answer
An invented pond takes up 3.0 kg of carbon per day from the air. Plants in the pond take 5.0 kg/day of dissolved carbon by photosynthesis, respiration and decay return 4.5 kg/day to the water, and the pond releases 2.0 kg/day to the air. What is the net daily change in the dissolved carbon of the pond water?
Inflows to the water: 3.0 + 4.5 = 7.5 kg/day. Outflows: 5.0 + 2.0 = 7.0 kg/day. Net change = +0.5 kg of carbon per day: the water is gaining dissolved carbon.
Question 16 of 20 · Short Answer
In the class model, the land sink of 3 GtC/yr is photosynthesis (120) minus plant respiration (60) minus decomposition (57). Suppose photosynthesis is really 124 GtC/yr, a difference of about 3%, with the other two fluxes unchanged. What is the land sink now, and what does this tell you about uncertainty in the model?
Land sink = 124 - 60 - 57 = 7 GtC/yr, more than twice the class value. A small percentage error in a large gross flux becomes a large error in the net flux, because the net is a small difference between big numbers. That is why net sinks are often estimated from other evidence, such as the measured rise in the atmosphere and ocean measurements, rather than by subtracting gross fluxes.
Question 17 of 20 · Short Answer
Atmospheric CO2 rose by 2.9 ppm in an invented year when emissions were 12 GtC. How many GtC did the atmosphere gain, and what was the airborne fraction?
Gain = 2.9 × 2.12 ≈ 6.1 GtC. Airborne fraction = 6.1 ÷ 12 ≈ 0.51, or about 51%.
Question 18 of 20 · Short Answer
Trace one carbon atom through all four spheres, starting in a limestone cliff. Name the process at each step.
A sample answer: rain made slightly acidic by CO2weathers the limestone (geosphere), and the carbon is carried as dissolved bicarbonate by a river to the sea (hydrosphere). Plankton take it up and one is eaten by a fish (biosphere). The fish's respiration releases the carbon as CO2, which moves from the water to the air (atmosphere). Any path that names all four spheres and a real process at each step earns credit.
Question 19 of 20 · Short Answer
In a revised model, next year's atmosphere = this year + emissions - 0.02 × (this year - 590). Start at 900 GtC with emissions of 10 GtC/yr. What does the model give for next year?
Explain how humans fit into the carbon cycle model as part of the biosphere, and name two fluxes humans have changed that start in two different spheres.
Humans are living things, so our breathing and food are part of the biosphere's fluxes, but our largest effect comes from the fluxes we control. Two examples: burning fossil fuels moves carbon from the geosphere to the atmosphere, and clearing forests moves carbon from the biosphere to the atmosphere. Making cement from limestone is another geosphere example.
0 of 20 answered · 0 correct
06
Frequently Asked Questions
10 Questions
What does HS-ESS2-6 mean?
HS-ESS2-6 asks students to develop a model with numbers that shows how carbon moves among the ocean, the air, rocks and living things. Students track how much carbon each reservoir holds and how fast it moves, then use inflow minus outflow to explain changes. The code stands for High School, Earth and Space Science, core idea 2 (Earth's Systems), performance expectation 6.
What makes the HS-ESS2-6 model quantitative?
The reservoirs and fluxes have values and units, such as 870 GtC in the atmosphere and 120 GtC per year of photosynthesis, and the model uses them in calculations. A labeled drawing of the carbon cycle is a good start, but it becomes a quantitative model when students can compute a change, a residence time or a scenario result from it.
Is HS-ESS2-6 taught in Earth science, biology or chemistry?
It is usually taught in Earth and Space Science or Environmental Science. It links closely to the biology standard HS-LS2-5 on photosynthesis and respiration, and it uses chemistry ideas such as the mass of carbon in CO2, so it works well as a shared unit.
What is the difference between HS-ESS2-6 and HS-LS2-5?
HS-LS2-5 focuses on how photosynthesis and cellular respiration cycle carbon, with the biosphere at the center. HS-ESS2-6 asks for a quantitative model of all four spheres, including slow geosphere processes like weathering, burial and fossil fuels, and human activity. A lesson can build the biology first and then add the other spheres and the numbers.
Is there an assessment boundary for this standard?
No. HS-ESS2-6 has no official Assessment Boundary. The Clarification Statement puts the emphasis on modeling carbon through the ocean, atmosphere, soil and biosphere, including humans, so those should all appear in the model.
How exact are the carbon cycle numbers?
They are estimates. Scientists measure atmospheric CO2 very precisely, but the sizes of soil, plant and ocean reservoirs and the gross fluxes such as photosynthesis are known to within several percent or more. This page uses rounded values chosen so the budget balances, which is fine for modeling as long as students know the values are approximate.
What is a GtC and how does it relate to ppm?
A GtC is a gigatonne of carbon, 1012 kg or one billion metric tons. Because the atmosphere has a fixed total mass, each ppm of CO2 corresponds to about 2.12 GtC. Emissions are often reported in tonnes of CO2, which weigh 44/12 as much as the carbon they contain.
What safety steps do the carbon labs need?
Wear goggles for both labs. Vinegar is a weak acid, so rinse splashes with water and never mix it with other chemicals. For bromothymol blue, students blow out through their own straw and never suck in, and everyone washes hands afterward. Keep lamps away from water, and return Elodea to the classroom tank instead of releasing it outdoors.
Can students build the model without computers?
Yes. A table on paper works for five or ten years of the model, with one row per year and one column per reservoir. Students can also move counters such as dried beans between labeled cups. A spreadsheet helps for longer runs and for the version where uptake depends on the amount of carbon in the air.
How does the carbon cycle connect to climate?
CO2 in the atmosphere absorbs infrared, so moving carbon into or out of the air changes Earth's energy balance. That link is the focus of HS-ESS2-4, while HS-ESS2-6 is about where the carbon goes. Using the two models together helps students connect a flux in GtC per year to a change in climate.
07
Related Standards
6 standards
These standards connect to HS-ESS2-6: prerequisites to review first, parallel standards at the same level, and next steps that build on it.
Before this lesson
MS-LS2-3Prerequisite
Model the cycling of matter and flow of energy among living and nonliving parts of ecosystems
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