Follow carbon through the air, plants, animals, soil, ocean and rocks. Watch the 8 processes, label the diagram, test a snail and an Elodea water plant in a virtual BTB jar lab, graph real CO₂ data from Mauna Loa and balance Earth’s carbon budget. High school and AP students can then run the 3D Earth box model.
Jump to: Processes · Label lab · Carbon stores · BTB jar lab · Keeling Curve · Carbon budget · Sort · Quiz · 3D model (AP) · Quick reference · Going further · For teachers · Misconceptions · FAQ
This diagram shows the carbon cycle the way you learn it in school (middle and high school); the AP 3D model of the whole planet is further down, in section 9. Choose a process to watch carbon atoms move along its arrow, or play Follow a carbon atom: roll the dice, see where your atom goes, and name each process.
Tap a button or an arrow on the diagram. Carbon atoms (black dots) will move along the arrow, and this box explains what happens.
Your carbon atom starts in the air as part of a CO2 molecule. Roll the dice to see where it goes next. Each place has its own dice rules, just like the classroom game.
Press all 8 process buttons above (or Play all processes). Then predict: does each process add CO₂ to the air, remove it, or only move carbon between other stores? Press Watch it to check on the diagram.
The Follow a carbon atom dice game is an extra: try it after the missions.
This is the carbon cycle diagram you draw and label in a test. Switch between the labeled diagram, a blank one to test your memory, and a quiz where you place each label yourself. You can also download both versions or print a label worksheet.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back. On a phone, swipe the diagram sideways to see every number.
Open the Label quiz, place all 13 labels, then press Check answers. All right, or at least 80 % (11 of 13) on the first check, completes the mission. Try again starts a new round.
Here are seven of Earth’s big carbon stores. Put them in order from the smallest store to the largest with the ▲ and ▼ buttons, then press Check my order. The bar chart then shows the real sizes, in gigatonnes of carbon (1 GtC = 1 billion metric tons).
Log scale: each step on the axis is 10 times more carbon. “× the air” = how many times the carbon in the air (900 GtC).
Numbers: IPCC AR6 (2021), Figure 5.12, rounded. Soils do not include the ~1,400 GtC in permafrost.
Bromothymol blue (BTB) is a color indicator: blue in fresh water, green with some dissolved CO₂, yellow with a lot (CO₂ makes water slightly acidic). Four sealed tubes hold BTB water: A water only, B a snail, C an Elodea water plant, D a snail and an Elodea. Choose light or dark and the starting color, predict, then run 24 hours.
| Time | A water | B snail | C Elodea | D snail + Elodea |
|---|
Mission: at least 3 runs with predictions (light and dark), then 4 questions with at least 3 right on the first try.
Virtual snails: no animals harmed. The numbers are illustrative, but they follow real class results (snail tubes turn yellow overnight; Elodea in bright light turns breathed-in BTB green and then blue again within a day).
Since 1958, scientists have measured CO₂ in the air on the Mauna Loa volcano in Hawaii, far from cities and forests. The graph shows NOAA’s yearly averages, 1959–2023 in ppm (parts per million: 421 ppm = 421 CO₂ molecules in every million molecules of air). Move the year slider to read the graph.
Seasonal swing modeled from the typical Mauna Loa cycle (about ±3.3 ppm: highest in May, lowest in late September and October).
Fact: the May 2024 monthly average was 426.90 ppm (◆ on the graph), and the record keeps rising: in May 2025 it reached about 430.5 ppm (NOAA).
Data: NOAA Global Monitoring Laboratory, Mauna Loa CO₂ annual mean (co2_annmean_mlo), public domain.
Think of the air as a bathtub. Emissions are the tap; the ocean and land plants are the drain. The drain gets faster when there is more CO₂ in the air, but slower as the ocean and land fill up. Uptake = the carbon the ocean and plants take out of the air each year. Net emissions = what we add (fossil fuels + cutting forests) minus what newly planted trees take back. Start in 2023 at 421.08 ppm, set the sliders (GtC per year), and run the model to 2100.
Simple teaching model: uptake = 0.0734 × (CO₂ − “full” level) GtC per year, split ocean 58 % / land 42 %. The “full” level is 339 ppm in 2023 and rises as the ocean and land take up carbon (by 30 % of the carbon they take up, counted in ppm); 2.124 GtC = 1 ppm. Real sinks also change as the climate warms.
| Year | Emissions (GtC/yr) | Uptake (GtC/yr) | CO₂ (ppm) |
|---|
Press a card, then choose where it belongs. Adds CO₂ to the air, removes CO₂ from the air, or only moves carbon between other stores (the air does not change)? Your first choice counts: 11 of 14 completes the mission.
10 questions from a bank of 20. Score 7 or more to complete the mission. Each answer comes with an explanation.
For high school and AP Environmental Science: a simplified box model of the whole planet (atmosphere, vegetation, soil, ocean and fossil reserves, starting near 410 ppm) in which you change human emissions and rate constants, run scenarios and the story walkthrough, and export a CSV.
Your browser has 3D graphics (WebGL) turned off, so the 3D Earth tab shows the reservoir-flow view instead. The model and the other three tabs work normally.
Observed warming so far is about 1.3 °C; the ocean delays the rest. ΔT here is the long-term warming for the CO₂ level (3 °C per doubling, toy model).
| Store | Carbon | × the air |
|---|---|---|
| Living plants | ~450 | 0.5 |
| Atmosphere (2023) | ~900 | 1 |
| Surface ocean | ~900 | 1 |
| Fossil fuel reserves | ~930 | 1 |
| Soils (+ ~1,400 permafrost) | ~1,700 | 1.9 |
| Deep ocean | ~37,000 | 41 |
| Sedimentary rocks | > 60,000,000 | > 66,000 |
| Process | Effect on the air | Word equation or path |
|---|---|---|
| Photosynthesis | Removes CO₂ | carbon dioxide + water → glucose + oxygen (light energy) |
| Respiration | Adds CO₂ | glucose + oxygen → carbon dioxide + water (+ energy) |
| Feeding | Moves carbon | plant → animal → animal (food chains) |
| Death & waste | Moves carbon | living things → dead matter and waste in the soil |
| Decomposition | Adds CO₂ | decomposers respire: glucose + oxygen → carbon dioxide + water |
| Fossilization | Moves carbon | buried remains → coal, oil, gas; shells → limestone (millions of years) |
| Combustion | Adds CO₂ | fuel + oxygen → carbon dioxide + water (+ energy) |
| Ocean exchange | Both ways | CO₂ dissolves into cold water (removes); warm water releases it (adds) |
Fast cycle (days to centuries): photosynthesis, respiration, feeding, decomposition and ocean–air exchange move about 200 GtC every year each way between air, living things, soil and ocean, almost in balance. Slow cycle (millions of years): fossilization, rock formation, weathering and volcanoes move only a fraction of a GtC per year. Burning fossil fuels takes carbon from the slow cycle and puts it into the fast cycle in decades, so CO₂ in the air rises.
pH is a log scale: [H⁺] = 10−pH. When CO₂ dissolves it forms carbonic acid (CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻), and the extra H⁺ also turns carbonate (CO₃²⁻) into bicarbonate, leaving less carbonate for shells (CaCO₃).
Every year about 200 GtC moves from the air into plants (~120) and the ocean (~80), and almost as much comes back. So an individual CO₂ molecule stays in the air only about 900 ÷ 200 ≈ 4–5 years (its residence time). But an excess of CO₂ is removed only as fast as the net uptake, a few GtC per year: much of it takes decades to centuries, and about a fifth is still in the air after 1,000 years (the adjustment time). Swapping molecules is fast; draining the extra carbon is slow.
Plants prefer the lighter isotope carbon-12, so plant-made carbon (and the coal, oil and gas made from ancient plants) is low in carbon-13 and, being millions of years old, has no carbon-14 left. As fossil CO₂ is added, the share of ¹³C in the air (δ¹³C) falls and ¹⁴C is diluted. Both trends are measured, and they show that the extra CO₂ comes from fossil fuels, not from volcanoes or the ocean.
The Carbon Budget Lab uses two equations: dC/dt = (E − k(C − Cb)) ÷ 2.124 and Cb = 280 + 0.3 × S ÷ 2.124, where S is the carbon the ocean and land have taken up since 1850 (about 420 GtC by 2023). Check it by hand: in 2023, Cb = 280 + 0.3 × 420 ÷ 2.124 = 339.3 ppm and k(C − Cb) = 0.0734 × (421.08 − 339.3) = 6.0 GtC/yr, so with E = 11.6 the rise is 5.6 ÷ 2.124 = 2.6 ppm/yr and the airborne fraction is 5.6 ÷ 11.6 ≈ 48 %. The rising Cb is a simple stand-in for the surface ocean filling up; it is why CO₂ falls only slowly after emissions stop. Then use the 3D Earth model, which tracks vegetation, soil and ocean separately with rate constants kP, kR and kO, to see which assumption changes the 2100 result most.
Think of the atmosphere as a bathtub. The tap is everything that adds CO₂ — factories, cars, deforestation. The drain is everything that removes CO₂ — forests photosynthesizing, oceans dissolving it. For 10,000 years the tap and drain were balanced, so the water level stayed around 280 ppm. Since 1850 we've opened the tap much wider than the drain can handle, and the level keeps rising. The higher the level, the warmer the climate gets. That's the whole story.
| Slider | Real-world meaning |
|---|---|
| Fossil-fuel burning | How much coal/oil/gas humanity is burning per year. Bigger = tap wide open. |
| Deforestation | Cutting forests releases stored carbon. Negative values = planting forests (drain bigger). |
| Cement / industry | Making cement chemically releases CO₂ even before fuel is burned. |
| Photosynthesis kP | How efficiently plants soak up CO₂. Healthier biosphere = bigger drain. |
| Respiration kR | How fast plants/soil release CO₂ back. Warmer Earth = bigger leak back into the tub. |
| Ocean uptake kO | How quickly the ocean dissolves atmospheric CO₂. Slow but steady drain. |
Picture carbon as money that never gets created or destroyed — it only moves between accounts. There are four accounts: the air (a small checking account you spend from every day), living things and soil (a medium savings account), the ocean (a huge, slow vault), and fossil fuels underground (an old safe nobody touched for millions of years). For thousands of years the same amount flowed into the air account as flowed out, so its balance held steady near 280 ppm. Then humanity cracked open the old safe and started shoveling its carbon into the air faster than the savings and vault accounts can take it back.
To put numbers on it, think of one simple rule: change equals what comes in minus what goes out. Over a year the air's carbon $A$ changes by
Right now humans add about $11.6$ PgC (= GtC) each year (fossil fuels $\approx 10.3$ + land use $\approx 1.3$), while forests and oceans together pull back roughly $6$ PgC. So about $5.6$ PgC stays in the air per year. Multiply by the conversion $0.471$ ppm per PgC and you get $\approx 2.6$ ppm/yr, close to the 2.4–2.6 ppm/yr measured at Mauna Loa in recent years. That single subtraction is the whole crisis in one line.
The deeper, AP-level picture replaces those two lumps with one balance equation for every account. For the atmosphere,
Each slider in the panel above is one term here: Fossil-fuel burning sets $E_{ff}$, Deforestation sets $E_{lu}$ (negative values plant forests instead), Photosynthesis $k_P$ scales $P$, Respiration $k_R$ scales the leak-back $R_V$, and Ocean uptake $k_O$ controls how fast the vault drains the air toward its equilibrium $A_{eq}\approx 280$ ppm. Notice the ocean term only pulls while $A > A_{eq}$, and it pulls slowly — which is why the air stays high for centuries even after the sources stop.
New to the topic? Start with the processes, the BTB jar lab and the carbon budget lab.
Try this in the 3D model above. First press Run as-is and watch CO₂ and ΔT climb together. Then drag Fossil-fuel burning, Deforestation and Cement to $0$: CO₂ stops rising and then falls only slowly, because the sinks take up just a few PgC per year. Stopping emissions stops the rise; it does not quickly undo it. Finally set Deforestation negative (reforestation) and raise Ocean uptake $k_O$, and watch how much harder the sinks have to work just to bend the line back down.
Carbon is conserved. It does not disappear; it merely moves between four major reservoirs: the atmosphere (as CO₂ & CH₄), the terrestrial biosphere (plants & soils), the oceans (dissolved inorganic carbon), and lithospheric reserves (fossil fuels, carbonate rock). Burning fossil fuels and deforesting land take carbon out of slow reservoirs and dump it into the fastest one — the atmosphere — where it warms the climate until the slower reservoirs catch up.
Let A, V, S, O, F be carbon (in PgC = petagrams = 10¹⁵ g) in atmosphere, vegetation, soil, ocean, and fossil reserves. Mass conservation:
| Symbol | Meaning | Typical value |
|---|---|---|
| P | Photosynthesis (gross primary productivity) | kP·Aeq·(1 + 0.4 ln(A/Aeq))·tanh(V/600) ≈ 95 PgC/yr in the model (real gross photosynthesis ≈ 120) |
| RV | Autotrophic (plant) respiration | kR·V ≈ 44 PgC/yr |
| RS | Heterotrophic (soil) respiration | kS·S ≈ 48 PgC/yr |
| L | Litterfall (V → S) | kL·V ≈ 50 PgC/yr |
| ΦO↔A | Net ocean → atmosphere flux | kO·(A − Aeq) |
| Eff | Fossil-fuel + cement emissions | 10 PgC/yr (2025) |
| Elu | Land-use change (deforestation) | ~1 PgC/yr |
The ocean–atmosphere exchange is approximated as a relaxation toward chemical equilibrium with partial pressure of CO₂:
One petagram of atmospheric carbon corresponds to about 0.471 ppm (1 PgC = 10¹⁵ g ÷ 12 g/mol ≈ 8.3 × 10¹³ mol of carbon, so of CO₂; dry air ≈ 5.15 × 10¹⁸ kg ÷ 29 g/mol ≈ 1.78 × 10²⁰ mol; ratio ≈ 0.47 ppm). So when emissions push A up by 11.6 PgC, ppm rises by ~5.5, and because the ocean & biosphere together take up about half, we measure ~2.5 ppm/yr at Mauna Loa.
To turn CO₂ change into a temperature, the simulation uses the logarithmic forcing law and a 3 °C-per-doubling sensitivity:
Set fossil emissions to 30 PgC/yr (extreme growth) and press Run. In about a century (by 2125) the atmosphere climbs from ~905 to ~1,950 PgC, roughly 920 ppm: high enough for a long-term ΔT ≈ +5.1 °C above pre-industrial in this toy model. In the real Earth system, about 20–35 % of such a pulse would still be in the air after 2,000 years (Archer et al. 2009); this simple model cannot show that long tail. Try it: set the sliders, press Run, and watch.
Everything runs in the browser on classroom laptops, tablets and phones: no login, no data collected. Mission progress stays on each student’s device; the certificate prints with the student’s name.
All 8 missions in one lesson is tight; for younger classes use two periods (Missions 1–4, then 5–8).
| Time | Activity |
|---|---|
| 0–5 min | Hook: “Where does the carbon in a tree come from?” Quick vote (air, soil, water). Optional live demo: a student blows through a straw into BTB. |
| 5–12 min | Missions 1–2: watch the 8 processes and predict adds / removes / moves; label quiz. |
| 12–17 min | Mission 3: order the carbon stores; discuss the log scale (each step is 10 times more). |
| 17–27 min | Mission 4: BTB jar lab in pairs (one light run, one dark run, one breathed-in run); fill the data table on the worksheet. |
| 27–35 min | Mission 5: Keeling Curve tasks (read, rate of rise, zigzag, prediction). |
| 35–41 min | Mission 6: carbon budget challenges; class discussion of the scenarios. |
| 41–45 min | Missions 7–8: sort and quiz (or for homework). Exit ticket on the worksheet. |
One printable page per student (stores, BTB data table, Keeling tasks, budget challenges, sort, exit ticket) and a key with every value from the page’s models.
The older diagram worksheet (label the diagram, match processes, dice game) is still in section 1: Print worksheet + answer key.
NGSS: MS-LS2-3 (cycling of matter among living and nonliving parts of an ecosystem), MS-LS1-6 (photosynthesis), MS-LS1-7 (respiration), MS-ESS3-5 (rise in global temperatures), HS-LS2-5 (photosynthesis and respiration in the carbon cycle), HS-ESS2-6 (quantitative model of carbon cycling), HS-ESS3-6 (human activity and Earth systems). Practices: analyzing data, developing and using models, constructing explanations (CER).