HS-ESS2-6: Modeling the Carbon Cycle with Numbers
Also called NGSS HS-ESS2-6 · Topic: Earth's Systems · Earth and Space Science
The Official Wording
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.
NGSS Lead States, 2013 · Read it on nextgenscience.org · checked October 15, 2026
What HS-ESS2-6 Means in Plain English
Students build a model with numbers that tracks carbon as it moves among the air, the oceans, rocks and soil, and living things. They compare how much each reservoir holds and how fast carbon flows between them, including the flows people add.
No formal limit is set, but the model must use quantities, not arrows alone. It builds on photosynthesis and respiration cycling carbon in ecosystems (HS-LS2-5) and feeds models of human impact on Earth's systems (HS-ESS3-6).
HS-ESS2-6 I Can Statements
- I can name the main places carbon is stored on Earth.
- I can describe the flows that move carbon between reservoirs.
- I can use a carbon budget to explain rising CO₂ levels.
HS-ESS2-6 Worked Example
A spreadsheet carbon budget built from rounded recent global estimates.
Of the carbon people release each year, how much stays in the air and how much do the oceans and land take up?
Set It Up: A spreadsheet with rounded yearly carbon flows in gigatonnes of carbon (GtC) and the conversion that 2.13 GtC raises atmospheric CO₂ by 1 ppm.
- Enter the yearly carbon released by people.Fossil fuels and cement about 10 GtC, land clearing about 1 GtC, total 11
- Enter the yearly uptake by the oceans and by land plants and soils.Ocean about 3 GtC, land about 3 GtC
- Subtract the uptake from the release to find what stays in the air.11 − 3 − 3 = 5 GtC per year
- Convert that carbon to a change in atmospheric CO₂.5 ÷ 2.13 ≈ 2.3 ppm per year
- Compare with the measured yearly rise in CO₂.Recent yearly rises run about 2 to 3 ppm
About half of the carbon people release each year stays in the atmosphere, and the rest is taken up by the oceans and by land plants and soils.
Why it works: Carbon is conserved, so whatever enters the air and is not drawn down by ocean chemistry or photosynthesis must pile up there, raising CO₂.
Common Mistakes with HS-ESS2-6
Thinking carbon gets used up
“Burning fuel destroys the carbon.”
Try this: Carbon atoms are never destroyed. Burning moves carbon from rock-stored fuel into the air as CO₂, where it joins the rest of the cycle.
Leaving out the slow reservoirs
A cycle drawn with only plants, animals and air
Try this: Include oceans, which hold far more carbon than the air, and rocks, which hold the most of all and release it slowly.
Mixing CO₂ mass with carbon mass
Treating 10 gigatonnes of CO₂ as 10 gigatonnes of carbon
Try this: One tonne of carbon makes about 3.67 tonnes of CO₂, so convert with the ratio 44 to 12 before comparing.
Where HS-ESS2-6 Fits
Practice HS-ESS2-6
Quick Check
- Which reservoir holds more carbon, the atmosphere or the ocean?
- People release 11 GtC a year and the oceans and land take up 6. How much stays in the air?
- How do plants move carbon out of the atmosphere?