B. Biology: Sustainable Ecosystems and Climate Change · Sustainable Ecosystems and Climate Change

The Carbon Cycle: Stores, Flows and Balance

B2.2 B2.6 A1.1About 70 minutes

Learning goals

I can describe the main carbon stores (reservoirs) in all four spheres.
I can explain the processes that move carbon between stores, in the fast and slow carbon cycles.
I can explain how the carbon cycle keeps ecosystems in dynamic equilibrium, using the seasonal CO₂ pattern as evidence.
I can explain how burning fossil fuels and clearing forests upset the balance.

Words to know

carbon reservoir (store)
A place where carbon is held for a time, such as the atmosphere, oceans, living things, soil or rock.

carbon sink
A reservoir that takes in more carbon than it releases, such as a growing forest, a peatland or the ocean.

carbon source
Something that releases more carbon than it takes in, such as burning fuel or a decomposing log.

combustion
Burning: a fuel reacts with oxygen, releasing energy, carbon dioxide and water.

fossil fuel
Coal, oil or natural gas, formed over millions of years from the remains of ancient organisms.

peatland
A wetland where dead plants build up faster than they decompose, forming thick, carbon-rich peat.

parts per million (ppm)
A unit for very small concentrations: 400 ppm means 400 molecules out of every million.

Remember from Grade 7

In Grade 7 you learned that carbon moves through photosynthesis, respiration, decomposition and combustion, and that forests and oceans are carbon sinks. In Grade 7 Heat in the Environment you learned that carbon dioxide is a greenhouse gas. Now you will map every major carbon store, separate the fast and slow carbon cycles, and read real atmospheric data as evidence of a dynamic equilibrium that humans are changing.

Let's start with you

About 18% of your body mass is carbon. The carbon atoms in your fingernails were in your breakfast a few weeks ago. Before that they were in a wheat field or a cow, and before that they were floating in the air as carbon dioxide. A carbon atom can stay in one place for seconds (in your breath) or for millions of years (in limestone). Following where carbon goes, and how long it stays, is the key to understanding both ecosystems and climate change.

Where is the carbon? Reservoirs in all four spheres

Atmosphere: mostly carbon dioxide (CO₂), plus a little methane (CH₄).
Biosphere: carbon is the backbone of every living molecule: sugars, fats, proteins, DNA. Forests hold a lot of it in wood.
Hydrosphere: the oceans hold much more carbon than the atmosphere, as dissolved CO₂ and related ions, and in the shells of sea creatures.
Lithosphere: by far the biggest store. Limestone rock (made from ancient shells), fossil fuels, and soil organic matter. Peatlands store carbon from plants that never fully decomposed. The Hudson Bay Lowlands in northern Ontario contain one of the largest areas of peatland in the world.

How carbon moves

The fast carbon cycle (days to centuries) moves carbon through living things, the air, the surface ocean and soil:

Photosynthesis: producers take CO₂ from the air (or water) and build sugar.
Eating: consumers take in carbon compounds from their food.
Cellular respiration: all organisms (plants too!) break down sugar and release CO₂.
Decomposition: decomposers break down dead matter and wastes and release CO₂ (or methane, where there is no oxygen, as in waterlogged soil).
Ocean–air exchange: CO₂ dissolves into cold surface water and comes out of warm water.

The slow carbon cycle (thousands to millions of years) moves carbon in and out of rock:

Shells and dead plankton sink and form limestone; buried plants and plankton can slowly become coal, oil and gas.
Rain (slightly acidic from dissolved CO₂) weathers rock, eventually carrying carbon to the ocean.
Volcanoes release CO₂ from deep inside Earth.

Combustion links the two. Wildfires burn living carbon. Burning fossil fuels takes carbon that was locked away for millions of years and puts it into the air in a few seconds.

The carbon cycle. Solid arrows show the fast cycle (days to centuries). The dashed arrow shows part of the slow cycle: shells and dead plankton settle and, over millions of years, form limestone. The thick arrow is combustion of fossil fuels, which moves ancient carbon into the air quickly.

Dynamic equilibrium in the carbon cycle

Before the Industrial Revolution (around 1750 to 1850), the amount of CO₂ in the air stayed close to 280 ppm for thousands of years. Carbon was moving constantly, but the flows in and out of the atmosphere roughly balanced: photosynthesis took in about as much as respiration and decomposition gave back. That is a dynamic equilibrium.

You can actually see the cycle at work. In the Northern Hemisphere, which has most of the world's land and forests, plants grow in spring and summer. Photosynthesis is faster than respiration, so CO₂ in the air drops. In fall and winter, leaves fall and decompose, photosynthesis slows, and CO₂ rises. This makes a yearly zigzag in the data.

Carbon dioxide in the air at Mauna Loa, Hawaii, drawn from rounded yearly values (about 317 ppm in 1960, 371 ppm in 2000 and 414 ppm in 2020) with the seasonal zigzag at its typical size (about 6 ppm from low to high). The zigzag is plants "breathing"; the long climb is extra CO₂ from human activities.

Humans tip the balance

Since the Industrial Revolution, people have added carbon to the atmosphere much faster than sinks can remove it:

Burning fossil fuels for transportation, heating, electricity and industry moves slow-cycle carbon into the fast cycle.
Deforestation removes a carbon sink, and burning or decomposing the trees releases their carbon.
Draining wetlands and peatlands lets oxygen in, so the peat decomposes and releases CO₂.
Making cement (from limestone) releases CO₂.

The oceans and land plants still absorb a large part of the extra CO₂ every year, roughly half combined, but not all of it. The rest builds up. CO₂ in the air is now more than 420 ppm, about 50% higher than before the Industrial Revolution, and it has been rising by more than 2 ppm per year recently. The extra CO₂ that dissolves in the ocean also makes seawater more acidic (Lesson 11).

Sink or source? It depends

A young, growing forest is a carbon sink: it takes in more CO₂ than it releases. A forest that is burning or has many dying trees (for example, after an insect outbreak) can become a carbon source. The same reservoir can switch roles, so scientists measure the net flow (in minus out).

Worked example

Let's read the CO₂ graph

Use the graph: CO₂ was about 317 ppm in 1960 and about 414 ppm in 2020. (a) What was the average rate of increase? (b) By what percentage did CO₂ increase over the 60 years?

(a) Change = 414 − 317 = 97 ppm. Time = 2020 − 1960 = 60 years.
Rate = 97 ppm ÷ 60 years ≈ 1.6 ppm per year (on average).
(b) Percentage increase = (change ÷ original) × 100% = (97 ÷ 317) × 100%.
97 ÷ 317 ≈ 0.306, so the increase is about 31%.
Notice: the curve gets steeper over time, so the rate is now faster than the 60-year average.

(a) About 1.6 ppm per year on average. (b) About a 31% increase from 1960 to 2020.

Worked example

Let's follow one carbon atom

Describe a possible path for a carbon atom that starts in a piece of coal burned 150 years ago and ends up in a maple leaf that falls this autumn.

Combustion: the coal burns, and the carbon atom joins two oxygen atoms to become CO₂ in the atmosphere.
It may spend years drifting in the air, maybe dissolving in the ocean and coming back out.
Photosynthesis: a sugar maple takes in the CO₂ through tiny pores (stomata) in a leaf and builds it into glucose.
The glucose is used to build cellulose in a new leaf.
In autumn the leaf falls. Decomposers will break it down and release the carbon as CO₂ again, continuing the fast cycle.

Coal (lithosphere) → CO₂ (atmosphere) by combustion → sugar in a maple (biosphere) by photosynthesis → leaf litter (soil) → back to CO₂ by decomposition.

Hands-on activity

The carbon atom's journey game

You need: one six-sided die, paper, a pencil. Optional: a family member to play against.

Make six stations on paper: Atmosphere, Plant, Animal, Soil, Ocean, Rock/fossil fuel. Start at Atmosphere. Roll and move using these rules, recording each move and the process that caused it:

Atmosphere: 1–2 Plant (photosynthesis); 3 Ocean (dissolves); 4–6 stay (still in the air).
Plant: 1–2 Atmosphere (respiration); 3 Animal (eaten); 4 Soil (leaf falls, plant dies); 5–6 stay (stored in wood).
Animal: 1–3 Atmosphere (respiration); 4–5 Soil (waste or death); 6 stay.
Soil: 1–3 Atmosphere (decomposition); 4 Plant (no: carbon doesn't go straight from soil into plants, so stay); 5–6 stay (stored in soil). Notice this rule and think about why.
Ocean: 1–2 Atmosphere (comes out of water); 3 Plant (taken up by algae; count algae as Plant); 4–5 stay; 6 Rock (shells sink and form limestone).
Rock/fossil fuel: 1 Atmosphere (volcano, or humans burn it); 2–6 stay (millions of years).

Play 15 rolls. Then answer:

Which stations did your atom visit most? Which one is hardest to leave?
Change the Rock rule to: 1–4 Atmosphere (humans burning fossil fuels). Play 15 more rolls with a new atom starting in Rock. What happens to the amount of carbon in the atmosphere if everyone plays this way?
Why can't carbon move straight from soil into a plant? (Plants get carbon from CO₂ in the air, not through their roots.)

What to look for: the Rock station traps the atom for a long time (slow cycle); changing the rule for burning moves rock carbon into the air much faster, which models the human effect.

Just for fun

Since about 18% of your body is carbon, you are officially a temporary carbon reservoir. Please return your atoms to the cycle in good condition (eventually, no rush).

Your turn

Name one carbon reservoir in each of the four spheres.

Which two processes move carbon INTO the atmosphere in the fast carbon cycle, and which one moves it OUT?

Explain the difference between the fast and slow carbon cycles, with an example of each.

Why does CO₂ in the air fall every Northern Hemisphere summer?

Why is burning fossil fuels such a big change to the carbon cycle?

Is a drained peatland a carbon sink or source? Explain.

About what was the CO₂ concentration before the Industrial Revolution, and about what is it now?

More practice: warm-up

What is a carbon sink?

Which process do plants use to take in CO₂?

Do plants carry out cellular respiration?

Name the three fossil fuels.

What rock is made mostly from ancient shells?

More practice: on your own

CO₂ was about 370 ppm in 2000 and about 414 ppm in 2020. Find the average rate of increase, and compare it with the 1960–2020 average of 1.6 ppm/yr.

A spruce budworm outbreak kills many trees in a forest. Explain how the forest could switch from a carbon sink to a carbon source.

Why do scientists measure CO₂ on a mountain in the middle of the Pacific Ocean?

Explain how the ocean is part of both the fast and slow carbon cycles.

Waterlogged soil in a wetland releases methane instead of CO₂ when plants decompose. Why?

Challenge

The seasonal zigzag in CO₂ is larger at Alert, Nunavut than at the South Pole. Suggest why.

People sometimes say "Burning wood is carbon neutral, but burning coal is not." Use the fast and slow carbon cycles to explain the reasoning, and one limit of it.

CO₂ in the air rose by about 97 ppm from 1960 to 2020, but humans emitted enough CO₂ to raise it by roughly twice that. Where did the rest go, and what problem does that cause?

Check your understanding

Quiz

Which reservoir holds the MOST carbon on Earth?

Which process releases CO₂ from dead leaves on the forest floor?

Why does atmospheric CO₂ dip each Northern Hemisphere summer?

Which is part of the SLOW carbon cycle?

CO₂ rose from 340 ppm to 420 ppm over 40 years. What was the average rate?

A peatland is drained to plant crops. What happens to its carbon?

For parents and teachers

Big idea (B2.2, links to B2.6): carbon is recycled between reservoirs in all four spheres. The fast cycle (photosynthesis, respiration, decomposition, ocean exchange) was in dynamic equilibrium for thousands of years, which shows up as a balanced seasonal "breathing" of the planet. Burning fossil fuels and clearing forests add carbon faster than sinks remove it. Reading and calculating from the CO₂ graph builds research and data skills (A1.1).

How to help: Play the carbon journey game together; it is quick and fun. Ask your teen to explain why the Rock station is so hard to leave.

Common difficulties:

Thinking plants only photosynthesize. Plants also respire, day and night.
Thinking plants get carbon from the soil. Plant carbon comes from CO₂ in the air.
Confusing ppm with percent. 420 ppm is 0.042%.

Sources: CO₂ values are rounded from the NOAA Mauna Loa record (annual means about 317 ppm in 1960 and 414 ppm in 2020; more than 420 ppm in the 2020s); pre-industrial value of about 280 ppm and roughly half of emissions absorbed by land and ocean sinks from IPCC and NOAA summaries.